linux-kernel - Re: [PATCH v2 2/3] sched/fair: Calculate the cache-hot time of the idle CPU

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Date:   Sat, 25 Nov 2023 12:40:18 +0530
From:   Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To:     Chen Yu <yu.c.chen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        6dccbf0f54cc4ee068a157b9eebfb4b5fa3cc4af.1700548379.git.yu.c.chen@...el.com
Cc:     Tim Chen <tim.c.chen@...el.com>, Aaron Lu <aaron.lu@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Chen Yu <yu.chen.surf@...il.com>, linux-kernel@...r.kernel.org,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [PATCH v2 2/3] sched/fair: Calculate the cache-hot time of the
 idle CPU

Hi Chen Yu,

On 21/11/23 13:09, Chen Yu wrote:
> When a CPU is about to become idle due to task dequeue, uses
> the dequeued task's average sleep time to set the cache
> hot timeout of this idle CPU. This information can facilitate
> SIS to skip the cache-hot idle CPU and scan for the next
> cache-cold one. When that task is woken up again, it can choose
> its previous CPU and reuses its hot-cache.
> 
> This is a preparation for the next patch to introduce SIS_CACHE
> based task wakeup.
> 
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
>  kernel/sched/fair.c     | 30 +++++++++++++++++++++++++++++-
>  kernel/sched/features.h |  1 +
>  kernel/sched/sched.h    |  1 +
>  3 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 672616503e35..c309b3d203c0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6853,8 +6853,17 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	util_est_update(&rq->cfs, p, task_sleep);
>  
>  	if (task_sleep) {
> -		p->last_dequeue_time = sched_clock_cpu(cpu_of(rq));
> +		u64 now = sched_clock_cpu(cpu_of(rq));
> +
> +		p->last_dequeue_time = now;
>  		p->last_dequeue_cpu = cpu_of(rq);
> +
> +#ifdef CONFIG_SMP
> +		/* this rq becomes idle, update its cache hot timeout */
> +		if (sched_feat(SIS_CACHE) && !rq->nr_running &&
> +		    p->avg_hot_dur)
> +			rq->cache_hot_timeout = max(rq->cache_hot_timeout, now + p->avg_hot_dur);

As per the discussion in the rfc patch, you mentioned that SIS_CACHE only honors the average sleep time
of the latest dequeued task and that we don't know how much of the cache is polluted by the latest task.

So I was wondering what made you to put max here.

Thanks and Regards 
Madadi Vineeth Reddy