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sched/fair: Add comments to explain select_idle_sibling() 

It's not entirely obvious how the main loop in select_idle_sibling()
works on first glance. Sprinkle a few comments to explain the design
and intention behind the loop based on some conversations with Mike
and Peter.

Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.com>
Cc: Mike Galbraith <mgalbraith@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1457535548-15329-1-git-send-email-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Matt Fleming 2016-03-09 14:59:08 +00:00 committed by Ingo Molnar
parent 3a47d5124a
commit d4335581dc
1 changed files with 18 additions and 1 deletions
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19
kernel/sched/fair.c
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@ -5055,7 +5055,19 @@ static int select_idle_sibling(struct task_struct *p, int target)
return i;
/*
* Otherwise, iterate the domains and find an elegible idle cpu.
* Otherwise, iterate the domains and find an eligible idle cpu.
*
* A completely idle sched group at higher domains is more
* desirable than an idle group at a lower level, because lower
* domains have smaller groups and usually share hardware
* resources which causes tasks to contend on them, e.g. x86
* hyperthread siblings in the lowest domain (SMT) can contend
* on the shared cpu pipeline.
*
* However, while we prefer idle groups at higher domains
* finding an idle cpu at the lowest domain is still better than
* returning 'target', which we've already established, isn't
* idle.
*/
sd = rcu_dereference(per_cpu(sd_llc, target));
for_each_lower_domain(sd) {
@ -5065,11 +5077,16 @@ static int select_idle_sibling(struct task_struct *p, int target)
tsk_cpus_allowed(p)))
goto next;
/* Ensure the entire group is idle */
for_each_cpu(i, sched_group_cpus(sg)) {
if (i == target || !idle_cpu(i))
goto next;
}
/*
* It doesn't matter which cpu we pick, the
* whole group is idle.
*/
target = cpumask_first_and(sched_group_cpus(sg),
tsk_cpus_allowed(p));
goto done;