linux-kernel - Re: [RFC PATCH v2 1/2] sched/fair: Introduce short duration task check

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Date:   Fri, 4 Nov 2022 12:09:12 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Vincent Guittot <vincent.guittot@...aro.org>,
        Tim Chen <tim.c.chen@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Aaron Lu <aaron.lu@...el.com>,
        Abel Wu <wuyun.abel@...edance.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Yicong Yang <yangyicong@...ilicon.com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Ingo Molnar <mingo@...hat.com>,
        "Dietmar Eggemann" <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Hillf Danton <hdanton@...a.com>,
        Honglei Wang <wanghonglei@...ichuxing.com>,
        Len Brown <len.brown@...el.com>,
        Chen Yu <yu.chen.surf@...il.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 1/2] sched/fair: Introduce short duration task
 check

Hi Peter,
thanks for your review,
On 2022-11-03 at 13:44:12 +0100, Peter Zijlstra wrote:
> On Sun, Oct 23, 2022 at 11:32:31PM +0800, Chen Yu wrote:
> 
> > +	if (sched_feat(SIS_SHORT) && !prev->on_rq) {
> > +		/*
> > +		 * sum_exec_runtime has been updated in update_curr()
> > +		 * because we reach here via dequeue.
> > +		 */
> > +		this_dur_avg = se->sum_exec_runtime - se->prev_sum_runtime_vol;
> > +		/*
> > +		 * Record the accumulated runtime when task voluntarily
> > +		 * switches out. End of old duration period, a new period
> > +		 * starts.
> > +		 */
> > +		se->prev_sum_runtime_vol = se->sum_exec_runtime;
> > +
> > +		last_dur_avg = se->dur_avg;
> > +		delta = this_dur_avg - last_dur_avg;
> > +		/* consider large change to avoid frequent update */
> > +		if (abs(delta) >= sysctl_sched_min_granularity) {
> > +			/*
> > +			 * If it is the first time the task starts to
> > +			 * record dur_avg, discard the initial value 0.
> > +			 * Otherwise, calculate the EWMA.
> > +			 */
> > +			if (unlikely(!this_dur_avg))
> > +				se->dur_avg = this_dur_avg;
> > +			else
> > +				update_avg(&se->dur_avg, this_dur_avg);
> > +		}
> > +	}
> 
> This seems excessively convoluted; what's wrong with something like:
> 
> 	if (sched_feat(SIS_SHORT) && !prev->on_rq) {
> 		u64 this_dur = se->sum_exec_runtime - se->prev_sum_exec_runtime_vol;
> 		se->prev_sum_exec_runtime_vol = se->sum_exec_runtime;
> 
> 		update_avg(&se->dur_avg, this_dur);
> 	}
> 
> All that other stuff just makes it unreadable and probably slower.
OK, I'll try your suggestion in next version.

thanks,
Chenyu