Re: [PATCH] Sched/fair: Block nohz tick_stop when cfs bandwidth in use

[Phil Auld]

On Thu, Jun 22, 2023 at 01:49:52PM -0700 Benjamin Segall wrote:
> Phil Auld <pauld@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> > Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> > Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> > Cc: Ben Segall <bsegall@xxxxxxxxxx>
> > ---
> >  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.

But yes, I suppose you could end up in a state that is just as bad as today.

Maybe I could add a redundant check in sched_can_stop_tick for when
nr_running == 1 and curr is not null and make sure the bit does not get
cleared. I'll look into that.


Thanks,
Phil

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