Messages in this thread Patch in this message | | | Date | Mon, 24 Mar 2014 13:18:48 +0100 | From | Nicholas Mc Guire <> | Subject | [PATCH RT] dont calculate hweight in update_migrate_disable |
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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@hofr.at> --- 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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