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

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Date:   Wed, 20 Dec 2017 10:17:12 +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, patrick.bellasi@....com
Subject: Re: [PATCH 2/4] sched: cpufreq: Keep track of cpufreq utilization
 update flags

On Wed, Dec 20, 2017 at 02:18:59PM +0530, Viresh Kumar wrote:
> On 20-12-17, 09:31, Peter Zijlstra wrote:
> What about this case: A CFS task is running currently and an RT task
> is enqueued.
> 
> - Is it always the case that the CFS task is preempted immediately and
>   the CPU is given to RT task ? I was talking to Vincent earlier and
>   he told me that for certain configurations the CFS task may keep
>   running until the next tick.
> 
> - What if the CFS task has disabled preemption ?
> 
> - More corner cases like this ?

If there is a runnable RT task we'll schedule to it ASAP. This means
for:

 CONFIG_PREEMPT=y

   either immediately or when next preempt_enable() hits a 0
   preempt_count.

 CONFIG_PREEMPT=n

   on any next kernel preemption point or when returning to userspace.

But this is not something we should worry about.