Commit 3029ede3 authored Sep 15, 2013 by Vladimir Davydov Committed by Ingo Molnar Sep 20, 2013

sched/balancing: Fix 'local->avg_load > busiest->avg_load' case in fix_small_imbalance()



In busiest->group_imb case we can come to fix_small_imbalance() with
local->avg_load > busiest->avg_load. This can result in wrong imbalance
fix-up, because there is the following check there where all the
members are unsigned:

if (busiest->avg_load - local->avg_load + scaled_busy_load_per_task >=
    (scaled_busy_load_per_task * imbn)) {
	env->imbalance = busiest->load_per_task;
	return;
}

As a result we can end up constantly bouncing tasks from one cpu to
another if there are pinned tasks.

Fix it by substituting the subtraction with an equivalent addition in
the check.

[ The bug can be caught by running 2*N cpuhogs pinned to two logical cpus
  belonging to different cores on an HT-enabled machine with N logical
  cpus: just look at se.nr_migrations growth. ]

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/ef167822e5c5b2d96cf5b0e3e4f4bdff3f0414a2.1379252740.git.vdavydov@parallels.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent b1885550

kernel/sched/fair.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -4823,8 +4823,8 @@ void fix_small_imbalance(struct lb_env env, struct sd_lb_stats sds)
		(busiest->load_per_task * SCHED_POWER_SCALE) /
		busiest->group_power;

		if (busiest->avg_load - local->avg_load + scaled_busy_load_per_task >=
		(scaled_busy_load_per_task * imbn)) {
		if (busiest->avg_load + scaled_busy_load_per_task >=
		local->avg_load + (scaled_busy_load_per_task * imbn)) {
		env->imbalance = busiest->load_per_task;
		return;
		}

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