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Date: Thu, 13 Jul 2023 09:25:28 -0400
From: Phil Auld <pauld@...hat.com>
To: Benjamin Segall <bsegall@...gle.com>
Cc: linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Valentin Schneider <vschneid@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Mel Gorman <mgorman@...e.de>,
Frederic Weisbecker <frederic@...nel.org>,
Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v6 2/2] Sched/fair: Block nohz tick_stop when cfs
bandwidth in use
On Wed, Jul 12, 2023 at 03:11:32PM -0700 Benjamin Segall wrote:
> Phil Auld <pauld@...hat.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.
>
> I think this looks good now.
> Reviewed-By: Ben Segall <bsegall@...gle.com>
>
Thanks for your help and patience!
Cheers,
Phil
> >
> > Signed-off-by: Phil Auld <pauld@...hat.com>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Vincent Guittot <vincent.guittot@...aro.org>
> > Cc: Juri Lelli <juri.lelli@...hat.com>
> > Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> > Cc: Valentin Schneider <vschneid@...hat.com>
> > Cc: Ben Segall <bsegall@...gle.com>
> > Cc: Frederic Weisbecker <frederic@...nel.org>
> > ---
> > v6: restore check for fair_sched_class
> >
> > v5: Reworked checks to use newly-fixed cfs_b->hierarchical_quota to
> > check for bw
> > constraints.
> >
> > v4: Made checks for runtime_enabled hierarchical.
> >
> > v3: Moved sched_cfs_bandwidth_active() prototype to sched.h outside of
> > guards to
> > silence -Wmissing-prototypes.
> >
> > v2: Ben pointed out that the bit could get cleared in the dequeue path
> > if we migrate a newly enqueued task without preempting curr. Added a
> > check for that edge case to sched_can_stop_tick. Removed the call to
> > sched_can_stop_tick from sched_fair_update_stop_tick since it was
> > redundant.
> >
> > kernel/sched/core.c | 26 ++++++++++++++++++++++
> > kernel/sched/fair.c | 49 +++++++++++++++++++++++++++++++++++++++++
> > kernel/sched/features.h | 2 ++
> > kernel/sched/sched.h | 1 +
> > 4 files changed, 78 insertions(+)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index f80697a79baf..8a2ed4c0b709 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1194,6 +1194,20 @@ static void nohz_csd_func(void *info)
> > #endif /* CONFIG_NO_HZ_COMMON */
> >
> > #ifdef CONFIG_NO_HZ_FULL
> > +static inline bool __need_bw_check(struct rq *rq, struct task_struct *p)
> > +{
> > + if (rq->nr_running != 1)
> > + return false;
> > +
> > + if (p->sched_class != &fair_sched_class)
> > + return false;
> > +
> > + if (!task_on_rq_queued(p))
> > + return false;
> > +
> > + return true;
> > +}
> > +
> > bool sched_can_stop_tick(struct rq *rq)
> > {
> > int fifo_nr_running;
> > @@ -1229,6 +1243,18 @@ 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) && __need_bw_check(rq, rq->curr)) {
> > + if (cfs_task_bw_constrained(rq->curr))
> > + return false;
> > + }
> > +
> > return true;
> > }
> > #endif /* CONFIG_NO_HZ_FULL */
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index d9b3d4617e16..acd9f317aad1 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6140,6 +6140,46 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
> > rcu_read_unlock();
> > }
> >
> > +bool cfs_task_bw_constrained(struct task_struct *p)
> > +{
> > + struct cfs_rq *cfs_rq = task_cfs_rq(p);
> > +
> > + if (!cfs_bandwidth_used())
> > + return false;
> > +
> > + if (cfs_rq->runtime_enabled ||
> > + tg_cfs_bandwidth(cfs_rq->tg)->hierarchical_quota != RUNTIME_INF)
> > + return true;
> > +
> > + return false;
> > +}
> > +
> > +#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)
> > +{
> > + 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)
> > + 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_task_bw_constrained(p))
> > + tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
> > +}
> > +#endif
> > +
> > #else /* CONFIG_CFS_BANDWIDTH */
> >
> > static inline bool cfs_bandwidth_used(void)
> > @@ -6182,9 +6222,17 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> > static inline void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
> > static inline void update_runtime_enabled(struct rq *rq) {}
> > static inline void unthrottle_offline_cfs_rqs(struct rq *rq) {}
> > +bool cfs_task_bw_constrained(struct task_struct *p)
> > +{
> > + return false;
> > +}
> >
> > #endif /* CONFIG_CFS_BANDWIDTH */
> >
> > +#if !defined(CONFIG_CFS_BANDWIDTH) || !defined(CONFIG_NO_HZ_FULL)
> > +static inline void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p) {}
> > +#endif
> > +
> > /**************************************************
> > * CFS operations on tasks:
> > */
> > @@ -8098,6 +8146,7 @@ done: __maybe_unused;
> > hrtick_start_fair(rq, p);
> >
> > update_misfit_status(p, rq);
> > + sched_fair_update_stop_tick(rq, p);
> >
> > return p;
> >
> > diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> > index ee7f23c76bd3..6fdf1fdf6b17 100644
> > --- a/kernel/sched/features.h
> > +++ b/kernel/sched/features.h
> > @@ -101,3 +101,5 @@ SCHED_FEAT(LATENCY_WARN, false)
> >
> > SCHED_FEAT(ALT_PERIOD, true)
> > SCHED_FEAT(BASE_SLICE, true)
> > +
> > +SCHED_FEAT(HZ_BW, false)
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 63822c9238cc..d6d346bc78aa 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -465,6 +465,7 @@ extern void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth
> > extern void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b);
> > extern void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
> > extern void unthrottle_cfs_rq(struct cfs_rq *cfs_rq);
> > +extern bool cfs_task_bw_constrained(struct task_struct *p);
> >
> > extern void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
> > struct sched_rt_entity *rt_se, int cpu,
>
--
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