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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>
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@ -5055,7 +5055,19 @@ static int select_idle_sibling(struct task_struct *p, int target)
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return i;
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/*
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* Otherwise, iterate the domains and find an elegible idle cpu.
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* Otherwise, iterate the domains and find an eligible idle cpu.
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*
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* A completely idle sched group at higher domains is more
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* desirable than an idle group at a lower level, because lower
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* domains have smaller groups and usually share hardware
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* resources which causes tasks to contend on them, e.g. x86
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* hyperthread siblings in the lowest domain (SMT) can contend
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* on the shared cpu pipeline.
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*
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* However, while we prefer idle groups at higher domains
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* finding an idle cpu at the lowest domain is still better than
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* returning 'target', which we've already established, isn't
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* idle.
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*/
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sd = rcu_dereference(per_cpu(sd_llc, target));
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for_each_lower_domain(sd) {
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@ -5065,11 +5077,16 @@ static int select_idle_sibling(struct task_struct *p, int target)
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tsk_cpus_allowed(p)))
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goto next;
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/* Ensure the entire group is idle */
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for_each_cpu(i, sched_group_cpus(sg)) {
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if (i == target || !idle_cpu(i))
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goto next;
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}
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/*
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* It doesn't matter which cpu we pick, the
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* whole group is idle.
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*/
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target = cpumask_first_and(sched_group_cpus(sg),
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tsk_cpus_allowed(p));
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goto done;
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