linux-kernel - Re: [PATCH AUTOSEL 5.12 62/80] sched/fair: Take thermal pressure into account while estimating energy

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Date:   Mon, 5 Jul 2021 06:12:43 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Greg KH <greg@...ah.com>
Subject: Re: [PATCH AUTOSEL 5.12 62/80] sched/fair: Take thermal pressure
 into account while estimating energy

On Mon, Jul 05, 2021 at 09:13:59AM +0100, Lukasz Luba wrote:
>Hi Sasha,
>
>+CC Greg
>
>On 7/5/21 12:05 AM, Sasha Levin wrote:
>>From: Lukasz Luba <lukasz.luba@....com>
>>
>>[ Upstream commit 489f16459e0008c7a5c4c5af34bd80898aa82c2d ]
>>
>>Energy Aware Scheduling (EAS) needs to be able to predict the frequency
>>requests made by the SchedUtil governor to properly estimate energy used
>>in the future. It has to take into account CPUs utilization and forecast
>>Performance Domain (PD) frequency. There is a corner case when the max
>>allowed frequency might be reduced due to thermal. SchedUtil is aware of
>>that reduced frequency, so it should be taken into account also in EAS
>>estimations.
>>
>>SchedUtil, as a CPUFreq governor, knows the maximum allowed frequency of
>>a CPU, thanks to cpufreq_driver_resolve_freq() and internal clamping
>>to 'policy::max'. SchedUtil is responsible to respect that upper limit
>>while setting the frequency through CPUFreq drivers. This effective
>>frequency is stored internally in 'sugov_policy::next_freq' and EAS has
>>to predict that value.
>>
>>In the existing code the raw value of arch_scale_cpu_capacity() is used
>>for clamping the returned CPU utilization from effective_cpu_util().
>>This patch fixes issue with too big single CPU utilization, by introducing
>>clamping to the allowed CPU capacity. The allowed CPU capacity is a CPU
>>capacity reduced by thermal pressure raw value.
>>
>>Thanks to knowledge about allowed CPU capacity, we don't get too big value
>>for a single CPU utilization, which is then added to the util sum. The
>>util sum is used as a source of information for estimating whole PD energy.
>>To avoid wrong energy estimation in EAS (due to capped frequency), make
>>sure that the calculation of util sum is aware of allowed CPU capacity.
>>
>>This thermal pressure might be visible in scenarios where the CPUs are not
>>heavily loaded, but some other component (like GPU) drastically reduced
>>available power budget and increased the SoC temperature. Thus, we still
>>use EAS for task placement and CPUs are not over-utilized.
>>
>>Signed-off-by: Lukasz Luba <lukasz.luba@....com>
>>Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>>Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
>>Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
>>Link: https://lore.kernel.org/r/20210614191128.22735-1-lukasz.luba@arm.com
>>Signed-off-by: Sasha Levin <sashal@...nel.org>
>>---
>>  kernel/sched/fair.c | 11 ++++++++---
>>  1 file changed, 8 insertions(+), 3 deletions(-)
>
>It has been picked up automatically right?
>To make it fully working you need also this patch:
>https://lore.kernel.org/linux-pm/20210614191030.22241-1-lukasz.luba@arm.com/
>
>It makes sure that the thermal pressure signal gets proper
>information also for CPUs which were offline and then wake-up.
>It has a proper fix tagging with commit hash id.
>That patch can be ported to stable: v5.6+
>I can send it to stable list. Please let me know if you need
>any help.
>
>The same applies to patch which I found for v5.13-stable:
>[PATCH AUTOSEL 5.13 65/85] sched/fair: Take thermal pressure into 
>account while estimating energy
>https://lore.kernel.org/stable/20210704230420.1488358-65-sashal@kernel.org/T/#u

I took that patch for 5.13-5.10, thanks!

-- 
Thanks,
Sasha