Messages in this thread | | | From | Benjamin Segall <> | Subject | Re: [PATCH v5 2/2] Sched/fair: Block nohz tick_stop when cfs bandwidth in use | Date | Mon, 10 Jul 2023 16:54:58 -0700 |
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Phil Auld <pauld@redhat.com> writes:
> CFS bandwidth limits and NOHZ full don't play well together. Tasks > can easily run well past their quotas before a remote tick does > accounting. This leads to long, multi-period stalls before such > tasks can run again. Currently, when presented with these conflicting > requirements the scheduler is favoring nohz_full and letting the tick > be stopped. However, nohz tick stopping is already best-effort, there > are a number of conditions that can prevent it, whereas cfs runtime > bandwidth is expected to be enforced. > > Make the scheduler favor bandwidth over stopping the tick by setting > TICK_DEP_BIT_SCHED when the only running task is a cfs task with > runtime limit enabled. We use cfs_b->hierarchical_quota to > determine if the task requires the tick. > > Add check in pick_next_task_fair() as well since that is where > we have a handle on the task that is actually going to be running. > > Add check in sched_can_stop_tick() to cover some edge cases such > as nr_running going from 2->1 and the 1 remains the running task. > > Add sched_feat HZ_BW (off by default) to control the tick_stop > behavior. > > Signed-off-by: Phil Auld <pauld@redhat.com> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Vincent Guittot <vincent.guittot@linaro.org> > Cc: Juri Lelli <juri.lelli@redhat.com> > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> > Cc: Valentin Schneider <vschneid@redhat.com> > Cc: Ben Segall <bsegall@google.com> > Cc: Frederic Weisbecker <frederic@kernel.org> > --- > kernel/sched/core.c | 12 ++++++++++ > kernel/sched/fair.c | 49 +++++++++++++++++++++++++++++++++++++++++ > kernel/sched/features.h | 2 ++ > kernel/sched/sched.h | 1 + > 4 files changed, 64 insertions(+) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 1b214e10c25d..4b8534abdf4f 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -1229,6 +1229,18 @@ bool sched_can_stop_tick(struct rq *rq) > if (rq->nr_running > 1) > return false; > > + /* > + * If there is one task and it has CFS runtime bandwidth constraints > + * and it's on the cpu now we don't want to stop the tick. > + * This check prevents clearing the bit if a newly enqueued task here is > + * dequeued by migrating while the constrained task continues to run. > + * E.g. going from 2->1 without going through pick_next_task(). > + */ > + if (sched_feat(HZ_BW) && rq->nr_running == 1 && task_on_rq_queued(rq->curr)) { > + if (cfs_task_bw_constrained(rq->curr)) > + return false; > + } > +
I think we still need the fair_sched_class check with the bit being on cfs_rq/tg rather than task.
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