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 <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> --- kernel/sched/fair.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2703,11 +2703,10 @@ static long calc_cfs_shares(struct cfs_r 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);
