Commit f9f9ffc2 authored Oct 16, 2013 by Ben Segall Committed by Ingo Molnar Oct 29, 2013

sched: Avoid throttle_cfs_rq() racing with period_timer stopping



throttle_cfs_rq() doesn't check to make sure that period_timer is running,
and while update_curr/assign_cfs_runtime does, a concurrently running
period_timer on another cpu could cancel itself between this cpu's
update_curr and throttle_cfs_rq(). If there are no other cfs_rqs running
in the tg to restart the timer, this causes the cfs_rq to be stranded
forever.

Fix this by calling __start_cfs_bandwidth() in throttle if the timer is
inactive.

(Also add some sched_debug lines for cfs_bandwidth.)

Tested: make a run/sleep task in a cgroup, loop switching the cgroup
between 1ms/100ms quota and unlimited, checking for timer_active=0 and
throttled=1 as a failure. With the throttle_cfs_rq() change commented out
this fails, with the full patch it passes.

Signed-off-by: Ben Segall <bsegall@google.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: pjt@google.com
Link: http://lkml.kernel.org/r/20131016181632.22647.84174.stgit@sword-of-the-dawn.mtv.corp.google.com


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

parent 0ac9b1c2

kernel/sched/debug.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -229,6 +229,14 @@ void print_cfs_rq(struct seq_file m, int cpu, struct cfs_rq cfs_rq)
		atomic_read(&cfs_rq->tg->runnable_avg));
		#endif
		#endif
		#ifdef CONFIG_CFS_BANDWIDTH
		SEQ_printf(m, " .%-30s: %d\n", "tg->cfs_bandwidth.timer_active",
		cfs_rq->tg->cfs_bandwidth.timer_active);
		SEQ_printf(m, " .%-30s: %d\n", "throttled",
		cfs_rq->throttled);
		SEQ_printf(m, " .%-30s: %d\n", "throttle_count",
		cfs_rq->throttle_count);
		#endif

		#ifdef CONFIG_FAIR_GROUP_SCHED
		print_cfs_group_stats(m, cpu, cfs_rq->tg);

kernel/sched/fair.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -3112,6 +3112,8 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
		cfs_rq->throttled_clock = rq_clock(rq);
		raw_spin_lock(&cfs_b->lock);
		list_add_tail_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq);
		if (!cfs_b->timer_active)
		__start_cfs_bandwidth(cfs_b);
		raw_spin_unlock(&cfs_b->lock);
		}

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