When we fail to mutex_trylock(), we release the pool spin_lock and do
mutex_lock(). After that, we should regrab the pool spin_lock, but,
regrabbing is missed in current code. So correct it.
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This patch adds system wide workqueues aligned towards power saving. This is
done by allocating them with WQ_UNBOUND flag if 'wq_power_efficient' is set to
'true'.
tj: updated comments a bit.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Workqueues can be performance or power-oriented. Currently, most workqueues are
bound to the CPU they were created on. This gives good performance (due to cache
effects) at the cost of potentially waking up otherwise idle cores (Idle from
scheduler's perspective. Which may or may not be physically idle) just to
process some work. To save power, we can allow the work to be rescheduled on a
core that is already awake.
Workqueues created with the WQ_UNBOUND flag will allow some power savings.
However, we don't change the default behaviour of the system. To enable
power-saving behaviour, a new config option CONFIG_WQ_POWER_EFFICIENT needs to
be turned on. This option can also be overridden by the
workqueue.power_efficient boot parameter.
tj: Updated config description and comments. Renamed
CONFIG_WQ_POWER_EFFICIENT to CONFIG_WQ_POWER_EFFICIENT_DEFAULT.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
One of the problems that arise when converting dedicated custom
threadpool to workqueue is that the shared worker pool used by workqueue
anonimizes each worker making it more difficult to identify what the
worker was doing on which target from the output of sysrq-t or debug
dump from oops, BUG() and friends.
This patch implements set_worker_desc() which can be called from any
workqueue work function to set its description. When the worker task is
dumped for whatever reason - sysrq-t, WARN, BUG, oops, lockdep assertion
and so on - the description will be printed out together with the
workqueue name and the worker function pointer.
The printing side is implemented by print_worker_info() which is called
from functions in task dump paths - sched_show_task() and
dump_stack_print_info(). print_worker_info() can be safely called on
any task in any state as long as the task struct itself is accessible.
It uses probe_*() functions to access worker fields. It may print
garbage if something went very wrong, but it wouldn't cause (another)
oops.
The description is currently limited to 24bytes including the
terminating \0. worker->desc_valid and workder->desc[] are added and
the 64 bytes marker which was already incorrect before adding the new
fields is moved to the correct position.
Here's an example dump with writeback updated to set the bdi name as
worker desc.
Hardware name: Bochs
Modules linked in:
Pid: 7, comm: kworker/u9:0 Not tainted 3.9.0-rc1-work+ #1
Workqueue: writeback bdi_writeback_workfn (flush-8:0)
ffffffff820a3ab0 ffff88000f6e9cb8 ffffffff81c61845 ffff88000f6e9cf8
ffffffff8108f50f 0000000000000000 0000000000000000 ffff88000cde16b0
ffff88000cde1aa8 ffff88001ee19240 ffff88000f6e9fd8 ffff88000f6e9d08
Call Trace:
[<ffffffff81c61845>] dump_stack+0x19/0x1b
[<ffffffff8108f50f>] warn_slowpath_common+0x7f/0xc0
[<ffffffff8108f56a>] warn_slowpath_null+0x1a/0x20
[<ffffffff81200150>] bdi_writeback_workfn+0x2a0/0x3b0
...
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Acked-by: Jan Kara <jack@suse.cz>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
memory allocated by kmem_cache_alloc() should be freed using
kmem_cache_free(), not kfree().
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>
destroy_workqueue() performs several sanity checks before proceeding
with destruction of a workqueue. One of the checks verifies that
refcnt of each pwq (pool_workqueue) is over 1 as at that point there
should be no in-flight work items and the only holder of pwq refs is
the workqueue itself.
This worked fine as a workqueue used to hold only one reference to its
pwqs; however, since 4c16bd327c ("workqueue: implement NUMA affinity
for unbound workqueues"), a workqueue may hold multiple references to
its default pwq triggering this sanity check spuriously.
Fix it by not triggering the pwq->refcnt assertion on default pwqs.
An example spurious WARN trigger follows.
WARNING: at kernel/workqueue.c:4201 destroy_workqueue+0x6a/0x13e()
Hardware name: 4286C12
Modules linked in: sdhci_pci sdhci mmc_core usb_storage i915 drm_kms_helper drm i2c_algo_bit i2c_core video
Pid: 361, comm: umount Not tainted 3.9.0-rc5+ #29
Call Trace:
[<c04314a7>] warn_slowpath_common+0x7c/0x93
[<c04314e0>] warn_slowpath_null+0x22/0x24
[<c044796a>] destroy_workqueue+0x6a/0x13e
[<c056dc01>] ext4_put_super+0x43/0x2c4
[<c04fb7b8>] generic_shutdown_super+0x4b/0xb9
[<c04fb848>] kill_block_super+0x22/0x60
[<c04fb960>] deactivate_locked_super+0x2f/0x56
[<c04fc41b>] deactivate_super+0x2e/0x31
[<c050f1e6>] mntput_no_expire+0x103/0x108
[<c050fdce>] sys_umount+0x2a2/0x2c4
[<c050fe0e>] sys_oldumount+0x1e/0x20
[<c085ba4d>] sysenter_do_call+0x12/0x38
tj: Rewrote description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
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Merge tag 'v3.9-rc5' into wq/for-3.10
Writeback conversion to workqueue will be based on top of wq/for-3.10
branch to take advantage of custom attrs and NUMA support for unbound
workqueues. Mainline currently contains two commits which result in
non-trivial merge conflicts with wq/for-3.10 and because
block/for-3.10/core is based on v3.9-rc3 which contains one of the
conflicting commits, we need a pre-merge-window merge anyway. Let's
pull v3.9-rc5 into wq/for-3.10 so that the block tree doesn't suffer
from workqueue merge conflicts.
The two conflicts and their resolutions:
* e68035fb65 ("workqueue: convert to idr_alloc()") in mainline changes
worker_pool_assign_id() to use idr_alloc() instead of the old idr
interface. worker_pool_assign_id() goes through multiple locking
changes in wq/for-3.10 causing the following conflict.
static int worker_pool_assign_id(struct worker_pool *pool)
{
int ret;
<<<<<<< HEAD
lockdep_assert_held(&wq_pool_mutex);
do {
if (!idr_pre_get(&worker_pool_idr, GFP_KERNEL))
return -ENOMEM;
ret = idr_get_new(&worker_pool_idr, pool, &pool->id);
} while (ret == -EAGAIN);
=======
mutex_lock(&worker_pool_idr_mutex);
ret = idr_alloc(&worker_pool_idr, pool, 0, 0, GFP_KERNEL);
if (ret >= 0)
pool->id = ret;
mutex_unlock(&worker_pool_idr_mutex);
>>>>>>> c67bf5361e
return ret < 0 ? ret : 0;
}
We want locking from the former and idr_alloc() usage from the
latter, which can be combined to the following.
static int worker_pool_assign_id(struct worker_pool *pool)
{
int ret;
lockdep_assert_held(&wq_pool_mutex);
ret = idr_alloc(&worker_pool_idr, pool, 0, 0, GFP_KERNEL);
if (ret >= 0) {
pool->id = ret;
return 0;
}
return ret;
}
* eb2834285c ("workqueue: fix possible pool stall bug in
wq_unbind_fn()") updated wq_unbind_fn() such that it has single
larger for_each_std_worker_pool() loop instead of two separate loops
with a schedule() call inbetween. wq/for-3.10 renamed
pool->assoc_mutex to pool->manager_mutex causing the following
conflict (earlier function body and comments omitted for brevity).
static void wq_unbind_fn(struct work_struct *work)
{
...
spin_unlock_irq(&pool->lock);
<<<<<<< HEAD
mutex_unlock(&pool->manager_mutex);
}
=======
mutex_unlock(&pool->assoc_mutex);
>>>>>>> c67bf5361e
schedule();
<<<<<<< HEAD
for_each_cpu_worker_pool(pool, cpu)
=======
>>>>>>> c67bf5361e
atomic_set(&pool->nr_running, 0);
spin_lock_irq(&pool->lock);
wake_up_worker(pool);
spin_unlock_irq(&pool->lock);
}
}
The resolution is mostly trivial. We want the control flow of the
latter with the rename of the former.
static void wq_unbind_fn(struct work_struct *work)
{
...
spin_unlock_irq(&pool->lock);
mutex_unlock(&pool->manager_mutex);
schedule();
atomic_set(&pool->nr_running, 0);
spin_lock_irq(&pool->lock);
wake_up_worker(pool);
spin_unlock_irq(&pool->lock);
}
}
Signed-off-by: Tejun Heo <tj@kernel.org>
Unbound workqueues are now NUMA aware. Let's add some control knobs
and update sysfs interface accordingly.
* Add kernel param workqueue.numa_disable which disables NUMA affinity
globally.
* Replace sysfs file "pool_id" with "pool_ids" which contain
node:pool_id pairs. This change is userland-visible but "pool_id"
hasn't seen a release yet, so this is okay.
* Add a new sysf files "numa" which can toggle NUMA affinity on
individual workqueues. This is implemented as attrs->no_numa whichn
is special in that it isn't part of a pool's attributes. It only
affects how apply_workqueue_attrs() picks which pools to use.
After "pool_ids" change, first_pwq() doesn't have any user left.
Removed.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Currently, an unbound workqueue has single current, or first, pwq
(pool_workqueue) to which all new work items are queued. This often
isn't optimal on NUMA machines as workers may jump around across node
boundaries and work items get assigned to workers without any regard
to NUMA affinity.
This patch implements NUMA affinity for unbound workqueues. Instead
of mapping all entries of numa_pwq_tbl[] to the same pwq,
apply_workqueue_attrs() now creates a separate pwq covering the
intersecting CPUs for each NUMA node which has online CPUs in
@attrs->cpumask. Nodes which don't have intersecting possible CPUs
are mapped to pwqs covering whole @attrs->cpumask.
As CPUs come up and go down, the pool association is changed
accordingly. Changing pool association may involve allocating new
pools which may fail. To avoid failing CPU_DOWN, each workqueue
always keeps a default pwq which covers whole attrs->cpumask which is
used as fallback if pool creation fails during a CPU hotplug
operation.
This ensures that all work items issued on a NUMA node is executed on
the same node as long as the workqueue allows execution on the CPUs of
the node.
As this maps a workqueue to multiple pwqs and max_active is per-pwq,
this change the behavior of max_active. The limit is now per NUMA
node instead of global. While this is an actual change, max_active is
already per-cpu for per-cpu workqueues and primarily used as safety
mechanism rather than for active concurrency control. Concurrency is
usually limited from workqueue users by the number of concurrently
active work items and this change shouldn't matter much.
v2: Fixed pwq freeing in apply_workqueue_attrs() error path. Spotted
by Lai.
v3: The previous version incorrectly made a workqueue spanning
multiple nodes spread work items over all online CPUs when some of
its nodes don't have any desired cpus. Reimplemented so that NUMA
affinity is properly updated as CPUs go up and down. This problem
was spotted by Lai Jiangshan.
v4: destroy_workqueue() was putting wq->dfl_pwq and then clearing it;
however, wq may be freed at any time after dfl_pwq is put making
the clearing use-after-free. Clear wq->dfl_pwq before putting it.
v5: apply_workqueue_attrs() was leaking @tmp_attrs, @new_attrs and
@pwq_tbl after success. Fixed.
Retry loop in wq_update_unbound_numa_attrs() isn't necessary as
application of new attrs is excluded via CPU hotplug. Removed.
Documentation on CPU affinity guarantee on CPU_DOWN added.
All changes are suggested by Lai Jiangshan.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Factor out lock pool, put_pwq(), unlock sequence into
put_pwq_unlocked(). The two existing places are converted and there
will be more with NUMA affinity support.
This is to prepare for NUMA affinity support for unbound workqueues
and doesn't introduce any functional difference.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Factor out pool_workqueue linking and installation into numa_pwq_tbl[]
from apply_workqueue_attrs() into numa_pwq_tbl_install(). link_pwq()
is made safe to call multiple times. numa_pwq_tbl_install() links the
pwq, installs it into numa_pwq_tbl[] at the specified node and returns
the old entry.
@last_pwq is removed from link_pwq() as the return value of the new
function can be used instead.
This is to prepare for NUMA affinity support for unbound workqueues.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Use kmem_cache_alloc_node() with @pool->node instead of
kmem_cache_zalloc() when allocating a pool_workqueue so that it's
allocated on the same node as the associated worker_pool. As there's
no no kmem_cache_zalloc_node(), move zeroing to init_pwq().
This was suggested by Lai Jiangshan.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Break init_and_link_pwq() into init_pwq() and link_pwq() and move
unbound-workqueue specific handling into apply_workqueue_attrs().
Also, factor out unbound pool and pool_workqueue allocation into
alloc_unbound_pwq().
This reorganization is to prepare for NUMA affinity and doesn't
introduce any functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Currently, an unbound workqueue has only one "current" pool_workqueue
associated with it. It may have multple pool_workqueues but only the
first pool_workqueue servies new work items. For NUMA affinity, we
want to change this so that there are multiple current pool_workqueues
serving different NUMA nodes.
Introduce workqueue->numa_pwq_tbl[] which is indexed by NUMA node and
points to the pool_workqueue to use for each possible node. This
replaces first_pwq() in __queue_work() and workqueue_congested().
numa_pwq_tbl[] is currently initialized to point to the same
pool_workqueue as first_pwq() so this patch doesn't make any behavior
changes.
v2: Use rcu_dereference_raw() in unbound_pwq_by_node() as the function
may be called only with wq->mutex held.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Move wq->flags and ->cpu_pwqs to the end of workqueue_struct and align
them to the cacheline. These two fields are used in the work item
issue path and thus hot. The scheduled NUMA affinity support will add
dispatch table at the end of workqueue_struct and relocating these two
fields will allow us hitting only single cacheline on hot paths.
Note that wq->pwqs isn't moved although it currently is being used in
the work item issue path for unbound workqueues. The dispatch table
mentioned above will replace its use in the issue path, so it will
become cold once NUMA support is implemented.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Currently workqueue->name[] is of flexible length. We want to use the
flexible field for something more useful and there isn't much benefit
in allowing arbitrary name length anyway. Make it fixed len capping
at 24 bytes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Currently, when exposing attrs of an unbound workqueue via sysfs, the
workqueue_attrs of first_pwq() is used as that should equal the
current state of the workqueue.
The planned NUMA affinity support will make unbound workqueues make
use of multiple pool_workqueues for different NUMA nodes and the above
assumption will no longer hold. Introduce workqueue->unbound_attrs
which records the current attrs in effect and use it for sysfs instead
of first_pwq()->attrs.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
When worker tasks are created using kthread_create_on_node(),
currently only per-cpu ones have the matching NUMA node specified.
All unbound workers are always created with NUMA_NO_NODE.
Now that an unbound worker pool may have an arbitrary cpumask
associated with it, this isn't optimal. Add pool->node which is
determined by the pool's cpumask. If the pool's cpumask is contained
inside a NUMA node proper, the pool is associated with that node, and
all workers of the pool are created on that node.
This currently only makes difference for unbound worker pools with
cpumask contained inside single NUMA node, but this will serve as
foundation for making all unbound pools NUMA-affine.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Currently, all workqueue workers which have negative nice value has
'H' postfixed to their names. This is necessary for per-cpu workers
as they use the CPU number instead of pool->id to identify the pool
and the 'H' postfix is the only thing distinguishing normal and
highpri workers.
As workers for unbound pools use pool->id, the 'H' postfix is purely
informational. TASK_COMM_LEN is 16 and after the static part and
delimiters, there are only five characters left for the pool and
worker IDs. We're expecting to have more unbound pools with the
scheduled NUMA awareness support. Let's drop the non-essential 'H'
postfix from unbound kworker name.
While at it, restructure kthread_create*() invocation to help future
NUMA related changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Unbound workqueues are going to be NUMA-affine. Add wq_numa_tbl_len
and wq_numa_possible_cpumask[] in preparation. The former is the
highest NUMA node ID + 1 and the latter is masks of possibles CPUs for
each NUMA node.
This patch only introduces these. Future patches will make use of
them.
v2: NUMA initialization move into wq_numa_init(). Also, the possible
cpumask array is not created if there aren't multiple nodes on the
system. wq_numa_enabled bool added.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
The scheduled NUMA affinity support for unbound workqueues would need
to walk workqueues list and pool related operations on each workqueue.
Move wq_pool_mutex locking out of get/put_unbound_pool() to their
callers so that pool operations can be performed while walking the
workqueues list, which is also protected by wq_pool_mutex.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
29c91e9912 ("workqueue: implement attribute-based unbound worker_pool
management") implemented attrs based worker_pool matching. It tried
to avoid false negative when comparing cpumasks with custom hash
function; unfortunately, the hash and comparison functions fail to
ignore CPUs which are not possible. It incorrectly assumed that
bitmap_copy() skips leftover bits in the last word of bitmap and
cpumask_equal() ignores impossible CPUs.
This patch updates attrs->cpumask handling such that impossible CPUs
are properly ignored.
* Hash and copy functions no longer do anything special. They expect
their callers to clear impossible CPUs.
* alloc_workqueue_attrs() initializes the cpumask to cpu_possible_mask
instead of setting all bits and explicit cpumask_setall() for
unbound_std_wq_attrs[] in init_workqueues() is dropped.
* apply_workqueue_attrs() is now responsible for ignoring impossible
CPUs. It makes a copy of @attrs and clears impossible CPUs before
doing anything else.
Signed-off-by: Tejun Heo <tj@kernel.org>
8864b4e59 ("workqueue: implement get/put_pwq()") implemented pwq
(pool_workqueue) refcnting which frees workqueue when the last pwq
goes away. It determined whether it was the last pwq by testing
wq->pwqs is empty. Unfortunately, the test was done outside wq->mutex
and multiple pwq release could race and try to free wq multiple times
leading to oops.
Test wq->pwqs emptiness while holding wq->mutex.
Signed-off-by: Tejun Heo <tj@kernel.org>
To simplify locking, the previous patches expanded wq->mutex to
protect all fields of each workqueue instance including the pwqs list
leaving pwq_lock without any user. Remove the unused pwq_lock.
tj: Rebased on top of the current dev branch. Updated description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
We're expanding wq->mutex to cover all fields specific to each
workqueue with the end goal of replacing pwq_lock which will make
locking simpler and easier to understand.
This patch makes wq->saved_max_active protected by wq->mutex instead
of pwq_lock. As pwq_lock locking around pwq_adjust_mac_active() is no
longer necessary, this patch also replaces pwq_lock lockings of
for_each_pwq() around pwq_adjust_max_active() to wq->mutex.
tj: Rebased on top of the current dev branch. Updated description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
We're expanding wq->mutex to cover all fields specific to each
workqueue with the end goal of replacing pwq_lock which will make
locking simpler and easier to understand.
init_and_link_pwq() and pwq_unbound_release_workfn() already grab
wq->mutex when adding or removing a pwq from wq->pwqs list. This
patch makes it official that the list is wq->mutex protected for
writes and updates readers accoridingly. Explicit IRQ toggles for
sched-RCU read-locking in flush_workqueue_prep_pwqs() and
drain_workqueues() are removed as the surrounding wq->mutex can
provide sufficient synchronization.
Also, assert_rcu_or_pwq_lock() is renamed to assert_rcu_or_wq_mutex()
and checks for wq->mutex too.
pwq_lock locking and assertion are not removed by this patch and a
couple of for_each_pwq() iterations are still protected by it.
They'll be removed by future patches.
tj: Rebased on top of the current dev branch. Updated description.
Folded in assert_rcu_or_wq_mutex() renaming from a later patch
along with associated comment updates.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
We're expanding wq->mutex to cover all fields specific to each
workqueue with the end goal of replacing pwq_lock which will make
locking simpler and easier to understand.
wq->nr_drainers and ->flags are specific to each workqueue. Protect
->nr_drainers and ->flags with wq->mutex instead of pool_mutex.
tj: Rebased on top of the current dev branch. Updated description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Currently pwq->flush_mutex protects many fields of a workqueue
including, especially, the pwqs list. We're going to expand this
mutex to protect most of a workqueue and eventually replace pwq_lock,
which will make locking simpler and easier to understand.
Drop the "flush_" prefix in preparation.
This patch is pure rename.
tj: Rebased on top of the current dev branch. Updated description.
Use WQ: and WR: instead of Q: and QR: for synchronization labels.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
wq->flush_mutex will be renamed to wq->mutex and cover all fields
specific to each workqueue and eventually replace pwq_lock, which will
make locking simpler and easier to understand.
Rename wq_mutex to wq_pool_mutex to avoid confusion with wq->mutex.
After the scheduled changes, wq_pool_mutex won't be protecting
anything specific to each workqueue instance anyway.
This patch is pure rename.
tj: s/wqs_mutex/wq_pool_mutex/. Rewrote description.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
If lockdep complains something for other subsystem, lockdep_is_held()
can be false negative, so we need to also test debug_locks before
triggering WARN.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
rcu_read_lock_sched() is better than preempt_disable() if the code is
protected by RCU_SCHED.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
If pwq_adjust_max_active() changes max_active from 0 to
saved_max_active, it needs to wakeup worker. This is already done by
thaw_workqueues().
If pwq_adjust_max_active() increases max_active for an unbound wq,
while not strictly necessary for correctness, it's still desirable to
wake up a worker so that the requested concurrency level is reached
sooner.
Move wake_up_worker() call from thaw_workqueues() to
pwq_adjust_max_active() so that it can handle both of the above two
cases. This also makes thaw_workqueues() simpler.
tj: Updated comments and description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
We can test worker->recue_wq instead of reaching into
current_pwq->wq->rescuer and then comparing it to self.
tj: Commit message.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
get_unbound_pool() forgot to set POOL_FREEZING if workqueue_freezing
is set and a new pool could go out of sync with the global freezing
state.
Fix it by adding POOL_FREEZING if workqueue_freezing. wq_mutex is
already held so no further locking is necessary. This also removes
the unused static variable warning when !CONFIG_FREEZER.
tj: Updated commit message.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
With the recent addition of the custom attributes support, unbound
pools may have allowed cpumask which isn't full. As long as some of
CPUs in the cpumask are online, its workers will maintain cpus_allowed
as set on worker creation; however, once no online CPU is left in
cpus_allowed, the scheduler will reset cpus_allowed of any workers
which get scheduled so that they can execute.
To remain compliant to the user-specified configuration, CPU affinity
needs to be restored when a CPU becomes online for an unbound pool
which doesn't currently have any online CPUs before.
This patch implement restore_unbound_workers_cpumask(), which is
called from CPU_ONLINE for all unbound pools, checks whether the
coming up CPU is the first allowed online one, and, if so, invokes
set_cpus_allowed_ptr() with the configured cpumask on all workers.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Rebinding workers of a per-cpu pool after a CPU comes online involves
a lot of back-and-forth mostly because only the task itself could
adjust CPU affinity if PF_THREAD_BOUND was set.
As CPU_ONLINE itself couldn't adjust affinity, it had to somehow
coerce the workers themselves to perform set_cpus_allowed_ptr(). Due
to the various states a worker can be in, this led to three different
paths a worker may be rebound. worker->rebind_work is queued to busy
workers. Idle ones are signaled by unlinking worker->entry and call
idle_worker_rebind(). The manager isn't covered by either and
implements its own mechanism.
PF_THREAD_BOUND has been relaced with PF_NO_SETAFFINITY and CPU_ONLINE
itself now can manipulate CPU affinity of workers. This patch
replaces the existing rebind mechanism with direct one where
CPU_ONLINE iterates over all workers using for_each_pool_worker(),
restores CPU affinity, and clears WORKER_UNBOUND.
There are a couple subtleties. All bound idle workers should have
their runqueues set to that of the bound CPU; however, if the target
task isn't running, set_cpus_allowed_ptr() just updates the
cpus_allowed mask deferring the actual migration to when the task
wakes up. This is worked around by waking up idle workers after
restoring CPU affinity before any workers can become bound.
Another subtlety is stems from matching @pool->nr_running with the
number of running unbound workers. While DISASSOCIATED, all workers
are unbound and nr_running is zero. As workers become bound again,
nr_running needs to be adjusted accordingly; however, there is no good
way to tell whether a given worker is running without poking into
scheduler internals. Instead of clearing UNBOUND directly,
rebind_workers() replaces UNBOUND with another new NOT_RUNNING flag -
REBOUND, which will later be cleared by the workers themselves while
preparing for the next round of work item execution. The only change
needed for the workers is clearing REBOUND along with PREP.
* This patch leaves for_each_busy_worker() without any user. Removed.
* idle_worker_rebind(), busy_worker_rebind_fn(), worker->rebind_work
and rebind logic in manager_workers() removed.
* worker_thread() now looks at WORKER_DIE instead of testing whether
@worker->entry is empty to determine whether it needs to do
something special as dying is the only special thing now.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
rebind_workers() will be reimplemented in a way which makes it mostly
decoupled from the rest of worker management. Move rebind_workers()
so that it's located with other CPU hotplug related functions.
This patch is pure function relocation.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Make worker_ida an idr - worker_idr and use it to implement
for_each_pool_worker() which will be used to simplify worker rebinding
on CPU_ONLINE.
pool->worker_idr is protected by both pool->manager_mutex and
pool->lock so that it can be iterated while holding either lock.
* create_worker() allocates ID without installing worker pointer and
installs the pointer later using idr_replace(). This is because
worker ID is needed when creating the actual task to name it and the
new worker shouldn't be visible to iterations before fully
initialized.
* In destroy_worker(), ID removal is moved before kthread_stop().
This is again to guarantee that only fully working workers are
visible to for_each_pool_worker().
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
PF_THREAD_BOUND was originally used to mark kernel threads which were
bound to a specific CPU using kthread_bind() and a task with the flag
set allows cpus_allowed modifications only to itself. Workqueue is
currently abusing it to prevent userland from meddling with
cpus_allowed of workqueue workers.
What we need is a flag to prevent userland from messing with
cpus_allowed of certain kernel tasks. In kernel, anyone can
(incorrectly) squash the flag, and, for worker-type usages,
restricting cpus_allowed modification to the task itself doesn't
provide meaningful extra proection as other tasks can inject work
items to the task anyway.
This patch replaces PF_THREAD_BOUND with PF_NO_SETAFFINITY.
sched_setaffinity() checks the flag and return -EINVAL if set.
set_cpus_allowed_ptr() is no longer affected by the flag.
This will allow simplifying workqueue worker CPU affinity management.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Pull workqueue fix from Tejun Heo:
"Lai's patch to fix highly unlikely but still possible workqueue stall
during CPU hotunplug."
* 'for-3.9-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq:
workqueue: fix possible pool stall bug in wq_unbind_fn()
With the recent locking updates, the only thing protected by
workqueue_lock is workqueue->maydays list. Rename workqueue_lock to
wq_mayday_lock.
This patch is pure rename.
Signed-off-by: Tejun Heo <tj@kernel.org>
This patch continues locking cleanup from the previous patch. It
breaks out pool_workqueue synchronization from workqueue_lock into a
new spinlock - pwq_lock. The followings are protected by pwq_lock.
* workqueue->pwqs
* workqueue->saved_max_active
The conversion is straight-forward. workqueue_lock usages which cover
the above two are converted to pwq_lock. New locking label PW added
for things protected by pwq_lock and FR is updated to mean flush_mutex
+ pwq_lock + sched-RCU.
This patch shouldn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Currently, workqueue_lock protects most shared workqueue resources -
the pools, workqueues, pool_workqueues, draining, ID assignments,
mayday handling and so on. The coverage has grown organically and
there is no identified bottleneck coming from workqueue_lock, but it
has grown a bit too much and scheduled rebinding changes need the
pools and workqueues to be protected by a mutex instead of a spinlock.
This patch breaks out pool and workqueue synchronization from
workqueue_lock into a new mutex - wq_mutex. The followings are
protected by wq_mutex.
* worker_pool_idr and unbound_pool_hash
* pool->refcnt
* workqueues list
* workqueue->flags, ->nr_drainers
Most changes are mostly straight-forward. workqueue_lock is replaced
with wq_mutex where applicable and workqueue_lock lock/unlocks are
added where wq_mutex conversion leaves data structures not protected
by wq_mutex without locking. irq / preemption flippings were added
where the conversion affects them. Things worth noting are
* New WQ and WR locking lables added along with
assert_rcu_or_wq_mutex().
* worker_pool_assign_id() now expects to be called under wq_mutex.
* create_mutex is removed from get_unbound_pool(). It now just holds
wq_mutex.
This patch shouldn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
When a manager creates or destroys workers, the operations are always
done with the manager_mutex held; however, initial worker creation or
worker destruction during pool release don't grab the mutex. They are
still correct as initial worker creation doesn't require
synchronization and grabbing manager_arb provides enough exclusion for
pool release path.
Still, let's make everyone follow the same rules for consistency and
such that lockdep annotations can be added.
Update create_and_start_worker() and put_unbound_pool() to grab
manager_mutex around thread creation and destruction respectively and
add lockdep assertions to create_worker() and destroy_worker().
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
get_unbound_pool(), workqueue_cpu_up_callback() and init_workqueues()
have similar code pieces to create and start the initial worker factor
those out into create_and_start_worker().
This patch doesn't introduce any functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Manager operations are currently governed by two mutexes -
pool->manager_arb and ->assoc_mutex. The former is used to decide who
gets to be the manager and the latter to exclude the actual manager
operations including creation and destruction of workers. Anyone who
grabs ->manager_arb must perform manager role; otherwise, the pool
might stall.
Grabbing ->assoc_mutex blocks everyone else from performing manager
operations but doesn't require the holder to perform manager duties as
it's merely blocking manager operations without becoming the manager.
Because the blocking was necessary when [dis]associating per-cpu
workqueues during CPU hotplug events, the latter was named
assoc_mutex. The mutex is scheduled to be used for other purposes, so
this patch gives it a more fitting generic name - manager_mutex - and
updates / adds comments to explain synchronization around the manager
role and operations.
This patch is pure rename / doc update.
Signed-off-by: Tejun Heo <tj@kernel.org>
There's no reason to make these trivial wrappers full (exported)
functions. Inline the followings.
queue_work()
queue_delayed_work()
mod_delayed_work()
schedule_work_on()
schedule_work()
schedule_delayed_work_on()
schedule_delayed_work()
keventd_up()
Signed-off-by: Tejun Heo <tj@kernel.org>
Rename @id argument of for_each_pool() to @pi so that it doesn't get
reused accidentally when for_each_pool() is used in combination with
other iterators.
This patch is purely cosmetic.
Signed-off-by: Tejun Heo <tj@kernel.org>
* Update incorrect and add missing synchronization labels.
* Update incorrect or misleading comments. Add new comments where
clarification is necessary. Reformat / rephrase some comments.
* drain_workqueue() can be used separately from destroy_workqueue()
but its warning message was incorrectly referring to destruction.
Other than the warning message change, this patch doesn't make any
functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Since 9e8cd2f589 ("workqueue: implement apply_workqueue_attrs()"),
init_and_link_pwq() may be called to initialize a new pool_workqueue
for a workqueue which is already online, but the function was setting
pwq->max_active to wq->saved_max_active without proper
synchronization.
Fix it by calling pwq_adjust_max_active() under proper locking instead
of manually setting max_active.
Signed-off-by: Tejun Heo <tj@kernel.org>
Rename pwq_set_max_active() to pwq_adjust_max_active() and move
pool_workqueue->max_active synchronization and max_active
determination logic into it.
The new function should be called with workqueue_lock held for stable
workqueue->saved_max_active, determines the current max_active value
the target pool_workqueue should be using from @wq->saved_max_active
and the state of the associated pool, and applies it with proper
synchronization.
The current two users - workqueue_set_max_active() and
thaw_workqueues() - are updated accordingly. In addition, the manual
freezing handling in __alloc_workqueue_key() and
freeze_workqueues_begin() are replaced with calls to
pwq_adjust_max_active().
This centralizes max_active handling so that it's less error-prone.
Signed-off-by: Tejun Heo <tj@kernel.org>
pwq_set_max_active() is gonna be modified and used during
pool_workqueue init. Move it above init_and_link_pwq().
This patch is pure code reorganization and doesn't introduce any
functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
idr_get_new*() and friends are about to be deprecated. Convert to the
new idr_alloc() interface.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Implement a function which queries whether it currently is running off
a workqueue rescuer. This will be used to convert writeback to
workqueue.
Signed-off-by: Tejun Heo <tj@kernel.org>
There are cases where workqueue users want to expose control knobs to
userland. e.g. Unbound workqueues with custom attributes are
scheduled to be used for writeback workers and depending on
configuration it can be useful to allow admins to tinker with the
priority or allowed CPUs.
This patch implements workqueue_sysfs_register(), which makes the
workqueue visible under /sys/bus/workqueue/devices/WQ_NAME. There
currently are two attributes common to both per-cpu and unbound pools
and extra attributes for unbound pools including nice level and
cpumask.
If alloc_workqueue*() is called with WQ_SYSFS,
workqueue_sysfs_register() is called automatically as part of
workqueue creation. This is the preferred method unless the workqueue
user wants to apply workqueue_attrs before making the workqueue
visible to userland.
v2: Disallow exposing ordered workqueues as ordered workqueues can't
be tuned in any way.
Signed-off-by: Tejun Heo <tj@kernel.org>
Adjusting max_active of or applying new workqueue_attrs to an ordered
workqueue breaks its ordering guarantee. The former is obvious. The
latter is because applying attrs creates a new pwq (pool_workqueue)
and there is no ordering constraint between the old and new pwqs.
Make apply_workqueue_attrs() and workqueue_set_max_active() trigger
WARN_ON() if those operations are requested on an ordered workqueue
and fail / ignore respectively.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
We're gonna add another internal WQ flag. Let's make the distinction
clear. Prefix WQ_DRAINING with __ and move it to bit 16.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Implement apply_workqueue_attrs() which applies workqueue_attrs to the
specified unbound workqueue by creating a new pwq (pool_workqueue)
linked to worker_pool with the specified attributes.
A new pwq is linked at the head of wq->pwqs instead of tail and
__queue_work() verifies that the first unbound pwq has positive refcnt
before choosing it for the actual queueing. This is to cover the case
where creation of a new pwq races with queueing. As base ref on a pwq
won't be dropped without making another pwq the first one,
__queue_work() is guaranteed to make progress and not add work item to
a dead pwq.
init_and_link_pwq() is updated to return the last first pwq the new
pwq replaced, which is put by apply_workqueue_attrs().
Note that apply_workqueue_attrs() is almost identical to unbound pwq
part of alloc_and_link_pwqs(). The only difference is that there is
no previous first pwq. apply_workqueue_attrs() is implemented to
handle such cases and replaces unbound pwq handling in
alloc_and_link_pwqs().
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Because per-cpu workqueues have multiple pwqs (pool_workqueues) to
serve the CPUs, to guarantee that a single work item isn't queued on
one pwq while still executing another, __queue_work() takes a look at
the previous pool the target work item was on and if it's still
executing there, queue the work item on that pool.
To support changing workqueue_attrs on the fly, unbound workqueues too
will have multiple pwqs and thus need non-reentrancy test when
queueing. This patch modifies __queue_work() such that the reentrancy
test is performed regardless of the workqueue type.
per_cpu_ptr(wq->cpu_pwqs, cpu) used to be used to determine the
matching pwq for the last pool. This can't be used for unbound
workqueues and is replaced with worker->current_pwq which also happens
to be simpler.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Unbound pwqs (pool_workqueues) will be dynamically created and
destroyed with the scheduled unbound workqueue w/ custom attributes
support. This patch synchronizes pwq linking and unlinking against
flush_workqueue() so that its operation isn't disturbed by pwqs coming
and going.
Linking and unlinking a pwq into wq->pwqs is now protected also by
wq->flush_mutex and a new pwq's work_color is initialized to
wq->work_color during linking. This ensures that pwqs changes don't
disturb flush_workqueue() in progress and the new pwq's work coloring
stays in sync with the rest of the workqueue.
flush_mutex during unlinking isn't strictly necessary but it's simpler
to do it anyway.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Add pool_workqueue->refcnt along with get/put_pwq(). Both per-cpu and
unbound pwqs have refcnts and any work item inserted on a pwq
increments the refcnt which is dropped when the work item finishes.
For per-cpu pwqs the base ref is never dropped and destroy_workqueue()
frees the pwqs as before. For unbound ones, destroy_workqueue()
simply drops the base ref on the first pwq. When the refcnt reaches
zero, pwq_unbound_release_workfn() is scheduled on system_wq, which
unlinks the pwq, puts the associated pool and frees the pwq and wq as
necessary. This needs to be done from a work item as put_pwq() needs
to be protected by pool->lock but release can't happen with the lock
held - e.g. put_unbound_pool() involves blocking operations.
Unbound pool->locks are marked with lockdep subclas 1 as put_pwq()
will schedule the release work item on system_wq while holding the
unbound pool's lock and triggers recursive locking warning spuriously.
This will be used to implement dynamic creation and destruction of
unbound pwqs.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
* Move initialization and linking of pool_workqueues into
init_and_link_pwq().
* Make the failure path use destroy_workqueue() once pool_workqueue
initialization succeeds.
These changes are to prepare for dynamic management of pool_workqueues
and don't introduce any functional changes.
While at it, convert list_del(&wq->list) to list_del_init() as a
precaution as scheduled changes will make destruction more complex.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
WQ_RESCUER is superflous. WQ_MEM_RECLAIM indicates that the user
wants a rescuer and testing wq->rescuer for NULL can answer whether a
given workqueue has a rescuer or not. Drop WQ_RESCUER and test
wq->rescuer directly.
This will help simplifying __alloc_workqueue_key() failure path by
allowing it to use destroy_workqueue() on a partially constructed
workqueue, which in turn will help implementing dynamic management of
pool_workqueues.
While at it, clear wq->rescuer after freeing it in
destroy_workqueue(). This is a precaution as scheduled changes will
make destruction more complex.
This patch doesn't introduce any functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
There are gonna be multiple unbound pools. Include pool ID in the
name of unbound kworkers.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
All per-cpu pools are standard, so there's no need to use both "cpu"
and "std" and for_each_std_worker_pool() is confusing in that it can
be used only for per-cpu pools.
* s/cpu_std_worker_pools/cpu_worker_pools/
* s/for_each_std_worker_pool()/for_each_cpu_worker_pool()/
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Workqueue no longer makes use of unbound_std_worker_pools[]. All
unbound worker_pools are created dynamically and there's nothing
special about the standard ones. With unbound_std_worker_pools[]
unused, workqueue no longer has places where it needs to treat the
per-cpu pools-cpu and unbound pools together.
Remove unbound_std_worker_pools[] and the helpers wrapping it to
present unified per-cpu and unbound standard worker_pools.
* for_each_std_worker_pool() now only walks through per-cpu pools.
* for_each[_online]_wq_cpu() which don't have any users left are
removed.
* std_worker_pools() and std_worker_pool_pri() are unused and removed.
* get_std_worker_pool() is removed. Its only user -
alloc_and_link_pwqs() - only used it for per-cpu pools anyway. Open
code per_cpu access in alloc_and_link_pwqs() instead.
This patch doesn't introduce any functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
This patch makes unbound worker_pools reference counted and
dynamically created and destroyed as workqueues needing them come and
go. All unbound worker_pools are hashed on unbound_pool_hash which is
keyed by the content of worker_pool->attrs.
When an unbound workqueue is allocated, get_unbound_pool() is called
with the attributes of the workqueue. If there already is a matching
worker_pool, the reference count is bumped and the pool is returned.
If not, a new worker_pool with matching attributes is created and
returned.
When an unbound workqueue is destroyed, put_unbound_pool() is called
which decrements the reference count of the associated worker_pool.
If the refcnt reaches zero, the worker_pool is destroyed in sched-RCU
safe way.
Note that the standard unbound worker_pools - normal and highpri ones
with no specific cpumask affinity - are no longer created explicitly
during init_workqueues(). init_workqueues() only initializes
workqueue_attrs to be used for standard unbound pools -
unbound_std_wq_attrs[]. The pools are spawned on demand as workqueues
are created.
v2: - Comment added to init_worker_pool() explaining that @pool should
be in a condition which can be passed to put_unbound_pool() even
on failure.
- pool->refcnt reaching zero and the pool being removed from
unbound_pool_hash should be dynamic. pool->refcnt is converted
to int from atomic_t and now manipulated inside workqueue_lock.
- Removed an incorrect sanity check on nr_idle in
put_unbound_pool() which may trigger spuriously.
All changes were suggested by Lai Jiangshan.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Introduce struct workqueue_attrs which carries worker attributes -
currently the nice level and allowed cpumask along with helper
routines alloc_workqueue_attrs() and free_workqueue_attrs().
Each worker_pool now carries ->attrs describing the attributes of its
workers. All functions dealing with cpumask and nice level of workers
are updated to follow worker_pool->attrs instead of determining them
from other characteristics of the worker_pool, and init_workqueues()
is updated to set worker_pool->attrs appropriately for all standard
pools.
Note that create_worker() is updated to always perform set_user_nice()
and use set_cpus_allowed_ptr() combined with manual assertion of
PF_THREAD_BOUND instead of kthread_bind(). This simplifies handling
random attributes without affecting the outcome.
This patch doesn't introduce any behavior changes.
v2: Missing cpumask_var_t definition caused build failure on some
archs. linux/cpumask.h included.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
This will be used to implement unbound pools with custom attributes.
This patch doesn't introduce any functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
POOL_MANAGING_WORKERS is used to synchronize the manager role.
Synchronizing among workers doesn't need blocking and that's why it's
implemented as a flag.
It got converted to a mutex a while back to add blocking wait from CPU
hotplug path - 6037315269 ("workqueue: use mutex for global_cwq
manager exclusion"). Later it turned out that synchronization among
workers and cpu hotplug need to be done separately. Eventually,
POOL_MANAGING_WORKERS is restored and workqueue->manager_mutex got
morphed into workqueue->assoc_mutex - 552a37e936 ("workqueue: restore
POOL_MANAGING_WORKERS") and b2eb83d123 ("workqueue: rename
manager_mutex to assoc_mutex").
Now, we're gonna need to be able to lock out managers from
destroy_workqueue() to support multiple unbound pools with custom
attributes making it again necessary to be able to block on the
manager role. This patch replaces POOL_MANAGING_WORKERS with
worker_pool->manager_arb.
This patch doesn't introduce any behavior changes.
v2: s/manager_mutex/manager_arb/
Signed-off-by: Tejun Heo <tj@kernel.org>
Make worker_pool_idr protected by workqueue_lock for writes and
sched-RCU protected for reads. Lockdep assertions are added to
for_each_pool() and get_work_pool() and all their users are converted
to either hold workqueue_lock or disable preemption/irq.
worker_pool_assign_id() is updated to hold workqueue_lock when
allocating a pool ID. As idr_get_new() always performs RCU-safe
assignment, this is enough on the writer side.
As standard pools are never destroyed, there's nothing to do on that
side.
The locking is superflous at this point. This is to help
implementation of unbound pools/pwqs with custom attributes.
This patch doesn't introduce any behavior changes.
v2: Updated for_each_pwq() use if/else for the hidden assertion
statement instead of just if as suggested by Lai. This avoids
confusing the following else clause.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Make workqueue->pwqs protected by workqueue_lock for writes and
sched-RCU protected for reads. Lockdep assertions are added to
for_each_pwq() and first_pwq() and all their users are converted to
either hold workqueue_lock or disable preemption/irq.
alloc_and_link_pwqs() is updated to use list_add_tail_rcu() for
consistency which isn't strictly necessary as the workqueue isn't
visible. destroy_workqueue() isn't updated to sched-RCU release pwqs.
This is okay as the workqueue should have on users left by that point.
The locking is superflous at this point. This is to help
implementation of unbound pools/pwqs with custom attributes.
This patch doesn't introduce any behavior changes.
v2: Updated for_each_pwq() use if/else for the hidden assertion
statement instead of just if as suggested by Lai. This avoids
confusing the following else clause.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
get_pwq() takes @cpu, which can also be WORK_CPU_UNBOUND, and @wq and
returns the matching pwq (pool_workqueue). We want to move away from
using @cpu for identifying pools and pwqs for unbound pools with
custom attributes and there is only one user - workqueue_congested() -
which makes use of the WQ_UNBOUND conditional in get_pwq(). All other
users already know whether they're dealing with a per-cpu or unbound
workqueue.
Replace get_pwq() with explicit per_cpu_ptr(wq->cpu_pwqs, cpu) for
per-cpu workqueues and first_pwq() for unbound ones, and open-code
WQ_UNBOUND conditional in workqueue_congested().
Note that this makes workqueue_congested() behave sligntly differently
when @cpu other than WORK_CPU_UNBOUND is specified. It ignores @cpu
for unbound workqueues and always uses the first pwq instead of
oopsing.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
workqueue->pool_wq union is used to point either to percpu pwqs
(pool_workqueues) or single unbound pwq. As the first pwq can be
accessed via workqueue->pwqs list, there's no reason for the single
pointer anymore.
Use list_first_entry(workqueue->pwqs) to access the unbound pwq and
drop workqueue->pool_wq.single pointer and the pool_wq union. It
simplifies the code and eases implementing multiple unbound pools w/
custom attributes.
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Workqueue is mixing unsigned int and int for @cpu variables. There's
no point in using unsigned int for cpus - many of cpu related APIs
take int anyway. Consistently use int for @cpu variables so that we
can use negative values to mark special ones.
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Similar to how pool_workqueue iteration used to be, raising and
servicing mayday requests is based on CPU numbers. It's hairy because
cpumask_t may not be able to handle WORK_CPU_UNBOUND and cpumasks are
assumed to be always set on UP. This is ugly and can't handle
multiple unbound pools to be added for unbound workqueues w/ custom
attributes.
Add workqueue_struct->maydays. When a pool_workqueue needs rescuing,
it gets chained on the list through pool_workqueue->mayday_node and
rescuer_thread() consumes the list until it's empty.
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
The three freeze/thaw related functions - freeze_workqueues_begin(),
freeze_workqueues_busy() and thaw_workqueues() - need to iterate
through all pool_workqueues of all freezable workqueues. They did it
by first iterating pools and then visiting all pwqs (pool_workqueues)
of all workqueues and process it if its pwq->pool matches the current
pool. This is rather backwards and done this way partly because
workqueue didn't have fitting iteration helpers and partly to avoid
the number of lock operations on pool->lock.
Workqueue now has fitting iterators and the locking operation overhead
isn't anything to worry about - those locks are unlikely to be
contended and the same CPU visiting the same set of locks multiple
times isn't expensive.
Restructure the three functions such that the flow better matches the
logical steps and pwq iteration is done using for_each_pwq() inside
workqueue iteration.
* freeze_workqueues_begin(): Setting of FREEZING is moved into a
separate for_each_pool() iteration. pwq iteration for clearing
max_active is updated as described above.
* freeze_workqueues_busy(): pwq iteration updated as described above.
* thaw_workqueues(): The single for_each_wq_cpu() iteration is broken
into three discrete steps - clearing FREEZING, restoring max_active,
and kicking workers. The first and last steps use for_each_pool()
and the second step uses pwq iteration described above.
This makes the code easier to understand and removes the use of
for_each_wq_cpu() for walking pwqs, which can't support multiple
unbound pwqs which will be needed to implement unbound workqueues with
custom attributes.
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
With the scheduled unbound pools with custom attributes, there will be
multiple unbound pools, so it wouldn't be able to use
for_each_wq_cpu() + for_each_std_worker_pool() to iterate through all
pools.
Introduce for_each_pool() which iterates through all pools using
worker_pool_idr and use it instead of for_each_wq_cpu() +
for_each_std_worker_pool() combination in freeze_workqueues_begin().
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Introduce for_each_pwq() which iterates all pool_workqueues of a
workqueue using the recently added workqueue->pwqs list and replace
for_each_pwq_cpu() usages with it.
This is primarily to remove the single unbound CPU assumption from pwq
iteration for the scheduled unbound pools with custom attributes
support which would introduce multiple unbound pwqs per workqueue;
however, it also simplifies iterator users.
Note that pwq->pool initialization is moved to alloc_and_link_pwqs()
as that now is the only place which is explicitly handling the two pwq
types.
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Add workqueue_struct->pwqs list and chain all pool_workqueues
belonging to a workqueue there. This will be used to implement
generic pool_workqueue iteration and handle multiple pool_workqueues
for the scheduled unbound pools with custom attributes.
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
pool_workqueues need to be aligned to 1 << WORK_STRUCT_FLAG_BITS as
the lower bits of work->data are used for flags when they're pointing
to pool_workqueues.
Due to historical reasons, unbound pool_workqueues are allocated using
kzalloc() with sufficient buffer area for alignment and aligned
manually. The original pointer is stored at the end which free_pwqs()
retrieves when freeing it.
There's no reason for this hackery anymore. Set alignment of struct
pool_workqueue to 1 << WORK_STRUCT_FLAG_BITS, add kmem_cache for
pool_workqueues with proper alignment and replace the hacky alloc and
free implementation with plain kmem_cache_zalloc/free().
In case WORK_STRUCT_FLAG_BITS gets shrunk too much and makes fields of
pool_workqueues misaligned, trigger WARN if the alignment of struct
pool_workqueue becomes smaller than that of long long.
Note that assertion on IS_ALIGNED() is removed from alloc_pwqs(). We
already have another one in pwq init loop in __alloc_workqueue_key().
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
workqueue_lock will be used to synchronize areas which require
irq-safety and there isn't much benefit in keeping it not irq-safe.
Make it irq-safe.
This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Workqueue has been using mostly BUG_ON()s for sanity checks, which
fail unnecessarily harshly when the assertion doesn't hold. Most
assertions can converted to be less drastic such that things can limp
along instead of dying completely. Convert BUG_ON()s to
WARN_ON[_ONCE]()s with softer failure behaviors - e.g. if assertion
check fails in destroy_worker(), trigger WARN and silently ignore
destruction request.
Most conversions are trivial. Note that sanity checks in
destroy_workqueue() are moved above removal from workqueues list so
that it can bail out without side-effects if assertion checks fail.
This patch doesn't introduce any visible behavior changes during
normal operation.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Since multiple pools per cpu have been introduced, wq_unbind_fn() has
a subtle bug which may theoretically stall work item processing. The
problem is two-fold.
* wq_unbind_fn() depends on the worker executing wq_unbind_fn() itself
to start unbound chain execution, which works fine when there was
only single pool. With multiple pools, only the pool which is
running wq_unbind_fn() - the highpri one - is guaranteed to have
such kick-off. The other pool could stall when its busy workers
block.
* The current code is setting WORKER_UNBIND / POOL_DISASSOCIATED of
the two pools in succession without initiating work execution
inbetween. Because setting the flags requires grabbing assoc_mutex
which is held while new workers are created, this could lead to
stalls if a pool's manager is waiting for the previous pool's work
items to release memory. This is almost purely theoretical tho.
Update wq_unbind_fn() such that it sets WORKER_UNBIND /
POOL_DISASSOCIATED, goes over schedule() and explicitly kicks off
execution for a pool and then moves on to the next one.
tj: Updated comments and description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
Rescuers visit different worker_pools to process work items from pools
under pressure. Currently, rescuer->pool is updated outside any
locking and when an outsider looks at a rescuer, there's no way to
tell when and whether rescuer->pool is gonna change. While this
doesn't currently cause any problem, it is nasty.
With recent worker_maybe_bind_and_lock() changes, we can move
rescuer->pool updates inside pool locks such that if rescuer->pool
equals a locked pool, it's guaranteed to stay that way until the pool
is unlocked.
Move rescuer->pool inside pool->lock.
This patch doesn't introduce any visible behavior difference.
tj: Updated the description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
worker_maybe_bind_and_lock() currently takes @worker but only cares
about @worker->pool. This patch updates worker_maybe_bind_and_lock()
to take @pool instead of @worker. This will be used to better define
synchronization rules regarding rescuer->pool updates.
This doesn't introduce any functional change.
tj: Updated the comments and description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
worker_maybe_bind_and_lock() uses both @worker->task and @current at
the same time. As worker_maybe_bind_and_lock() can only be called by
the current worker task, they are always the same.
Update worker_maybe_bind_and_lock() to use %current consistently.
This doesn't introduce any functional change.
tj: Massaged the description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
I'm not sure why, but the hlist for each entry iterators were conceived
list_for_each_entry(pos, head, member)
The hlist ones were greedy and wanted an extra parameter:
hlist_for_each_entry(tpos, pos, head, member)
Why did they need an extra pos parameter? I'm not quite sure. Not only
they don't really need it, it also prevents the iterator from looking
exactly like the list iterator, which is unfortunate.
Besides the semantic patch, there was some manual work required:
- Fix up the actual hlist iterators in linux/list.h
- Fix up the declaration of other iterators based on the hlist ones.
- A very small amount of places were using the 'node' parameter, this
was modified to use 'obj->member' instead.
- Coccinelle didn't handle the hlist_for_each_entry_safe iterator
properly, so those had to be fixed up manually.
The semantic patch which is mostly the work of Peter Senna Tschudin is here:
@@
iterator name hlist_for_each_entry, hlist_for_each_entry_continue, hlist_for_each_entry_from, hlist_for_each_entry_rcu, hlist_for_each_entry_rcu_bh, hlist_for_each_entry_continue_rcu_bh, for_each_busy_worker, ax25_uid_for_each, ax25_for_each, inet_bind_bucket_for_each, sctp_for_each_hentry, sk_for_each, sk_for_each_rcu, sk_for_each_from, sk_for_each_safe, sk_for_each_bound, hlist_for_each_entry_safe, hlist_for_each_entry_continue_rcu, nr_neigh_for_each, nr_neigh_for_each_safe, nr_node_for_each, nr_node_for_each_safe, for_each_gfn_indirect_valid_sp, for_each_gfn_sp, for_each_host;
type T;
expression a,c,d,e;
identifier b;
statement S;
@@
-T b;
<+... when != b
(
hlist_for_each_entry(a,
- b,
c, d) S
|
hlist_for_each_entry_continue(a,
- b,
c) S
|
hlist_for_each_entry_from(a,
- b,
c) S
|
hlist_for_each_entry_rcu(a,
- b,
c, d) S
|
hlist_for_each_entry_rcu_bh(a,
- b,
c, d) S
|
hlist_for_each_entry_continue_rcu_bh(a,
- b,
c) S
|
for_each_busy_worker(a, c,
- b,
d) S
|
ax25_uid_for_each(a,
- b,
c) S
|
ax25_for_each(a,
- b,
c) S
|
inet_bind_bucket_for_each(a,
- b,
c) S
|
sctp_for_each_hentry(a,
- b,
c) S
|
sk_for_each(a,
- b,
c) S
|
sk_for_each_rcu(a,
- b,
c) S
|
sk_for_each_from
-(a, b)
+(a)
S
+ sk_for_each_from(a) S
|
sk_for_each_safe(a,
- b,
c, d) S
|
sk_for_each_bound(a,
- b,
c) S
|
hlist_for_each_entry_safe(a,
- b,
c, d, e) S
|
hlist_for_each_entry_continue_rcu(a,
- b,
c) S
|
nr_neigh_for_each(a,
- b,
c) S
|
nr_neigh_for_each_safe(a,
- b,
c, d) S
|
nr_node_for_each(a,
- b,
c) S
|
nr_node_for_each_safe(a,
- b,
c, d) S
|
- for_each_gfn_sp(a, c, d, b) S
+ for_each_gfn_sp(a, c, d) S
|
- for_each_gfn_indirect_valid_sp(a, c, d, b) S
+ for_each_gfn_indirect_valid_sp(a, c, d) S
|
for_each_host(a,
- b,
c) S
|
for_each_host_safe(a,
- b,
c, d) S
|
for_each_mesh_entry(a,
- b,
c, d) S
)
...+>
[akpm@linux-foundation.org: drop bogus change from net/ipv4/raw.c]
[akpm@linux-foundation.org: drop bogus hunk from net/ipv6/raw.c]
[akpm@linux-foundation.org: checkpatch fixes]
[akpm@linux-foundation.org: fix warnings]
[akpm@linux-foudnation.org: redo intrusive kvm changes]
Tested-by: Peter Senna Tschudin <peter.senna@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
commit d8e794dfd5 ("workqueue: set
delayed_work->timer function on initialization") exports function
delayed_work_timer_fn() only for GPL modules. This makes delayed-works
unusable for non-GPL modules, because initialization macro now requires
GPL symbol. For example schedule_delayed_work() available for non-GPL.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # 3.7
workqueue has moved away from global_cwqs to worker_pools and with the
scheduled custom worker pools, wforkqueues will be associated with
pools which don't have anything to do with CPUs. The workqueue code
went through significant amount of changes recently and mass renaming
isn't likely to hurt much additionally. Let's replace 'cpu' with
'pool' so that it reflects the current design.
* s/struct cpu_workqueue_struct/struct pool_workqueue/
* s/cpu_wq/pool_wq/
* s/cwq/pwq/
This patch is purely cosmetic.
Signed-off-by: Tejun Heo <tj@kernel.org>
is_chained_work() was added before current_wq_worker() and implemented
its own ham-fisted way of finding out whether %current is a workqueue
worker - it iterates through all possible workers.
Drop the custom implementation and reimplement using
current_wq_worker().
Signed-off-by: Tejun Heo <tj@kernel.org>
c9e7cf273f ("workqueue: move busy_hash from global_cwq to
worker_pool") incorrectly converted is_chained_work() to use
get_gcwq() inside for_each_gcwq_cpu() while removing get_gcwq().
As cwq might not exist for all possible workqueue CPUs, @cwq can be
NULL and the following cwq deferences can lead to oops.
Fix it by using for_each_cwq_cpu() instead, which is the better one to
use anyway as we only need to check pools that the wq is associated
with.
Signed-off-by: Tejun Heo <tj@kernel.org>
Currently, __queue_work() chooses the pool to queue a work item to and
then determines cwq from the target wq and the chosen pool. This is a
bit backwards in that we can determine cwq first and simply use
cwq->pool. This way, we can skip get_std_worker_pool() in queueing
path which will be a hurdle when implementing custom worker pools.
Update __queue_work() such that it chooses the target cwq and then use
cwq->pool instead of the other way around. While at it, add missing
{} in an if statement.
This patch doesn't introduce any functional changes.
tj: The original patch had two get_cwq() calls - the first to
determine the pool by doing get_cwq(cpu, wq)->pool and the second
to determine the matching cwq from get_cwq(pool->cpu, wq).
Updated the function such that it chooses cwq instead of pool and
removed the second call. Rewrote the description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
get_work_pool_id() currently first obtains pool using get_work_pool()
and then return pool->id. For an off-queue work item, this involves
obtaining pool ID from worker->data, performing idr_find() to find the
matching pool and then returning its pool->id which of course is the
same as the one which went into idr_find().
Just open code WORK_STRUCT_CWQ case and directly return pool ID from
work->data.
tj: The original patch dropped on-queue work item handling and renamed
the function to offq_work_pool_id(). There isn't much benefit in
doing so. Handling it only requires a single if() and we need at
least BUG_ON(), which is also a branch, even if we drop on-queue
handling. Open code WORK_STRUCT_CWQ case and keep the function in
line with get_work_pool(). Rewrote the description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
As nr_running is likely to be accessed from other CPUs during
try_to_wake_up(), it was kept outside worker_pool; however, while less
frequent, other fields in worker_pool are accessed from other CPUs
for, e.g., non-reentrancy check. Also, with recent pool related
changes, accessing nr_running matching the worker_pool isn't as simple
as it used to be.
Move nr_running inside worker_pool. Keep it aligned to cacheline and
define CPU pools using DEFINE_PER_CPU_SHARED_ALIGNED(). This should
give at least the same cacheline behavior.
get_pool_nr_running() is replaced with direct pool->nr_running
accesses.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Joonsoo Kim <js1304@gmail.com>
With the recent is-work-queued-here test simplification, the nested
if() in try_to_grab_pending() can be collapsed. Collapse it.
This patch is purely cosmetic.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Currently, determining whether a work item is queued on a locked pool
involves somewhat convoluted memory barrier dancing. It goes like the
following.
* When a work item is queued on a pool, work->data is updated before
work->entry is linked to the pending list with a wmb() inbetween.
* When trying to determine whether a work item is currently queued on
a pool pointed to by work->data, it locks the pool and looks at
work->entry. If work->entry is linked, we then do rmb() and then
check whether work->data points to the current pool.
This works because, work->data can only point to a pool if it
currently is or were on the pool and,
* If it currently is on the pool, the tests would obviously succeed.
* It it left the pool, its work->entry was cleared under pool->lock,
so if we're seeing non-empty work->entry, it has to be from the work
item being linked on another pool. Because work->data is updated
before work->entry is linked with wmb() inbetween, work->data update
from another pool is guaranteed to be visible if we do rmb() after
seeing non-empty work->entry. So, we either see empty work->entry
or we see updated work->data pointin to another pool.
While this works, it's convoluted, to put it mildly. With recent
updates, it's now guaranteed that work->data points to cwq only while
the work item is queued and that updating work->data to point to cwq
or back to pool is done under pool->lock, so we can simply test
whether work->data points to cwq which is associated with the
currently locked pool instead of the convoluted memory barrier
dancing.
This patch replaces the memory barrier based "are you still here,
really?" test with much simpler "does work->data points to me?" test -
if work->data points to a cwq which is associated with the currently
locked pool, the work item is guaranteed to be queued on the pool as
work->data can start and stop pointing to such cwq only under
pool->lock and the start and stop coincide with queue and dequeue.
tj: Rewrote the comments and description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
