Messages in this thread | | | Date | Fri, 23 Jun 2023 15:59:47 -0400 | From | Phil Auld <> | Subject | Re: [PATCH] Sched/fair: Block nohz tick_stop when cfs bandwidth in use |
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On Fri, Jun 23, 2023 at 11:59:09AM -0700 Benjamin Segall wrote: > Phil Auld <pauld@redhat.com> writes: > > > On Thu, Jun 22, 2023 at 05:37:30PM -0400 Phil Auld wrote: > >> On Thu, Jun 22, 2023 at 01:49:52PM -0700 Benjamin Segall wrote: > >> > 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. Currentlyi, 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. > >> > > > >> > > Add sched_feat HZ_BW (off by default) to control this 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> > >> > > --- > >> > > kernel/sched/fair.c | 33 ++++++++++++++++++++++++++++++++- > >> > > kernel/sched/features.h | 2 ++ > >> > > 2 files changed, 34 insertions(+), 1 deletion(-) > >> > > > >> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> > > index 373ff5f55884..880eadfac330 100644 > >> > > --- a/kernel/sched/fair.c > >> > > +++ b/kernel/sched/fair.c > >> > > @@ -6139,6 +6139,33 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq) > >> > > rcu_read_unlock(); > >> > > } > >> > > > >> > > +#ifdef CONFIG_NO_HZ_FULL > >> > > +/* called from pick_next_task_fair() */ > >> > > +static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p) > >> > > +{ > >> > > + struct cfs_rq *cfs_rq = task_cfs_rq(p); > >> > > + int cpu = cpu_of(rq); > >> > > + > >> > > + if (!sched_feat(HZ_BW) || !cfs_bandwidth_used()) > >> > > + return; > >> > > + > >> > > + if (!tick_nohz_full_cpu(cpu)) > >> > > + return; > >> > > + > >> > > + if (rq->nr_running != 1 || !sched_can_stop_tick(rq)) > >> > > + return; > >> > > + > >> > > + /* > >> > > + * We know there is only one task runnable and we've just picked it. The > >> > > + * normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will > >> > > + * be otherwise able to stop the tick. Just need to check if we are using > >> > > + * bandwidth control. > >> > > + */ > >> > > + if (cfs_rq->runtime_enabled) > >> > > + tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED); > >> > > +} > >> > > +#endif > >> > > >> > So from a CFS_BANDWIDTH pov runtime_enabled && nr_running == 1 seems > >> > fine. But working around sched_can_stop_tick instead of with it seems > >> > sketchy in general, and in an edge case like "migrate a task onto the > >> > cpu and then off again" you'd get sched_update_tick_dependency resetting > >> > the TICK_DEP_BIT and then not call PNT (ie a task wakes up onto this cpu > >> > without preempting, and then another cpu goes idle and pulls it, causing > >> > this cpu to go into nohz_full). > >> > > >> > >> The information to make these tests is not available in sched_can_stop_tick. > >> I did start there. When that is called, and we are likely to go nohz_full, > >> curr is null so it's hard to find the right cfs_rq to make that > >> runtime_enabled test against. We could, maybe, plumb the task being enqueued > >> in but it would not be valid for the dequeue path and would be a bit messy. > >> > > > > Sorry, mispoke... rq->curr == rq-idle not null. But still we don't have > > access to the task and its cfs_rq which will have runtime_enabled set. > > > > That is unfortunate. I suppose then you'd wind up needing both this > extra bit in PNT to handle the switch into nr_running == 1 territory, > and a "HZ_BW && nr_running == 1 && curr is fair && curr->on_rq && > curr->cfs_rq->runtime_enabled" check in sched_can_stop_tick to catch > edge cases. (I think that would be sufficient, if an annoyingly long set > of conditionals) >
Right. That's more or less what the version I'm testing now does.
Thanks again.
Cheers, Phil
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