Messages in this thread | | | Date | Fri, 29 Sep 2017 10:04:34 +0100 | From | Morten Rasmussen <> | Subject | Re: [PATCH -v2 03/18] sched/fair: Cure calc_cfs_shares() vs reweight_entity() |
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On Fri, Sep 01, 2017 at 03:21:02PM +0200, Peter Zijlstra wrote: > 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> > --- > 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);
We use cfs_rq->tg_load_avg_contrib (the filtered version of cfs_rq->avg.load_avg) instead of cfs_rq->avg.load_avg further down, so I think we should here too for consistency.
+ load = max(scale_load_down(cfs_rq->load.weight), + cfs_rq->tg_load_avg_contrib);
With this change (5) almost becomes (3):
ge->load.weight =
tg->weight * max(grq->load.weight, grq->avg.load_avg) --------------------------------------------------------------------------- tg->load_avg - grq->avg.load_avg + max(grq->load.weight, grq->avg.load_avg)
The difference is that we boost ge->load.weight for if the grq has runnable tasks with se->avg.load_avg < se->load.weight, i.e. tasks that occasionally block. This means that the underestimate scenario I have in my reply for patch #2 is no longer possible. AFAICT, we are now guaranteed to over-estimate ge->load.weight. It is still quite sensitive to periodic high priority tasks though.
tg->weight = 1024 tg->load_avg = 2560 \Sum grq->load.weight = 2048
cpu 0 1 \Sum grq->avg.load_avg 1536 1024 grq->load.weight 1024 1024 load (max) 1536 1024 ge->load_weight (1) 512 512 1024 >= tg->weight ge->load_weight (3) 614 410 1024 >= tg->weight ge->load_weight (5) 512 410 922 < tg->weight ge->load_weight (5*) 614 410 1024 >= tg->weight
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