linux-kernel - Re: [PATCH] sched/fair: Dequeue sched_delayed tasks when waking to a busy CPU

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Message-ID: <20241108133452.GA43508@pauld.westford.csb>
Date: Fri, 8 Nov 2024 08:34:52 -0500
From: Phil Auld <pauld@...hat.com>
To: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
	kprateek.nayak@....com, wuyun.abel@...edance.com,
	youssefesmat@...omium.org, tglx@...utronix.de
Subject: Re: [PATCH] sched/fair: Dequeue sched_delayed tasks when waking to a
 busy CPU

On Fri, Nov 08, 2024 at 01:24:35AM +0100 Mike Galbraith wrote:
> On Thu, 2024-11-07 at 15:09 +0100, Peter Zijlstra wrote:
> > On Thu, Nov 07, 2024 at 03:02:36PM +0100, Mike Galbraith wrote:
> > > On Thu, 2024-11-07 at 10:46 +0100, Mike Galbraith wrote:
> > > > On Thu, 2024-11-07 at 05:03 +0100, Mike Galbraith wrote:
> > > > >
> > > > > I built that patch out of curiosity, and yeah, set_next_task_fair()
> > > > > finding a cfs_rq->curr ends play time pretty quickly.
> > > >
> > > > The below improved uptime, and trace_printk() says it's doing the
> > > > intended, so I suppose I'll add a feature and see what falls out.
> > >
> > > From netperf, I got.. number tabulation practice.  Three runs of each
> > > test with and without produced nothing but variance/noise.
> >
> > Make it go away then.
> >
> > If you could write a Changelog for you inspired bit and stick my cleaned
> > up version under it, I'd be much obliged.
> 
> Salut, much obliged for eyeball relief.
>

Thanks Mike (and Peter).  We have our full perf tests running on Mike's
original verion of this patch. Results probably Monday (there's a long
queue). We'll see if this blows up anything else then.  I'll queue up a
build with this cleaned up version as well but the results will be late
next week, probably.

At that point maybe some or all of these:

Suggested-by: Phil Auld <pauld@...hat.com> 
Reviewed-by: Phil Auld <pauld@...hat.com>
Tested-by: Jirka Hladky <jhladky@...hat.com>


Cheers,
Phil


> ---snip---
> 
> Phil Auld (Redhat) reported an fio benchmark regression having been found
> to have been caused by addition of the DELAY_DEQUEUE feature, suggested it
> may be related to wakees losing the ability to migrate, and confirmed that
> restoration of same indeed did restore previous performance.
> 
> (de-uglified-a-lot-by)
> 
> Reported-by: Phil Auld <pauld@...hat.com>
> Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
> Link: https://lore.kernel.org/lkml/20241101124715.GA689589@pauld.westford.csb/
> Signed-off-by: Mike Galbraith <efault@....de>
> ---
>  kernel/sched/core.c  |   48 +++++++++++++++++++++++++++++-------------------
>  kernel/sched/sched.h |    5 +++++
>  2 files changed, 34 insertions(+), 19 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3783,28 +3783,38 @@ ttwu_do_activate(struct rq *rq, struct t
>   */
>  static int ttwu_runnable(struct task_struct *p, int wake_flags)
>  {
> -	struct rq_flags rf;
> -	struct rq *rq;
> -	int ret = 0;
> -
> -	rq = __task_rq_lock(p, &rf);
> -	if (task_on_rq_queued(p)) {
> -		update_rq_clock(rq);
> -		if (p->se.sched_delayed)
> -			enqueue_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_DELAYED);
> -		if (!task_on_cpu(rq, p)) {
> -			/*
> -			 * When on_rq && !on_cpu the task is preempted, see if
> -			 * it should preempt the task that is current now.
> -			 */
> -			wakeup_preempt(rq, p, wake_flags);
> +	CLASS(__task_rq_lock, rq_guard)(p);
> +	struct rq *rq = rq_guard.rq;
> +
> +	if (!task_on_rq_queued(p))
> +		return 0;
> +
> +	update_rq_clock(rq);
> +	if (p->se.sched_delayed) {
> +		int queue_flags = ENQUEUE_DELAYED | ENQUEUE_NOCLOCK;
> +
> +		/*
> +		 * Since sched_delayed means we cannot be current anywhere,
> +		 * dequeue it here and have it fall through to the
> +		 * select_task_rq() case further along the ttwu() path.
> +		 */
> +		if (rq->nr_running > 1 && p->nr_cpus_allowed > 1) {
> +			dequeue_task(rq, p, DEQUEUE_SLEEP | queue_flags);
> +			return 0;
>  		}
> -		ttwu_do_wakeup(p);
> -		ret = 1;
> +
> +		enqueue_task(rq, p, queue_flags);
> +	}
> +	if (!task_on_cpu(rq, p)) {
> +		/*
> +		 * When on_rq && !on_cpu the task is preempted, see if
> +		 * it should preempt the task that is current now.
> +		 */
> +		wakeup_preempt(rq, p, wake_flags);
>  	}
> -	__task_rq_unlock(rq, &rf);
> +	ttwu_do_wakeup(p);
> 
> -	return ret;
> +	return 1;
>  }
> 
>  #ifdef CONFIG_SMP
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1779,6 +1779,11 @@ task_rq_unlock(struct rq *rq, struct tas
>  	raw_spin_unlock_irqrestore(&p->pi_lock, rf->flags);
>  }
> 
> +DEFINE_LOCK_GUARD_1(__task_rq_lock, struct task_struct,
> +		    _T->rq = __task_rq_lock(_T->lock, &_T->rf),
> +		    __task_rq_unlock(_T->rq, &_T->rf),
> +		    struct rq *rq; struct rq_flags rf)
> +
>  DEFINE_LOCK_GUARD_1(task_rq_lock, struct task_struct,
>  		    _T->rq = task_rq_lock(_T->lock, &_T->rf),
>  		    task_rq_unlock(_T->rq, _T->lock, &_T->rf),
> 
> 

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