Commit b1885550 authored Sep 15, 2013 by Vladimir Davydov Committed by Ingo Molnar Sep 20, 2013

sched/balancing: Fix 'local->avg_load > sds->avg_load' case in calculate_imbalance()



In busiest->group_imb case we can come to calculate_imbalance() with
local->avg_load >= busiest->avg_load >= sds->avg_load. This can result
in imbalance overflow, because it is calculated as follows

env->imbalance = min(
	max_pull * busiest->group_power,
	(sds->avg_load - local->avg_load) * local->group_power) / SCHED_POWER_SCALE;

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

Fix this by skipping the assignment and assuming imbalance=0 in case
local->avg_load > sds->avg_load.

[ 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/8f596cc6bc0e5e655119dc892c9bfcad26e971f4.1379252740.git.vdavydov@parallels.com


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

parent 7e28b271

kernel/sched/fair.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -4896,7 +4896,8 @@ static inline void calculate_imbalance(struct lb_env env, struct sd_lb_stats s
		* max load less than avg load(as we skip the groups at or below
		* its cpu_power, while calculating max_load..)
		*/
		if (busiest->avg_load < sds->avg_load) {
		if (busiest->avg_load <= sds->avg_load \|\|
		local->avg_load >= sds->avg_load) {
		env->imbalance = 0;
		return fix_small_imbalance(env, sds);
		}