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Date:   Wed, 20 Dec 2017 10:17:12 +0100
From:   Peter Zijlstra <>
To:     Viresh Kumar <>
Cc:     Rafael Wysocki <>, Ingo Molnar <>,,
        Vincent Guittot <>,,,,,,,
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


   either immediately or when next preempt_enable() hits a 0


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

But this is not something we should worry about.

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