lkml.org 
[lkml]   [2014]   [Mar]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH RT] 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 <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


\
 
 \ /
  Last update: 2014-03-24 14:01    [W:1.213 / U:0.168 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site