linux-kernel - Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked

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Date:   Tue, 22 May 2018 17:30:33 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        "Joel Fernandes (Google.)" <joelaf@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Todd Kjos <tkjos@...gle.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        kernel-team@...roid.com, Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when
 kthread kicked

On Tue, May 22, 2018 at 12:02 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Mon, May 21, 2018 at 6:13 PM, Joel Fernandes <joel@...lfernandes.org> wrote:
>> On Mon, May 21, 2018 at 10:29:52AM +0200, Rafael J. Wysocki wrote:
>>> On Mon, May 21, 2018 at 7:14 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>>> > On 18-05-18, 11:55, Joel Fernandes (Google.) wrote:
>>> >> From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
>>> >>
>>> >> Currently there is a chance of a schedutil cpufreq update request to be
>>> >> dropped if there is a pending update request. This pending request can
>>> >> be delayed if there is a scheduling delay of the irq_work and the wake
>>> >> up of the schedutil governor kthread.
>>> >>
>>> >> A very bad scenario is when a schedutil request was already just made,
>>> >> such as to reduce the CPU frequency, then a newer request to increase
>>> >> CPU frequency (even sched deadline urgent frequency increase requests)
>>> >> can be dropped, even though the rate limits suggest that its Ok to
>>> >> process a request. This is because of the way the work_in_progress flag
>>> >> is used.
>>> >>
>>> >> This patch improves the situation by allowing new requests to happen
>>> >> even though the old one is still being processed. Note that in this
>>> >> approach, if an irq_work was already issued, we just update next_freq
>>> >> and don't bother to queue another request so there's no extra work being
>>> >> done to make this happen.
>>> >
>>> > Now that this isn't an RFC anymore, you shouldn't have added below
>>> > paragraph here. It could go to the comments section though.
>>> >
>>> >> I had brought up this issue at the OSPM conference and Claudio had a
>>> >> discussion RFC with an alternate approach [1]. I prefer the approach as
>>> >> done in the patch below since it doesn't need any new flags and doesn't
>>> >> cause any other extra overhead.
>>> >>
>>> >> [1] https://patchwork.kernel.org/patch/10384261/
>>> >>
>>> >> LGTMed-by: Viresh Kumar <viresh.kumar@...aro.org>
>>> >> LGTMed-by: Juri Lelli <juri.lelli@...hat.com>
>>> >
>>> > Looks like a Tag you just invented ? :)
>>>
>>> Yeah.
>>>
>>> The LGTM from Juri can be converned into an ACK silently IMO.  That
>>> said I have added Looks-good-to: tags to a couple of commits. :-)
>>
>> Cool, I'll covert them to Acks :-)
>
> So it looks like I should expect an update of this patch, right?
>
> Or do you prefer the current one to be applied and work on top of it?
>

[cut]

>>
>> I just realized that on a single policy switch that uses the governor thread,
>> there will be 1 thread per-CPU. The sugov_update_single will be called on the
>> same CPU with interrupts disabled.
>
> sugov_update_single() doesn't have to run on the target CPU.

Which sadly is a bug IMO. :-/

>> In sugov_work, we are doing a
>> raw_spin_lock_irqsave which also disables interrupts. So I don't think
>> there's any possibility of a race happening on the same CPU between the
>> frequency update request and the sugov_work executing. In other words, I feel
>> we can drop the above if (..) statement for single policies completely and
>> only keep the changes for the shared policy. Viresh since you brought up the
>> single policy issue initially which made me add this if statememnt, could you
>> let me know if you agree with what I just said?
>
> Which is why you need the spinlock too.

And you totally have a point.  With the above bug fixed, disabling
interrupts should be sufficient to prevent concurrent updates from
occurring in the one-CPU policy case and the work_in_progress check in
sugov_update_single() isn't necessary.