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 <der.herr@xxxxxxx>
---
kernel/sched/core.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bf93f63..6c07fe9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -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;
--
1.7.10.4
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