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From: Quentin Perret <qperret@google.com>
To: vincent.donnefort@arm.com
Cc: peterz@infradead.org, mingo@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	linux-kernel@vger.kernel.org, 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 08:52:56 +0000	[thread overview]
Message-ID: <YDdlaLjOvGH+VLBh@google.com> (raw)
In-Reply-To: <20210225083612.1113823-2-vincent.donnefort@arm.com>

On Thursday 25 Feb 2021 at 08:36:11 (+0000), vincent.donnefort@arm.com wrote:
> From: Vincent Donnefort <vincent.donnefort@arm.com>
> 
> find_energy_efficient_cpu() (feec()) computes for each perf_domain (pd) an
> energy delta as follows:
> 
>   feec(task)
>     for_each_pd
>       base_energy = compute_energy(task, -1, pd)
>         -> for_each_cpu(pd)
>            -> cpu_util_next(cpu, task, -1)
> 
>       energy_delta = compute_energy(task, dst_cpu, pd)
>         -> for_each_cpu(pd)
>            -> cpu_util_next(cpu, task, dst_cpu)
>       energy_delta -= base_energy
> 
> Then it picks the best CPU as being the one that minimizes energy_delta.
> 
> 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)
> 
> This is problematic for feec(), as cpu_util_next() might give an unfair
> advantage to a CPU which is mostly busy (2) compared to one which is
> mostly idle (1). _task_util_est being always bigger than task_util in
> feec() (as the task is waking up), the task contribution to the energy
> might look smaller on certain CPUs (2) and this breaks the energy
> comparison.
> 
> This issue is, moreover, not sporadic. By starving idle CPUs, it keeps
> their cpu_util < _task_util_est (1) while others will maintain cpu_util >
> _task_util_est (2).
> 
> Fix this problem by always using max(task_util, _task_util_est) as a task
> contribution to the energy (ENERGY_UTIL). The new estimated CPU
> utilization for the energy would then be:
> 
>   max(cpu_util, cpu_util_est) + max(task_util, _task_util_est)
> 
> compute_energy() still needs to know which OPP would be selected if the
> task would be migrated in the perf_domain (FREQUENCY_UTIL). Hence,
> cpu_util_next() is still used to estimate the maximum util within the pd.
> 
> Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>

Reviewed-by: Quentin Perret <qperret@google.com>

Thanks,
Quentin

  reply	other threads:[~2021-02-25  8:54 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 [this message]
2021-02-25 11:45   ` Dietmar Eggemann
2021-02-25 11:58     ` Quentin Perret
2021-02-25 16:23     ` Vincent Donnefort
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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