sched/balancing: Fix 'local->avg_load > sds->avg_load' case in calculate_imbalance()
authorVladimir Davydov <vdavydov@parallels.com>
Sun, 15 Sep 2013 13:49:13 +0000 (17:49 +0400)
committerIngo Molnar <mingo@kernel.org>
Fri, 20 Sep 2013 09:59:36 +0000 (11:59 +0200)
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>
kernel/sched/fair.c

index 11cd13667359862c58872a9ce6091391a1d40b86..0b99aae339cb90fca133b234a836f1cf915fc06a 100644 (file)
@@ -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);
        }