Re: [PATCH v4] Sched/fair: Block nohz tick_stop when cfs bandwidth in use

[Phil Auld]

Hi Peter,

On Fri, Jun 30, 2023 at 05:06:41PM +0200 Peter Zijlstra wrote:
> On Fri, Jun 30, 2023 at 09:57:14AM -0400, Phil Auld wrote:
> 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index a68d1276bab0..2685373e12f1 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1229,6 +1229,16 @@ 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.
> > +	 */
> > +	if (sched_feat(HZ_BW) && rq->nr_running == 1 && rq->curr
> > +	    && rq->curr->sched_class == &fair_sched_class && task_on_rq_queued(rq->curr)) {
> 
> && goes at the end of the previous line
> 
> rq->curr is never NULL
> 
> But surely you can find a saner way to write this?
>

Okay, I'll try to clean that up.


> > +		if (sched_cfs_bandwidth_active(rq->curr))
> > +			return false;
> > +	}
> > +
> >  	return true;
> >  }
> >  #endif /* CONFIG_NO_HZ_FULL */
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 373ff5f55884..125b1ec4476f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6139,6 +6139,50 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
> >  	rcu_read_unlock();
> >  }
> >  
> > +#ifdef CONFIG_NO_HZ_FULL
> > +static inline bool cfs_se_bandwidth_enabled(struct sched_entity *se)
> > +{
> > +	int ret = 0;
> > +
> > +	for_each_sched_entity(se)
> > +		ret += cfs_rq_of(se)->runtime_enabled;
> > +
> > +	return ret != 0;
> > +}
> > +
> > +bool sched_cfs_bandwidth_active(struct task_struct *p)
> > +{
> > +	if (cfs_bandwidth_used() && cfs_se_bandwidth_enabled(&p->se))
> > +		return true;
> > +
> > +	return false;
> > +}
> > +
> > +/* 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_se_bandwidth_enabled(&p->se))
> > +		tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
> > +}
> 
> Yeah, I think not; pick_next_task_fair() just walked the cgroup
> hierarchy and now you do it again.
> 
> Also, why does this code exist at all? Both enqueue/dequeue already end
> up in sched_update_tick_depenency() and should be able to handle the
> nr_running==1 with bandwidth crap, no?
> 

No. Or at least not without plumbing the enqueued/dequeued task all the way
through.  I can do it that way if you prefer but that seemed a lot more
intrusive. When we are in sched_can_stop_tick() we don't have access to the
cfs task which will end up running. Curr is idle in that case.  We'd have to
essential run pick_next_task_fair() to find the task to check which seemed
wrong. Maybe there is a better way? 

The code in sched_can_stop_tick() was added to catch the edge case
of waking a second task and having it migrated before it runs so we don't
clear the dependency of the running bandwidth enabled task by the dequeue
of the transient waking task.


Thanks for taking a look. This is better than "OMG" :)


Cheers,
Phil

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