linux-kernel - Re: [PATCH 7/7] Propagate negative bias

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Date: Tue, 25 Jun 2024 10:18:35 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Hongyan Xia <hongyan.xia2@....com>
CC: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
	<dietmar.eggemann@....com>, Juri Lelli <juri.lelli@...hat.com>, Steven
 Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin
 Schneider <vschneid@...hat.com>, Qais Yousef <qyousef@...alina.io>, Morten
 Rasmussen <morten.rasmussen@....com>, Lukasz Luba <lukasz.luba@....com>,
	Christian Loehle <christian.loehle@....com>, Pierre Gondois
	<pierre.gondois@....com>, Youssef Esmat <youssefesmat@...gle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] Propagate negative bias

Hello Hongyan,

On 6/24/2024 3:53 PM, Hongyan Xia wrote:
> Negative bias is interesting, because dequeuing such a task will
> actually increase utilization.
> 
> Solve by applying PELT decay to negative biases as well. This in fact
> can be implemented easily with some math tricks.
> 
> Signed-off-by: Hongyan Xia <hongyan.xia2@....com>
> ---
>   kernel/sched/fair.c  | 40 ++++++++++++++++++++++++++++++++++++++++
>   kernel/sched/sched.h |  4 ++++
>   2 files changed, 44 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3bb077df52ae..d09af6abf464 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4878,6 +4878,45 @@ static inline unsigned long root_cfs_util_uclamp(struct rq *rq)
>   
>   	return max(ret, 0L);
>   }
> +
> +/*
> + * Negative biases are tricky. If we remove them right away then dequeuing a
> + * uclamp_max task has the interesting effect that dequeuing results in a higher
> + * rq utilization. Solve this by applying PELT decay to the bias itself.
> + *
> + * Keeping track of a PELT-decayed negative bias is extra overhead. However, we
> + * observe this interesting math property, where y is the decay factor and p is
> + * the number of periods elapsed:
> + *
> + *	util_new = util_old * y^p - neg_bias * y^p
> + *		 = (util_old - neg_bias) * y^p
> + *
> + * Therefore, we simply subtract the negative bias from util_avg the moment we
> + * dequeue, then the PELT signal itself is the total of util_avg and the decayed
> + * negative bias, and we no longer need to track the decayed bias separately.
> + */
> +static void propagate_negative_bias(struct task_struct *p)
> +{
> +	if (task_util_bias(p) < 0 && !task_on_rq_migrating(p)) {
> +		unsigned long neg_bias = -task_util_bias(p);
> +		struct sched_entity *se = &p->se;
> +		struct cfs_rq *cfs_rq;
> +
> +		p->se.avg.util_avg_bias = 0;
> +
> +		for_each_sched_entity(se) {
> +			u32 divider, neg_sum;
> +
> +			cfs_rq = cfs_rq_of(se);
> +			divider = get_pelt_divider(&cfs_rq->avg);
> +			neg_sum = neg_bias * divider;
> +			sub_positive(&se->avg.util_avg, neg_bias);
> +			sub_positive(&se->avg.util_sum, neg_sum);

Most cases where I've seen "get_pelt_divider()" followed by
"add_positive()" or "sub_positive()" on "util_avg" and "util_sum" I've
seen a correction step that does:

	util_sum = max_t(u32, util_sum, util_avg * PELT_MIN_DIVIDER)

There is a comment on its significance in "update_cfs_rq_load_avg()".
Would it also apply in this case?

> +			sub_positive(&cfs_rq->avg.util_avg, neg_bias);
> +			sub_positive(&cfs_rq->avg.util_sum, neg_sum);
> +		}
> +	}
> +}
>   #else
>   static inline long task_util_bias(struct task_struct *p)
>   {
> @@ -6869,6 +6908,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>   	/* At this point se is NULL and we are at root level*/
>   	sub_nr_running(rq, 1);
>   	util_bias_dequeue(rq, p);
> +	propagate_negative_bias(p);

Perhaps I'm pointing to a premature optimization but since the hierarchy
is traversed above in "dequeue_task_fair()", could the "neg_bias" and
"neg_sum" removal be done along the way above instead of
"propagate_negative_bias()" traversing the hierarchy again? I don't see
a dependency on "util_bias_dequeue()" (which modifies
"rq->cfs.avg.util_avg_bias") for "propagate_negative_bias()" (which
works purely with task_util_bias() or "p->se.avg.util_avg_bias") but if
I'm missing something please do let me know.

Since you mentioned this patch isn't strictly necessary in the cover
letter, I would wait for other folks to chime in before changing this :)

>   
>   	/* balance early to pull high priority tasks */
>   	if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
> [..snip..]

-- 
Thanks and Regards,
Prateek