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

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Date:   Thu, 27 Jul 2017 12:44:41 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Joel Fernandes (Google)" <joel.opensrc@...il.com>
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 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.

-- 
viresh