dont calculate hweight in update_migrate_disable

PATCH Applied Nicholas Mc Guire 3 participants 3 messages Mar 24, 2014 – Mar 28, 2014 Scheduler

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Nicholas Mc Guire

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2014-03-24 12:18 UTC #0 lore raw

PATCH 1/1 dont calculate hweight in update_migrate_disable

Hi !

Proposal for a minor optimization in update_migrate_disable - its only a few instructions saved but those are in the hot path of locks so it might be worth it

When being scheduled out while migrate_disable > 0 and migrate_disabled_updated is not yet set we end up here (kernel/sched/core.c):

static inline void update_migrate_disable(struct task_struct *p)

{
        ...

        mask = tsk_cpus_allowed(p);

        if (p->sched_class->set_cpus_allowed)
                p->sched_class->set_cpus_allowed(p, mask);
        p->nr_cpus_allowed = cpumask_weight(mask);

as we only can get here if migrate_disable > 0 there is no need to calculate the cpumask_weight(mask) as tsk_cpus_allowed in that case will return cpumask_of(task_cpu(p)) which only can have a hamming weight of 1 anyway. So we can simply do:

        p->nr_cpus_allowed = 1;

without changing the behavior.

Patch is on top of 3.12.13-rt21

Signed-off-by Nicholas Mc Guire <[email protected]>

---
 kernel/sched/core.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

kernel/sched/core.c +2 -1

index bf93f63..6c07fe9 100644
@@ -2405,7 +2405,8 @@ static inline void update_migrate_disable(struct task_struct *p)
 
 	if (p->sched_class->set_cpus_allowed)
 		p->sched_class->set_cpus_allowed(p, mask);
-	p->nr_cpus_allowed = cpumask_weight(mask);
+	/* mask==cpumask_of(task_cpu(p)) which has a cpumask_weight==1 */
+	p->nr_cpus_allowed = 1;
 
 	/* Let migrate_enable know to fix things back up */
 	p->migrate_disable |= MIGRATE_DISABLE_SET_AFFIN;

Signature · 1 line

1.7.10.4

Steven Rostedt

reviewer Reviewed-by on v1 1/1

2014-03-24 14:18 UTC #1 lore raw

Quoted 25 lines · On Mon, 24 Mar 2014 13:18:48 +0100 Nicholas Mc Guire <[email protected]> wrote:

Hi !
Proposal for a minor optimization in update_migrate_disable - its only a few instructions saved but those are in the hot path of locks so it might be worth it
When being scheduled out while migrate_disable > 0 and migrate_disabled_updated is not yet set we end up here (kernel/sched/core.c):
static inline void update_migrate_disable(struct task_struct *p)
{
        ...

        mask = tsk_cpus_allowed(p);

        if (p->sched_class->set_cpus_allowed)
                p->sched_class->set_cpus_allowed(p, mask);
        p->nr_cpus_allowed = cpumask_weight(mask);
as we only can get here if migrate_disable > 0 there is no need to calculate the cpumask_weight(mask) as tsk_cpus_allowed in that case will return cpumask_of(task_cpu(p)) which only can have a hamming weight of 1 anyway. So we can simply do:

Quoted 6 lines

        p->nr_cpus_allowed = 1;
without changing the behavior.
Patch is on top of 3.12.13-rt21

Reviewed-by Steven Rostedt <[email protected]>

-- Steve

Sebastian Andrzej Siewior

Applied

2014-03-28 09:35 UTC #2 lore raw

Quoted 7 lines · * Steven Rostedt | 2014-03-24 10:18:00 [-0400]:

Quoted 3 lines · On Mon, 24 Mar 2014 13:18:48 +0100 Nicholas Mc Guire <[email protected]> wrote:
Proposal for a minor optimization in update_migrate_disable - its only a few instructions saved but those are in the hot path of locks so it might be worth it
Reviewed-by Steven Rostedt <[email protected]>

Applied.

Quoted 2 lines

-- Steve

Sebastian