linux-kernel - Re: [Eas-dev] [PATCH V4 1/3] sched: cpufreq: Allow remote cpufreq callbacks

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Message-ID: <CAEi0qNmj+MDTo43GB+6NrSw26RgTmFbJHy7Y87krp++w4pKozA@mail.gmail.com>
Date:   Thu, 27 Jul 2017 20:34:41 -0700
From:   "Joel Fernandes (Google)" <joel.opensrc@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-pm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        smuckle.linux@...il.com, eas-dev@...ts.linaro.org,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [Eas-dev] [PATCH V4 1/3] sched: cpufreq: Allow remote cpufreq callbacks

On Thu, Jul 27, 2017 at 12:14 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 26-07-17, 23:13, Joel Fernandes (Google) wrote:
>> On Wed, Jul 26, 2017 at 10:50 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> > On 26-07-17, 22:34, Joel Fernandes (Google) wrote:
>> >> On Wed, Jul 26, 2017 at 2:22 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> >> > @@ -221,7 +226,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>> >> >         sugov_set_iowait_boost(sg_cpu, time, flags);
>> >> >         sg_cpu->last_update = time;
>> >> >
>> >> > -       if (!sugov_should_update_freq(sg_policy, time))
>> >> > +       if (!sugov_should_update_freq(sg_policy, time, hook->cpu))
>> >> >                 return;
>> >>
>> >> Since with the remote callbacks now possible, isn't it unsafe to
>> >> modify sg_cpu and sg_policy structures without a lock in
>> >> sugov_update_single?
>> >>
>> >> Unlike sugov_update_shared, we don't acquire any lock in
>> >> sugov_update_single before updating these structures. Did I miss
>> >> something?
>> >
>> > As Peter already mentioned it earlier, the callbacks are called with
>> > rq locks held and so sugov_update_single() wouldn't get called in
>> > parallel for a target CPU.
>>
>> Ah ok, I have to catch up with that discussion since I missed the
>> whole thing. Now that you will have me on CC, that shouldn't happen,
>> thanks and sorry about the noise.
>>
>> > That's the only race you were worried about ?
>>
>> Yes. So then in that case, makes sense to move raw_spin_lock in
>> sugov_update_shared further down? (Just discussing, this point is
>> independent of your patch), Something like:
>
> Even that was discussed tomorrow with Peter :)
>
> No it wouldn't work because sg_cpu->util we are updating here may be
> getting read from some other cpu that shares policy with sg_cpu.
>

Ok. yes you are right :) thank you Viresh and Peter for the clarification.

thanks,

-Joel