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Subject: Re: [PATCH] sched/fair: vruntime should normalize when switching from
 fair
To:     Peter Zijlstra <peterz@infradead.org>,
        Miguel de Dios <migueldedios@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        kernel-team@android.com, Todd Kjos <tkjos@google.com>,
        Paul Turner <pjt@google.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Patrick Bellasi <Patrick.Bellasi@arm.com>,
        Chris Redpath <Chris.Redpath@arm.com>,
        Morten Rasmussen <Morten.Rasmussen@arm.com>,
        John Dias <joaodias@google.com>
References: <20180817182728.76129-1-smuckle@google.com>
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 <20180824093227.GN24124@hirez.programming.kicks-ass.net>
 <20180824094742.GJ24142@hirez.programming.kicks-ass.net>
From:   Steve Muckle <smuckle@google.com>
Message-ID: <ae94fb88-646b-1d82-b61d-103792965dbf@google.com>
Date:   Fri, 24 Aug 2018 14:24:48 -0700
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On 08/24/2018 02:47 AM, Peter Zijlstra wrote:
>>> On 08/17/2018 11:27 AM, Steve Muckle wrote:
> 
>>>> When rt_mutex_setprio changes a task's scheduling class to RT,
>>>> we're seeing cases where the task's vruntime is not updated
>>>> correctly upon return to the fair class.
> 
>>>> Specifically, the following is being observed:
>>>> - task is deactivated while still in the fair class
>>>> - task is boosted to RT via rt_mutex_setprio, which changes
>>>>     the task to RT and calls check_class_changed.
>>>> - check_class_changed leads to detach_task_cfs_rq, at which point
>>>>     the vruntime_normalized check sees that the task's state is TASK_WAKING,
>>>>     which results in skipping the subtraction of the rq's min_vruntime
>>>>     from the task's vruntime
>>>> - later, when the prio is deboosted and the task is moved back
>>>>     to the fair class, the fair rq's min_vruntime is added to
>>>>     the task's vruntime, even though it wasn't subtracted earlier.
> 
> I'm thinking that is an incomplete scenario; where do we get to
> TASK_WAKING.

Yes there's a missing bit of context here at the beginning that the task 
to be boosted had already been put into TASK_WAKING.