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[PATCH] cpuset: use rcu directly optimization

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Optimize the cpuset impact on page allocation, the most performance critical
cpuset hook in the kernel.

On each page allocation, the cpuset hook needs to check for a possible change
in the current tasks cpuset.  It can now handle the common case, of no change,
without taking any spinlock or semaphore, thanks to RCU.

Convert a spinlock on the current task to an rcu_read_lock(), saving
approximately a memory barrier and an atomic op, depending on architecture.

This is done by adding rcu_assign_pointer() and synchronize_rcu() calls to the
write side of the task->cpuset pointer, in cpuset.c:attach_task(), to delay
freeing up a detached cpuset until after any critical sections referencing
that pointer.

Thanks to Andi Kleen, Nick Piggin and Eric Dumazet for ideas.

Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Paul Jackson 2006-01-08 01:02:02 -08:00 committed by Linus Torvalds
parent c417f0242e
commit 6b9c2603ce
1 changed files with 30 additions and 10 deletions
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40
kernel/cpuset.c
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@ -39,6 +39,7 @@
#include <linux/namei.h>
#include <linux/pagemap.h>
#include <linux/proc_fs.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
@ -248,6 +249,11 @@ static struct super_block *cpuset_sb;
* a tasks cpuset pointer we use task_lock(), which acts on a spinlock
* (task->alloc_lock) already in the task_struct routinely used for
* such matters.
*
* P.S. One more locking exception. RCU is used to guard the
* update of a tasks cpuset pointer by attach_task() and the
* access of task->cpuset->mems_generation via that pointer in
* the routine cpuset_update_task_memory_state().
*/
static DECLARE_MUTEX(manage_sem);
@ -610,12 +616,24 @@ static void guarantee_online_mems(const struct cpuset *cs, nodemask_t *pmask)
* cpuset pointer. This routine also might acquire callback_sem and
* current->mm->mmap_sem during call.
*
* The task_lock() is required to dereference current->cpuset safely.
* Without it, we could pick up the pointer value of current->cpuset
* in one instruction, and then attach_task could give us a different
* cpuset, and then the cpuset we had could be removed and freed,
* and then on our next instruction, we could dereference a no longer
* valid cpuset pointer to get its mems_generation field.
* Reading current->cpuset->mems_generation doesn't need task_lock
* to guard the current->cpuset derefence, because it is guarded
* from concurrent freeing of current->cpuset by attach_task(),
* using RCU.
*
* The rcu_dereference() is technically probably not needed,
* as I don't actually mind if I see a new cpuset pointer but
* an old value of mems_generation. However this really only
* matters on alpha systems using cpusets heavily. If I dropped
* that rcu_dereference(), it would save them a memory barrier.
* For all other arch's, rcu_dereference is a no-op anyway, and for
* alpha systems not using cpusets, another planned optimization,
* avoiding the rcu critical section for tasks in the root cpuset
* which is statically allocated, so can't vanish, will make this
* irrelevant. Better to use RCU as intended, than to engage in
* some cute trick to save a memory barrier that is impossible to
* test, for alpha systems using cpusets heavily, which might not
* even exist.
*
* This routine is needed to update the per-task mems_allowed data,
* within the tasks context, when it is trying to allocate memory
@ -627,11 +645,12 @@ void cpuset_update_task_memory_state()
{
int my_cpusets_mem_gen;
struct task_struct *tsk = current;
struct cpuset *cs = tsk->cpuset;
struct cpuset *cs;
task_lock(tsk);
rcu_read_lock();
cs = rcu_dereference(tsk->cpuset);
my_cpusets_mem_gen = cs->mems_generation;
task_unlock(tsk);
rcu_read_unlock();
if (my_cpusets_mem_gen != tsk->cpuset_mems_generation) {
down(&callback_sem);
@ -1131,7 +1150,7 @@ static int attach_task(struct cpuset *cs, char *pidbuf, char **ppathbuf)
return -ESRCH;
}
atomic_inc(&cs->count);
tsk->cpuset = cs;
rcu_assign_pointer(tsk->cpuset, cs);
task_unlock(tsk);
guarantee_online_cpus(cs, &cpus);
@ -1151,6 +1170,7 @@ static int attach_task(struct cpuset *cs, char *pidbuf, char **ppathbuf)
if (is_memory_migrate(cs))
do_migrate_pages(tsk->mm, &from, &to, MPOL_MF_MOVE_ALL);
put_task_struct(tsk);
synchronize_rcu();
if (atomic_dec_and_test(&oldcs->count))
check_for_release(oldcs, ppathbuf);
return 0;