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

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Date:   Thu, 10 Nov 2022 18:51:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Kajetan Puchalski <kajetan.puchalski@....com>,
        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 Thu, Nov 10, 2022 at 03:59:01PM +0100, Dietmar Eggemann wrote:
> On 10/11/2022 14:05, Peter Zijlstra wrote:
> > On Thu, Nov 10, 2022 at 12:16:26PM +0100, Dietmar Eggemann wrote:
> >> 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.
> > 
> > Duh, obviously I meant to measure the runtime of the current activation
> > and messed up.
> > 
> > We don't appear to have the right information to compute this atm :/
> 
> This would be:
> 
> u64 now = rq_clock_task(rq);
> u64 runtime = now - curr->se.exec_start;
> 
> but we don't hold the rq lock so we can't get `now`?

Not quite the same; that's the time since we got on-cpu last, but that's
not the same as the runtime of this activation (it is when you discount
preemption).