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locking/mutexes: Documentation update/rewrite

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Our mutexes have gone a long ways since the original
implementation back in 2005/2006. However, the mutex-design.txt
document is still stuck in the past, to the point where most of
the information there is practically useless and, more
important, simply incorrect. This patch pretty much rewrites it
to resemble what we have nowadays.

Since regular semaphores are almost much extinct in the kernel
(most users now rely on mutexes or rwsems), it no longer makes
sense to have such a close comparison, which was copied from
most of the cover letter when Ingo introduced the generic mutex
subsystem.

Note that ww_mutexes are intentionally left out, leaving things
as generic as possible.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: tim.c.chen@linux.intel.com
Cc: paulmck@linux.vnet.ibm.com
Cc: waiman.long@hp.com
Cc: jason.low2@hp.com
Cc: aswin@hp.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1401338203.2618.11.camel@buesod1.americas.hpqcorp.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Davidlohr Bueso 2014-05-28 21:36:43 -07:00 committed by Ingo Molnar
parent 0cc3d01164
commit 9161f54097
1 changed files with 131 additions and 113 deletions
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244
Documentation/mutex-design.txt
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@ -1,139 +1,157 @@
Generic Mutex Subsystem
started by Ingo Molnar <mingo@redhat.com>
updated by Davidlohr Bueso <davidlohr@hp.com>
"Why on earth do we need a new mutex subsystem, and what's wrong
with semaphores?"
What are mutexes?
-----------------
firstly, there's nothing wrong with semaphores. But if the simpler
mutex semantics are sufficient for your code, then there are a couple
of advantages of mutexes:
In the Linux kernel, mutexes refer to a particular locking primitive
that enforces serialization on shared memory systems, and not only to
the generic term referring to 'mutual exclusion' found in academia
or similar theoretical text books. Mutexes are sleeping locks which
behave similarly to binary semaphores, and were introduced in 2006[1]
as an alternative to these. This new data structure provided a number
of advantages, including simpler interfaces, and at that time smaller
code (see Disadvantages).
- 'struct mutex' is smaller on most architectures: E.g. on x86,
'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes.
A smaller structure size means less RAM footprint, and better
CPU-cache utilization.
[1] http://lwn.net/Articles/164802/
- tighter code. On x86 i get the following .text sizes when
switching all mutex-alike semaphores in the kernel to the mutex
subsystem:
Implementation
--------------
text data bss dec hex filename
3280380 868188 396860 4545428 455b94 vmlinux-semaphore
3255329 865296 396732 4517357 44eded vmlinux-mutex
Mutexes are represented by 'struct mutex', defined in include/linux/mutex.h
and implemented in kernel/locking/mutex.c. These locks use a three
state atomic counter (->count) to represent the different possible
transitions that can occur during the lifetime of a lock:
that's 25051 bytes of code saved, or a 0.76% win - off the hottest
codepaths of the kernel. (The .data savings are 2892 bytes, or 0.33%)
Smaller code means better icache footprint, which is one of the
major optimization goals in the Linux kernel currently.
1: unlocked
0: locked, no waiters
negative: locked, with potential waiters
- the mutex subsystem is slightly faster and has better scalability for
contended workloads. On an 8-way x86 system, running a mutex-based
kernel and testing creat+unlink+close (of separate, per-task files)
in /tmp with 16 parallel tasks, the average number of ops/sec is:
In its most basic form it also includes a wait-queue and a spinlock
that serializes access to it. CONFIG_SMP systems can also include
a pointer to the lock task owner (->owner) as well as a spinner MCS
lock (->osq), both described below in (ii).
Semaphores: Mutexes:
When acquiring a mutex, there are three possible paths that can be
taken, depending on the state of the lock:
$ ./test-mutex V 16 10 $ ./test-mutex V 16 10
8 CPUs, running 16 tasks. 8 CPUs, running 16 tasks.
checking VFS performance. checking VFS performance.
avg loops/sec: 34713 avg loops/sec: 84153
CPU utilization: 63% CPU utilization: 22%
(i) fastpath: tries to atomically acquire the lock by decrementing the
counter. If it was already taken by another task it goes to the next
possible path. This logic is architecture specific. On x86-64, the
locking fastpath is 2 instructions:
i.e. in this workload, the mutex based kernel was 2.4 times faster
than the semaphore based kernel, _and_ it also had 2.8 times less CPU
utilization. (In terms of 'ops per CPU cycle', the semaphore kernel
performed 551 ops/sec per 1% of CPU time used, while the mutex kernel
performed 3825 ops/sec per 1% of CPU time used - it was 6.9 times
more efficient.)
the scalability difference is visible even on a 2-way P4 HT box:
Semaphores: Mutexes:
$ ./test-mutex V 16 10 $ ./test-mutex V 16 10
4 CPUs, running 16 tasks. 8 CPUs, running 16 tasks.
checking VFS performance. checking VFS performance.
avg loops/sec: 127659 avg loops/sec: 181082
CPU utilization: 100% CPU utilization: 34%
(the straight performance advantage of mutexes is 41%, the per-cycle
efficiency of mutexes is 4.1 times better.)
- there are no fastpath tradeoffs, the mutex fastpath is just as tight
as the semaphore fastpath. On x86, the locking fastpath is 2
instructions:
c0377ccb <mutex_lock>:
c0377ccb: f0 ff 08 lock decl (%eax)
c0377cce: 78 0e js c0377cde <.text..lock.mutex>
c0377cd0: c3 ret
0000000000000e10 <mutex_lock>:
e21: f0 ff 0b lock decl (%rbx)
e24: 79 08 jns e2e <mutex_lock+0x1e>
the unlocking fastpath is equally tight:
c0377cd1 <mutex_unlock>:
c0377cd1: f0 ff 00 lock incl (%eax)
c0377cd4: 7e 0f jle c0377ce5 <.text..lock.mutex+0x7>
c0377cd6: c3 ret
0000000000000bc0 <mutex_unlock>:
bc8: f0 ff 07 lock incl (%rdi)
bcb: 7f 0a jg bd7 <mutex_unlock+0x17>
- 'struct mutex' semantics are well-defined and are enforced if
CONFIG_DEBUG_MUTEXES is turned on. Semaphores on the other hand have
virtually no debugging code or instrumentation. The mutex subsystem
checks and enforces the following rules:
* - only one task can hold the mutex at a time
* - only the owner can unlock the mutex
* - multiple unlocks are not permitted
* - recursive locking is not permitted
* - a mutex object must be initialized via the API
* - a mutex object must not be initialized via memset or copying
* - task may not exit with mutex held
* - memory areas where held locks reside must not be freed
* - held mutexes must not be reinitialized
* - mutexes may not be used in hardware or software interrupt
* contexts such as tasklets and timers
(ii) midpath: aka optimistic spinning, tries to spin for acquisition
while the lock owner is running and there are no other tasks ready
to run that have higher priority (need_resched). The rationale is
that if the lock owner is running, it is likely to release the lock
soon. The mutex spinners are queued up using MCS lock so that only
one spinner can compete for the mutex.
furthermore, there are also convenience features in the debugging
code:
The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spinlock
with the desirable properties of being fair and with each cpu trying
to acquire the lock spinning on a local variable. It avoids expensive
cacheline bouncing that common test-and-set spinlock implementations
incur. An MCS-like lock is specially tailored for optimistic spinning
for sleeping lock implementation. An important feature of the customized
MCS lock is that it has the extra property that spinners are able to exit
the MCS spinlock queue when they need to reschedule. This further helps
avoid situations where MCS spinners that need to reschedule would continue
waiting to spin on mutex owner, only to go directly to slowpath upon
obtaining the MCS lock.
* - uses symbolic names of mutexes, whenever they are printed in debug output
* - point-of-acquire tracking, symbolic lookup of function names
* - list of all locks held in the system, printout of them
* - owner tracking
* - detects self-recursing locks and prints out all relevant info
* - detects multi-task circular deadlocks and prints out all affected
* locks and tasks (and only those tasks)
(iii) slowpath: last resort, if the lock is still unable to be acquired,
the task is added to the wait-queue and sleeps until woken up by the
unlock path. Under normal circumstances it blocks as TASK_UNINTERRUPTIBLE.
While formally kernel mutexes are sleepable locks, it is path (ii) that
makes them more practically a hybrid type. By simply not interrupting a
task and busy-waiting for a few cycles instead of immediately sleeping,
the performance of this lock has been seen to significantly improve a
number of workloads. Note that this technique is also used for rw-semaphores.
Semantics
---------
The mutex subsystem checks and enforces the following rules:
- Only one task can hold the mutex at a time.
- Only the owner can unlock the mutex.
- Multiple unlocks are not permitted.
- Recursive locking/unlocking is not permitted.
- A mutex must only be initialized via the API (see below).
- A task may not exit with a mutex held.
- Memory areas where held locks reside must not be freed.
- Held mutexes must not be reinitialized.
- Mutexes may not be used in hardware or software interrupt
contexts such as tasklets and timers.
These semantics are fully enforced when CONFIG DEBUG_MUTEXES is enabled.
In addition, the mutex debugging code also implements a number of other
features that make lock debugging easier and faster:
- Uses symbolic names of mutexes, whenever they are printed
in debug output.
- Point-of-acquire tracking, symbolic lookup of function names,
list of all locks held in the system, printout of them.
- Owner tracking.
- Detects self-recursing locks and prints out all relevant info.
- Detects multi-task circular deadlocks and prints out all affected
locks and tasks (and only those tasks).
Interfaces
----------
Statically define the mutex:
DEFINE_MUTEX(name);
Dynamically initialize the mutex:
mutex_init(mutex);
Acquire the mutex, uninterruptible:
void mutex_lock(struct mutex *lock);
void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
int mutex_trylock(struct mutex *lock);
Acquire the mutex, interruptible:
int mutex_lock_interruptible_nested(struct mutex *lock,
unsigned int subclass);
int mutex_lock_interruptible(struct mutex *lock);
Acquire the mutex, interruptible, if dec to 0:
int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
Unlock the mutex:
void mutex_unlock(struct mutex *lock);
Test if the mutex is taken:
int mutex_is_locked(struct mutex *lock);
Disadvantages
-------------
The stricter mutex API means you cannot use mutexes the same way you
can use semaphores: e.g. they cannot be used from an interrupt context,
nor can they be unlocked from a different context that which acquired
it. [ I'm not aware of any other (e.g. performance) disadvantages from
using mutexes at the moment, please let me know if you find any. ]
Unlike its original design and purpose, 'struct mutex' is larger than
most locks in the kernel. E.g: on x86-64 it is 40 bytes, almost twice
as large as 'struct semaphore' (24 bytes) and 8 bytes shy of the
'struct rw_semaphore' variant. Larger structure sizes mean more CPU
cache and memory footprint.
Implementation of mutexes
-------------------------
When to use mutexes
-------------------
'struct mutex' is the new mutex type, defined in include/linux/mutex.h and
implemented in kernel/locking/mutex.c. It is a counter-based mutex with a
spinlock and a wait-list. The counter has 3 states: 1 for "unlocked", 0 for
"locked" and negative numbers (usually -1) for "locked, potential waiters
queued".
the APIs of 'struct mutex' have been streamlined:
DEFINE_MUTEX(name);
mutex_init(mutex);
void mutex_lock(struct mutex *lock);
int mutex_lock_interruptible(struct mutex *lock);
int mutex_trylock(struct mutex *lock);
void mutex_unlock(struct mutex *lock);
int mutex_is_locked(struct mutex *lock);
void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
int mutex_lock_interruptible_nested(struct mutex *lock,
unsigned int subclass);
int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
Unless the strict semantics of mutexes are unsuitable and/or the critical
region prevents the lock from being shared, always prefer them to any other
locking primitive.