Commit 3d4b60d3 authored May 11, 2017 by Peter Zijlstra Committed by Ingo Molnar Sep 29, 2017

sched/fair: Cure calc_cfs_shares() vs. reweight_entity()



Vincent reported that when running in a cgroup, his root
cfs_rq->avg.load_avg dropped to 0 on task idle.

This is because reweight_entity() will now immediately propagate the
weight change of the group entity to its cfs_rq, and as it happens,
our approxmation (5) for calc_cfs_shares() results in 0 when the group
is idle.

Avoid this by using the correct (3) as a lower bound on (5). This way
the empty cgroup will slowly decay instead of instantly drop to 0.

Reported-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent cef27403

kernel/sched/fair.c

+3 −4

Original line number	Diff line number	Diff line
		@@ -2763,11 +2763,10 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq)
		tg_shares = READ_ONCE(tg->shares);

		/*
		* This really should be: cfs_rq->avg.load_avg, but instead we use
		* cfs_rq->load.weight, which is its upper bound. This helps ramp up
		* the shares for small weight interactive tasks.
		* Because (5) drops to 0 when the cfs_rq is idle, we need to use (3)
		* as a lower bound.
		*/
		load = scale_load_down(cfs_rq->load.weight);
		load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);

		tg_weight = atomic_long_read(&tg->load_avg);