linux-kernel - Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

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Date:   Thu, 10 Nov 2022 12:16:26 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Kajetan Puchalski <kajetan.puchalski@....com>
Cc:     Jian-Min Liu <jian-min.liu@...iatek.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel@...r.kernel.org,
        Jonathan JMChen <jonathan.jmchen@...iatek.com>
Subject: Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

On 07/11/2022 14:41, Peter Zijlstra wrote:
> On Thu, Sep 29, 2022 at 03:41:47PM +0100, Kajetan Puchalski wrote:

[...]

> @@ -2956,13 +2958,26 @@ static inline unsigned long cpu_util_dl(struct rq *rq)
>   */
>  static inline unsigned long cpu_util_cfs(int cpu)
>  {
> +	struct rq *rq = cpu_rq(cpu);
>  	struct cfs_rq *cfs_rq;
>  	unsigned long util;
>  
> -	cfs_rq = &cpu_rq(cpu)->cfs;
> +	cfs_rq = &rq->cfs;
>  	util = READ_ONCE(cfs_rq->avg.util_avg);
>  
>  	if (sched_feat(UTIL_EST)) {
> +		if (sched_feat(UTIL_EST_FASTER)) {
> +			struct task_struct *curr;
> +
> +			rcu_read_lock();
> +			curr = rcu_dereference(rq->curr);
> +			if (likely(curr->sched_class == &fair_sched_class)) {
> +				u64 runtime = curr->se.sum_exec_runtime - curr->se.exec_start;

Don't we and up with gigantic runtime numbers here?

oot@...o:~# cat /proc/1676/task/1676/schedstat
36946300 1150620 11
root@...o:~# cat /proc/1676/task/1676/sched
rt-app (1676, #threads: 2)
-------------------------------------------------------------------
se.exec_start                                :         77766.964240 <- !
se.vruntime                                  :           563.587883
e.sum_exec_runtime                          :            36.946300  <- !
se.nr_migrations                             :                    0
...

I expect cpu_util_cfs() to be ~1024 almost all the time now.

> +				util = max_t(unsigned long, util,
> +					     faster_est_approx(runtime * 2));
> +			}
> +			rcu_read_unlock();
> +		}
>  		util = max_t(unsigned long, util,
>  			     READ_ONCE(cfs_rq->avg.util_est.enqueued));
>  	}

[...]