linux-kernel - Re: [PATCH 2/4] sched: cpufreq: Keep track of cpufreq utilization update flags

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Date:   Tue, 19 Dec 2017 20:25:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, Ingo Molnar <mingo@...hat.com>,
        linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, morten.rasmussen@....com,
        juri.lelli@...hat.com, tkjos@...roid.com, joelaf@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] sched: cpufreq: Keep track of cpufreq utilization
 update flags

On Wed, Dec 13, 2017 at 03:23:21PM +0530, Viresh Kumar wrote:
> Currently the schedutil governor overwrites the sg_cpu->flags field on
> every call to the utilization handler. It was pretty good as the initial
> implementation of utilization handlers, there are several drawbacks
> though.
> 
> The biggest drawback is that the sg_cpu->flags field doesn't always
> represent the correct type of tasks that are enqueued on a CPU's rq. For
> example, if a fair task is enqueued while a RT or DL task is running, we
> will overwrite the flags with value 0 and that may take the CPU to lower
> OPPs unintentionally. There can be other corner cases as well which we
> aren't aware of currently.
> 
> This patch changes the current implementation to keep track of all the
> task types that are currently enqueued to the CPUs rq. A new flag: CLEAR
> is introduced and is set by the scheduling classes when their last task
> is dequeued. When the CLEAR flag bit is set, the schedutil governor resets
> all the other flag bits that are present in the flags parameter. For
> now, the util update handlers return immediately if they were called to
> clear the flag.

Yeah, not happy about this either; we had code that did the right thing
without this extra tracking I think.

Also, we can look at the rq state if we want to, we don't need to
duplicate that state.