linux-kernel - Re: [PATCH] sched/fair: vruntime should normalize when switching from fair

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Date:   Fri, 24 Aug 2018 11:47:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Miguel de Dios <migueldedios@...gle.com>
Cc:     Steve Muckle <smuckle@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Todd Kjos <tkjos@...gle.com>, Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Patrick Bellasi <Patrick.Bellasi@....com>,
        Chris Redpath <Chris.Redpath@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        John Dias <joaodias@...gle.com>
Subject: Re: [PATCH] sched/fair: vruntime should normalize when switching
 from fair

> > 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.