linux-kernel - Re: [PATCH V3 1/3] sched: cpufreq: Allow remote cpufreq callbacks

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Message-ID: <20170726062912.GA352@vireshk-i7>
Date:   Wed, 26 Jul 2017 11:59:12 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        smuckle.linux@...il.com, juri.lelli@....com,
        Morten.Rasmussen@....com, patrick.bellasi@....com,
        eas-dev@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 1/3] sched: cpufreq: Allow remote cpufreq callbacks

On 24-07-17, 15:47, Peter Zijlstra wrote:
> I said nothing about the shared locking. That is indeed required. All I
> said is that those two tests you add could be left out.

I was right, I didn't understood your comment at all :(

> > > That would then continue to process the iowait and other accounting
> > > stuff, but stall the moment we call into the actual driver, which will
> > > then drop the request on the floor as per the first few hunks.
> > 
> > I am not sure I understood your comment completely though.
> 
> Since we call cpufreq_update_util(@rq, ...) with @rq->lock held, all
> such calls are in fact serialized for that cpu.

Yes, they are serialized but ..

> Therefore the cpu !=
> current_cpu test you add are pointless.

.. I didn't understand why you said so. This check isn't there to take
care of serialization but remote callbacks.

> Only once we get to the actual cpufreq driver (intel_pstate and others)
> do we run into the fact that we might not be able to service the request
> remotely.

We never check for remote callbacks in drivers.

> But since you also add a test there, that is sufficient.

No.

The diff for intel-pstate that you saw in this patch was for the case
where intel-pstate works directly with the scheduler (i.e. no
schedutil governor). The routine that gets called with schedutil is
intel_cpufreq_target(), which doesn't check for remoteness at all.

-- 
viresh