Revert "sched: Improve scalability via 'CPU buddies', which withstand random perturbations"
This reverts commit 970e178985cadbca660feb02f4d2ee3a09f7fdda.
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 970e178985ca ("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>
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 42d9df6..96e2b18 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2637,6 +2637,8 @@
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 @@
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;
}
