linux-kernel - Re: [Patch v4 2/6] sched: Add infrastructure to store and update instantaneous thermal pressure

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Message-ID: <5DBA029E.8020508@linaro.org>
Date:   Wed, 30 Oct 2019 17:37:34 -0400
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, ionela.voinescu@....com,
        vincent.guittot@...aro.org, rui.zhang@...el.com,
        edubezval@...il.com, qperret@...gle.com,
        linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, daniel.lezcano@...aro.org
Subject: Re: [Patch v4 2/6] sched: Add infrastructure to store and update
 instantaneous thermal pressure

Hello Peter,
Thanks for the review.


On 10/28/2019 11:21 AM, Peter Zijlstra wrote:
> On Tue, Oct 22, 2019 at 04:34:21PM -0400, Thara Gopinath wrote:
>> Add thermal.c and thermal.h files that provides interface
>> APIs to initialize, update/average, track, accumulate and decay
>> thermal pressure per cpu basis. A per cpu variable delta_capacity is
>> introduced to keep track of instantaneous per cpu thermal pressure.
>> Thermal pressure is the delta between maximum capacity and capped
>> capacity due to a thermal event.
> 
>> API trigger_thermal_pressure_average is called for periodic accumulate
>> and decay of the thermal pressure. It is to to be called from a
>> periodic tick function. This API passes on the instantaneous delta
>> capacity of a cpu to update_thermal_load_avg to do the necessary
>> accumulate, decay and average.
> 
>> API update_thermal_pressure is for the system to update the thermal
>> pressure by providing a capped frequency ratio.
> 
>> Considering, trigger_thermal_pressure_average reads delta_capacity and
>> update_thermal_pressure writes into delta_capacity, one can argue for
>> some sort of locking mechanism to avoid a stale value.
> 
>> But considering trigger_thermal_pressure_average can be called from a
>> system critical path like scheduler tick function, a locking mechanism
>> is not ideal. This means that it is possible the delta_capacity value
>> used to calculate average thermal pressure for a cpu can be
>> stale for upto 1 tick period.
> 
> Please use a blank line at the end of a paragraph.

Will do

> 
>> Signed-off-by: Thara Gopinath <thara.gopinath@...aro.org>
>> ---
> 
>>  include/linux/sched.h  |  8 ++++++++
>>  kernel/sched/Makefile  |  2 +-
>>  kernel/sched/thermal.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>>  kernel/sched/thermal.h | 13 +++++++++++++
>>  4 files changed, 67 insertions(+), 1 deletion(-)
>>  create mode 100644 kernel/sched/thermal.c
>>  create mode 100644 kernel/sched/thermal.h
> 
> These are some tiny files, do these functions really need their own
> little files?

I will merge them  into fair.c. There will be update_thermal_pressure
that will be called from cpu_cooling or other relevant framework.
> 
> 
>> diff --git a/kernel/sched/thermal.c b/kernel/sched/thermal.c
>> new file mode 100644
>> index 0000000..0c84960
>> --- /dev/null
>> +++ b/kernel/sched/thermal.c
>> @@ -0,0 +1,45 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Scheduler Thermal Interactions
>> + *
>> + *  Copyright (C) 2018 Linaro, Inc., Thara Gopinath <thara.gopinath@...aro.org>
>> + */
>> +
>> +#include <linux/sched.h>
>> +#include "sched.h"
>> +#include "pelt.h"
>> +#include "thermal.h"
>> +
>> +static DEFINE_PER_CPU(unsigned long, delta_capacity);
>> +
>> +/**
>> + * update_thermal_pressure: Update thermal pressure
>> + * @cpu: the cpu for which thermal pressure is to be updated for
>> + * @capped_freq_ratio: capped max frequency << SCHED_CAPACITY_SHIFT / max freq
>> + *
>> + * capped_freq_ratio is normalized into capped capacity and the delta between
>> + * the arch_scale_cpu_capacity and capped capacity is stored in per cpu
>> + * delta_capacity.
>> + */
>> +void update_thermal_pressure(int cpu, u64 capped_freq_ratio)
>> +{
>> +	unsigned long __capacity, delta;
>> +
>> +	/* Normalize the capped freq ratio */
>> +	__capacity = (capped_freq_ratio * arch_scale_cpu_capacity(cpu)) >>
>> +							SCHED_CAPACITY_SHIFT;
>> +	delta = arch_scale_cpu_capacity(cpu) -  __capacity;
>> +	pr_debug("updating cpu%d thermal pressure to %lu\n", cpu, delta);
> 
> Surely we can do without the pr_debug() here?

Will remove. I had it while developing to check if the thermal pressure
is calculated correct.
> 
>> +	per_cpu(delta_capacity, cpu) = delta;
>> +}


-- 
Warm Regards
Thara