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Revert "sched: Improve scalability via 'CPU buddies', which withstand random perturbations"

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This reverts commit 970e178985.

Nikolay Ulyanitsky reported thatthe 3.6-rc5 kernel has a 15-20%
performance drop on PostgreSQL 9.2 on his machine (running "pgbench").

Borislav Petkov was able to reproduce this, and bisected it to this
commit 970e178985 ("sched: Improve scalability via 'CPU buddies' ...")
apparently because the new single-idle-buddy model simply doesn't find
idle CPU's to reschedule on aggressively enough.

Mike Galbraith suspects that it is likely due to the user-mode spinlocks
in PostgreSQL not reacting well to preemption, but we don't really know
the details - I'll just revert the commit for now.

There are hopefully other approaches to improve scheduler scalability
without it causing these kinds of downsides.

Reported-by: Nikolay Ulyanitsky <lystor@gmail.com>
Bisected-by: Borislav Petkov <bp@alien8.de>
Acked-by: Mike Galbraith <efault@gmx.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds
2012-09-16 12:29:43 -07:00
parent 3f0c3c8fe3
commit 37407ea7f9
3 changed files with 22 additions and 46 deletions
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28
kernel/sched/fair.c
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@@ -2637,6 +2637,8 @@ static int select_idle_sibling(struct task_struct *p, int target)
int cpu = smp_processor_id();
int prev_cpu = task_cpu(p);
struct sched_domain *sd;
struct sched_group *sg;
int i;
/*
* If the task is going to be woken-up on this cpu and if it is
@@ -2653,17 +2655,29 @@ static int select_idle_sibling(struct task_struct *p, int target)
return prev_cpu;
/*
* Otherwise, check assigned siblings to find an elegible idle cpu.
* Otherwise, iterate the domains and find an elegible idle cpu.
*/
sd = rcu_dereference(per_cpu(sd_llc, target));
for_each_lower_domain(sd) {
if (!cpumask_test_cpu(sd->idle_buddy, tsk_cpus_allowed(p)))
continue;
if (idle_cpu(sd->idle_buddy))
return sd->idle_buddy;
}
sg = sd->groups;
do {
if (!cpumask_intersects(sched_group_cpus(sg),
tsk_cpus_allowed(p)))
goto next;
for_each_cpu(i, sched_group_cpus(sg)) {
if (!idle_cpu(i))
goto next;
}
target = cpumask_first_and(sched_group_cpus(sg),
tsk_cpus_allowed(p));
goto done;
next:
sg = sg->next;
} while (sg != sd->groups);
}
done:
return target;
}