Commit-ID: 0a702bb8af3c1b2dff355fb3c27e7f7d5285e30b Gitweb: http://git.kernel.org/tip/0a702bb8af3c1b2dff355fb3c27e7f7d5285e30b Author: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> AuthorDate: Fri, 8 Feb 2013 11:10:46 +0400 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Fri, 8 Feb 2013 15:14:38 +0100 sched: Initialize cfs_rq->runtime_remaining to non-zero on cfs bw set If cfs_rq->runtime_remaining is <= 0 then either - cfs_rq is throttled and waiting for quota redistribution, or - cfs_rq is currently executing and will be throttled on put_prev_entity, or - cfs_rq is not throttled and has not executed since its quota was set (runtime_remaining is set to 0 on cfs bandwidth reconfiguration). It is obvious that the last case is rather an exception from the rule "runtime_remaining<=0 iff cfs_rq is throttled or will be throttled as soon as it finishes its execution". Moreover, it can lead to a task hang as follows. If put_prev_task() is called immediately after first pick_next_task after quota was set, "immediately" meaning rq->clock in both functions is the same, then the corresponding cfs_rq will be throttled. Besides being unfair (the cfs_rq has not executed in fact), the quota refilling timer can be idle at that time and it won't be activated on put_prev_task because update_curr calls account_cfs_rq_runtime, which activates the timer, only if delta_exec is strictly positive. As a result we can get a task "running" inside a throttled cfs_rq which will probably never be unthrottled. To avoid the problem, the patch makes tg_set_cfs_bandwidth initialize runtime_remaining of each cfs_rq to 1 instead of 0 so that the cfs_rq will be throttled only if it has executed for some positive number of nanoseconds. Several times we had our customers encountered such hangs inside a VM (seems something is wrong or rather different in time accounting there). Analyzing crash dumps revealed that hung tasks were running inside cfs_rq's, which had the following setup: cfs_rq->throttled=1 cfs_rq->runtime_enabled=1 cfs_rq->runtime_remaining=0 cfs_rq->tg->cfs_bandwidth.idle=1 cfs_rq->tg->cfs_bandwidth.timer_active=0 which conforms pretty nice to the explanation given above. Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> Cc: <devel@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Paul Turner <pjt@xxxxxxxxxx> Link: http://lkml.kernel.org/r/1360307446-26978-1-git-send-email-vdavydov@xxxxxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- kernel/sched/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 26058d0..c7a078f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7686,7 +7686,7 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota) raw_spin_lock_irq(&rq->lock); cfs_rq->runtime_enabled = runtime_enabled; - cfs_rq->runtime_remaining = 0; + cfs_rq->runtime_remaining = 1; if (cfs_rq->throttled) unthrottle_cfs_rq(cfs_rq); -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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