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| author | Matt Fleming <matt@codeblueprint.co.uk> | 2016-03-09 14:59:08 +0000 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2016-03-21 10:52:51 +0100 |
| commit | d4335581dc30ec6545999c7443bb9fead274a980 (patch) | |
| tree | 35d41f13ec5e7e2f097bea13f3975381995992f6 /kernel | |
| parent | 3a47d5124a957358274e9ca7b115b2f3a914f56d (diff) | |
| download | linux-d4335581dc30ec6545999c7443bb9fead274a980.tar.gz | |
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>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/sched/fair.c | 19 |
1 files changed, 18 insertions, 1 deletions
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 3c114d971d84..303d6392b389 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -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; |