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

[Peter Zijlstra]

On Mon, Aug 20, 2018 at 04:54:25PM -0700, Miguel de Dios wrote:
> On 08/17/2018 11:27 AM, Steve Muckle wrote:
> > From: John Dias <joaodias@xxxxxxxxxx>
> > 
> > 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.
> > The immediate result is inflation of the task's vruntime, giving
> > it lower priority (starving it if there's enough available work).
> > The longer-term effect is inflation of all vruntimes because the
> > task's vruntime becomes the rq's min_vruntime when the higher
> > priority tasks go idle. That leads to a vicious cycle, where
> > the vruntime inflation repeatedly doubled.
> > 
> > The change here is to detect when vruntime_normalized is being
> > called when the task is waking but is waking in another class,
> > and to conclude that this is a case where vruntime has not
> > been normalized.
> > 
> > Signed-off-by: John Dias <joaodias@xxxxxxxxxx>
> > Signed-off-by: Steve Muckle <smuckle@xxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index b39fb596f6c1..14011d7929d8 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9638,7 +9638,8 @@ static inline bool vruntime_normalized(struct task_struct *p)
> >   	 * - A task which has been woken up by try_to_wake_up() and
> >   	 *   waiting for actually being woken up by sched_ttwu_pending().
> >   	 */
> > -	if (!se->sum_exec_runtime || p->state == TASK_WAKING)
> > +	if (!se->sum_exec_runtime ||
> > +	    (p->state == TASK_WAKING && p->sched_class == &fair_sched_class))
> >   		return true;
> >   	return false;

> The normalization of vruntime used to exist in task_waking but it was
> removed and the normalization was moved into migrate_task_rq_fair. The
> reasoning being that task_waking_fair was only hit when a task is queued
> onto a different core and migrate_task_rq_fair should do the same work.
> 
> However, we're finding that there's one case which migrate_task_rq_fair
> doesn't hit: that being the case where rt_mutex_setprio changes a task's
> scheduling class to RT when its scheduled out. The task never hits
> migrate_task_rq_fair because it is switched to RT and migrates as an RT
> task. Because of this we're getting an unbounded addition of min_vruntime
> when the task is re-attached to the CFS runqueue when it loses the inherited
> priority. The patch above works because now the kernel specifically checks
> for this case and normalizes accordingly.
> 
> Here's the patch I was talking about:
> https://lore.kernel.org/patchwork/patch/677689/. In our testing we were
> seeing vruntimes nearly double every time after rt_mutex_setprio boosts the
> task to RT.

Bah, patchwork is such shit... how do you get to the previus patch from
there? Because I think 2/3 is the actual commit that changed things, 3/3
just cleans up a bit.

That would be commit:

  b5179ac70de8 ("sched/fair: Prepare to fix fairness problems on migration")

But I'm still somewhat confused; how would task_waking_fair() have
helped if we're already changed to a different class?