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>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index dd565aeafc5a1e906edb6df5324f46c0ba673f43..63166a0ed85409f6ee314a4de683630b6cf5ded7 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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);