linux-kernel - Re: [PATCH v3] sched/topology: remove sysctl_sched_energy

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Message-ID: <xhsmhbkdzvfiq.mognet@vschneid.remote.csb>
Date:   Mon, 18 Sep 2023 14:21:01 +0200
From:   Valentin Schneider <vschneid@...hat.com>
To:     Pierre Gondois <pierre.gondois@....com>,
        Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
Cc:     dietmar.eggemann@....com, linux-kernel@...r.kernel.org,
        ionela.voinescu@....com, quentin.perret@....com,
        srikar@...ux.vnet.ibm.com, mgorman@...hsingularity.net,
        mingo@...nel.org, yu.c.chen@...el.com, tim.c.chen@...ux.intel.com,
        mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org
Subject: Re: [PATCH v3] sched/topology: remove sysctl_sched_energy_aware
 depending on the architecture

On 15/09/23 15:35, Pierre Gondois wrote:
> Hello Valentin,
>
> On 9/15/23 14:00, Valentin Schneider wrote:
>> On 14/09/23 23:26, Shrikanth Hegde wrote:
>>> On 9/14/23 9:51 PM, Valentin Schneider wrote:
>>>> On 13/09/23 17:18, Shrikanth Hegde wrote:
>>>>> sysctl_sched_energy_aware is available for the admin to disable/enable
>>>>> energy aware scheduling(EAS). EAS is enabled only if few conditions are
>>>>> met by the platform. They are, asymmetric CPU capacity, no SMT,
>>>>> valid cpufreq policy, frequency invariant load tracking. It is possible
>>>>> platform when booting may not have EAS capability, but can do that after.
>>>>> For example, changing/registering the cpufreq policy.
>>>>>
>>>>> At present, though platform doesn't support EAS, this sysctl is still
>>>>> present and it ends up calling rebuild of sched domain on write to 1 and
>>>>> NOP when writing to 0. That is confusing and un-necessary.
>>>>>
>>>>
>>>
>>> Hi Valentin, Thanks for taking a look at this patch.
>>>
>>>> But why would you write to it in the first place? Or do you mean to use
>>>> this as an indicator for userspace that EAS is supported?
>>>>
>>>
>>> Since this sysctl is present and its value being 1, it gives the
>>> impression to the user that EAS is supported when it is not.
>>> So its an attempt to correct that part.
>>>
>> 
>> Ah, I see. Then how about just making the sysctl return 0 when EAS isn't
>> supported? And on top of it, prevent all writes when EAS isn't supported
>> (perf domains cannot be built, so there would be no point in forcing a
>> rebuild that will do nothing).
>
> I think the issue comes from the fact there is no variable representing
> whether EAS is supported or not. sched_energy_enabled()/sched_energy_present
> tells whether EAS is actively running on the system instead.
>
> So on a system with EAS running, I think what would happen is:
> # Disable EAS and set sched_energy_present=0
> echo 0 > /proc/sys/kernel/sched_energy_aware
>
> # sched_energy_present==0, so we get -EOPNOTSUPP
> echo 1 > /proc/sys/kernel/sched_energy_aware
>

Ah, quite so, I didn't think this through!