linux-kernel - Re: [Patch v9 7/8] sched/fair: Enable tuning of decay period

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Date:   Tue, 18 Feb 2020 09:57:25 -0500
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>, mingo@...hat.com,
        ionela.voinescu@....com, vincent.guittot@...aro.org,
        rui.zhang@...el.com, qperret@...gle.com, daniel.lezcano@...aro.org,
        viresh.kumar@...aro.org, rostedt@...dmis.org, will@...nel.org,
        catalin.marinas@....com, sudeep.holla@....com,
        juri.lelli@...hat.com, corbet@....net,
        linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, amit.kucheria@...durent.com
Subject: Re: [Patch v9 7/8] sched/fair: Enable tuning of decay period



On 2/14/20 5:26 AM, Dietmar Eggemann wrote:
> On 13/02/2020 14:54, Thara Gopinath wrote:
>> On 02/10/2020 06:59 AM, Dietmar Eggemann wrote:
>>> On 07/02/2020 23:42, Thara Gopinath wrote:
>>>> On 02/04/2020 03:39 AM, Dietmar Eggemann wrote:
>>>>> On 03/02/2020 16:55, Peter Zijlstra wrote:
>>>>>> On Mon, Feb 03, 2020 at 07:07:57AM -0500, Thara Gopinath wrote:
>>>>>>> On 01/28/2020 06:56 PM, Randy Dunlap wrote:
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> On 1/28/20 2:36 PM, Thara Gopinath wrote:
> 
> [...]
> 
>>> is really not saying from which review comment the individual changes in
>>> the function name are coming from. And I don't see an answer to Ionela's
>>> email saying that her proposal will manifest in a particular part of
>>> this change.
>> Hi Dietmar,
>>
>> Like I said, don't want to argue on name. It is trivial for me. I have
>> v10 prepped with the name change. Will send it out shortly.
> 
> Thanks.
> 
> [...]
> 
>>> Cpu-invariant accounting can't be guarded with a kernel CONFIG switch.
>>> Frequency-invariant accounting could be with CONFIG_CPU_FREQ but this is
>>> enabled by default by Arm64 defconfig.
>>> Thermal pressure (accounting) (CONFIG_HAVE_SCHED_THERMAL_PRESSURE) is
>>> disabled by default so why should a per-cpu thermal_pressure be
>>> maintained on such a system (CONFIG_CPU_THERMAL=y by default)?
>>
>> I agree that there is no need for per-cpu thermal pressure to be
>> maintained if no averaging is happening in the scheduler, today. I don't
>> know if there will ever be an use for it.
> 
> All arch_scale_FOO() functions follow the approach to force the arch
> (currently x86, arm, arm64) to do
> 
> #define arch_scale_FOO BAR
> 
> to enable the FOO functionality.
> 
> There is no direct link between consumer and provider here.
> 
>   consumer (sched) -> arch <- provider (arch, counters, CPUfreq, CPU
>                                         cooling, etc.)
> 
> So IMHO, FOO=thermal_pressure should follow this design pattern too.
> 
> 'thermal_pressure' would be the only one which can be disabled by a
> kernel config switch at the consumer side.
> IMHO, it doesn't make sense to have the provider operating in this case.
> 
>> My issue has to do with using a config option meant for internal
>> scheduler code being used else where. To me, once this happens, the
>> entire work done to separate out reading and writing of instantaneous
>> thermal pressure to arch_topology makes no sense. We could have kept it
>> in scheduler itself.
> 
> You might see thermal_pressure more on the level of irq_load or
> [rt/dl]_rq_load and that could be why we have a different opinion here?
> 
> Now rt_rq_load and dl_rq_load are scheduler internal providers and
> irq_load is driven by 'irq_delta + steal' time (which is much closer to
> the scheduler than thermal for instance).

In this case, thermal pressure is quite close to scheduler as it reduces 
the maximum capacity available per cpu and hence affects scheduler 
placement of tasks

> 
> My assumption is that we don't want a direct link between the scheduler
> and e.g. a provider 'thermal'.

Exactly. Which is why the same CONFIG option should not be used between 
the provider and consumer.

> 
>> Another way I think about this whole thermal pressure framework  is that
>> it is the job of cooling device or cpufreq or any other entity to update
>> a throttle in maximum pressure to the scheduler. It should be
>> independent of what scheduler does with it. Scheduler can choose to
>> ignore it

-- 
Warm Regards
Thara