linux-kernel - Re: [PATCH 2/3] cpufreq: schedutil: move slow path from workqueue to SCHED

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Message-ID: <20161113194722.GC29583@graphite.smuckle.net>
Date:   Sun, 13 Nov 2016 11:47:22 -0800
From:   Steve Muckle <smuckle.linux@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Saravana Kannan <skannan@...eaurora.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <Juri.Lelli@....com>,
        Robin Randhawa <robin.randhawa@....com>
Subject: Re: [PATCH 2/3] cpufreq: schedutil: move slow path from workqueue to
 SCHED_FIFO task

On Sun, Nov 13, 2016 at 03:37:18PM +0100, Rafael J. Wysocki wrote:
> > Hold on a sec. I thought during LPC someone (Peter?) made a point that when
> > RT thread run, we should bump the frequency to max? So, schedutil is going
> > to trigger schedutil to bump up the frequency to max, right?
> 
> No, it isn't, or at least that is unlikely.
> 
> sugov_update_commit() sets sg_policy->work_in_progress before queuing
> the IRQ work and it is not cleared until the frequency changes in
> sugov_work().
> 
> OTOH, sugov_should_update_freq() checks sg_policy->work_in_progress
> upfront and returns false when it is set, so the governor won't see
> its own worker threads run, unless I'm overlooking something highly
> non-obvious.

FWIW my intention with the original version of this patch (which I
neglected to communicate to Viresh) was that it would depend on changing
the frequency policy for RT. I had been using rt_avg. It sounds like
during LPC there were talks of using another metric.

It does appear things would work okay without that but it also seems
a bit fragile. There's the window between when the work_in_progress
gets cleared and the RT kthread yields. I have not thought through the
various scenarios there, what is possible and tested to see if it is
significant enough to impact power-sensitive platforms.

thanks,
Steve