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Date:   Fri, 24 Aug 2018 11:47:42 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Miguel de Dios <migueldedios@google.com>
Cc:     Steve Muckle <smuckle@google.com>, 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>
Subject: Re: [PATCH] sched/fair: vruntime should normalize when switching
 from fair
Message-ID: <20180824094742.GJ24142@hirez.programming.kicks-ass.net>
References: <20180817182728.76129-1-smuckle@google.com>
 <dcd2fa21-7fef-d648-017d-9d42d11793e0@google.com>
 <20180824093227.GN24124@hirez.programming.kicks-ass.net>
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> > 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.