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From: Vincent Donnefort <vincent.donnefort@arm.com>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: peterz@infradead.org, mingo@redhat.com,
	vincent.guittot@linaro.org, linux-kernel@vger.kernel.org,
	qperret@google.com, patrick.bellasi@matbug.net,
	valentin.schneider@arm.com
Subject: Re: [PATCH v2 1/2] sched/fair: Fix task utilization accountability in compute_energy()
Date: Thu, 25 Feb 2021 16:23:46 +0000	[thread overview]
Message-ID: <20210225162346.GB1345245@e124901.cambridge.arm.com> (raw)
In-Reply-To: <f2f5cf8e-3a0b-7192-5293-bad576e7066b@arm.com>

On Thu, Feb 25, 2021 at 12:45:06PM +0100, Dietmar Eggemann wrote:
> On 25/02/2021 09:36, vincent.donnefort@arm.com wrote:
> > From: Vincent Donnefort <vincent.donnefort@arm.com>
> 
> [...]
> 
> > cpu_util_next() estimates the CPU utilization that would happen if the
> > task was placed on dst_cpu as follows:
> > 
> >   max(cpu_util + task_util, cpu_util_est + _task_util_est)
> > 
> > The task contribution to the energy delta can then be either:
> > 
> >   (1) _task_util_est, on a mostly idle CPU, where cpu_util is close to 0
> >       and _task_util_est > cpu_util.
> >   (2) task_util, on a mostly busy CPU, where cpu_util > _task_util_est.
> > 
> >   (cpu_util_est doesn't appear here. It is 0 when a CPU is idle and
> >    otherwise must be small enough so that feec() takes the CPU as a
> >    potential target for the task placement)
> 
> I still don't quite get the reasoning for (2) why task_util is used as
> task contribution.
> 
> So we use 'cpu_util + task_util' instead of 'cpu_util_est +
> _task_util_est' in (2).
> 
> I.e. since _task_util_est is always >= task_util during wakeup, cpu_util
> must be > cpu_util_est (by more than _task_util_est - task_util).
> 
> I can see it for a CPU whose cpu_util has a fair amount of contributions
> from blocked tasks which cpu_util_est wouldn't have.
> 
> [...]

Yes exactly. I discovered this issue in a trace where an overutilized happened.
Many tasks were migrated to the biggest CPU, but once EAS was back on, it was too
late. The big CPU had a high util_avg and the task_util _task_util_est
unfairness kept placing tasks on that one, despite being inefficient. All the
tasks enqueued on that CPU were enough to keep util_avg high enough and that
situation wasn't resolving.

> 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 7043bb0f2621..146ac9fec4b6 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6573,8 +6573,24 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
> >  	 * its pd list and will not be accounted by compute_energy().
> >  	 */
> >  	for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {
> > -		unsigned long cpu_util, util_cfs = cpu_util_next(cpu, p, dst_cpu);
> > -		struct task_struct *tsk = cpu == dst_cpu ? p : NULL;
> > +		unsigned long util_freq = cpu_util_next(cpu, p, dst_cpu);
> > +		unsigned long cpu_util, util_running = util_freq;
> > +		struct task_struct *tsk = NULL;
> > +
> > +		/*
> > +		 * When @p is placed on @cpu:
> > +		 *
> > +		 * util_running = max(cpu_util, cpu_util_est) +
> > +		 *		  max(task_util, _task_util_est)
> > +		 *
> > +		 * while cpu_util_next is: max(cpu_util + task_util,
> > +		 *			       cpu_util_est + _task_util_est)
> > +		 */
> 
> Nit pick:
> 
> s/on @cpu/on @dst_cpu ?

I meant @cpu. When dst_cpu == cpu, it means that we simulate the task being
placed on cpu. That's what I wanted to highlight. But I can remove it if you
think this is not necessary.

> 
> s/while cpu_util_next is/while cpu_util_next(cpu, p, cpu) would be
> 
> If dst_cpu != cpu (including dst_cpu == -1) task_util and _task_util_est
> are not added to util resp. util_est.
> 
> Not sure if this is clear from the source code here?
> 
> [...]
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

  parent reply	other threads:[~2021-02-25 16:24 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-25  8:36 [PATCH v2 0/2] Fix task utilization accountability for EAS vincent.donnefort
2021-02-25  8:36 ` [PATCH v2 1/2] sched/fair: Fix task utilization accountability in compute_energy() vincent.donnefort
2021-02-25  8:52   ` Quentin Perret
2021-02-25 11:45   ` Dietmar Eggemann
2021-02-25 11:58     ` Quentin Perret
2021-02-25 16:23     ` Vincent Donnefort [this message]
2021-03-02  9:01   ` [tip: sched/core] " tip-bot2 for Vincent Donnefort
2021-03-03  9:49   ` tip-bot2 for Vincent Donnefort
2021-03-06 11:42   ` tip-bot2 for Vincent Donnefort
2021-02-25  8:36 ` [PATCH v2 2/2] sched/fair: use lsub_positive in cpu_util_next() vincent.donnefort
2021-02-25  8:53   ` Quentin Perret
2021-02-25 11:46   ` Dietmar Eggemann
2021-03-02  9:01   ` [tip: sched/core] " tip-bot2 for Vincent Donnefort
2021-03-03  9:49   ` tip-bot2 for Vincent Donnefort
2021-03-06 11:42   ` tip-bot2 for Vincent Donnefort
2021-02-25 17:29 ` [PATCH v2 0/2] Fix task utilization accountability for EAS Peter Zijlstra

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