Messages in this thread Patch in this message | | | Date | Fri, 20 Sep 2013 06:46:39 -0700 | From | tip-bot for Vladimir Davydov <> | Subject | [tip:sched/core] sched/balancing: Fix 'local->avg_load > sds-> avg_load' case in calculate_imbalance() |
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Commit-ID: b18855500fc40da050512d9df82d2f1471e59642 Gitweb: http://git.kernel.org/tip/b18855500fc40da050512d9df82d2f1471e59642 Author: Vladimir Davydov <vdavydov@parallels.com> AuthorDate: Sun, 15 Sep 2013 17:49:13 +0400 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Fri, 20 Sep 2013 11:59:36 +0200
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> --- kernel/sched/fair.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 11cd136..0b99aae 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -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); }
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