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

[Dietmar Eggemann]

Hi,

On 08/21/2018 01:54 AM, 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
> > ÂÂ the task to RT and calls check_class_changed.
> > - check_class_changed leads to detach_task_cfs_rq, at which point
> > ÂÂ the vruntime_normalized check sees that the task's state is 
> > TASK_WAKING,
> > ÂÂ which results in skipping the subtraction of the rq's min_vruntime
> > ÂÂ from the task's vruntime
> > - later, when the prio is deboosted and the task is moved back
> > ÂÂ to the fair class, the fair rq's min_vruntime is added to
> > ÂÂ 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>
> > ---
> > Â kernel/sched/fair.c | 3 ++-
> > Â 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)
> > ÂÂÂÂÂÂ * - A task which has been woken up by try_to_wake_up() and
> > ÂÂÂÂÂÂ *ÂÂ waiting for actually being woken up by sched_ttwu_pending().
> > ÂÂÂÂÂÂ */
> > -ÂÂÂ if (!se->sum_exec_runtime || p->state == TASK_WAKING)
> > +ÂÂÂ if (!se->sum_exec_runtime ||
> > +ÂÂÂÂÂÂÂ (p->state == TASK_WAKING && p->sched_class == 
> > &fair_sched_class))
> > ÂÂÂÂÂÂÂÂÂ return true;
> > ÂÂÂÂÂ 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.
>
> Signed-off-by: Miguel de Dios <migueldedios@xxxxxxxxxx>
> Tested-by: Miguel de Dios <migueldedios@xxxxxxxxxx>

I tried to catch this issue on my Arm64 Juno board using pi_test (and a 
slightly adapted pip_test (usleep_val = 1500 and keep low as cfs)) from 
rt-tests but wasn't able to do so.

# pi_stress --inversions=1 --duration=1 --groups=1 --sched id=low,policy=cfs

Starting PI Stress Test
Number of thread groups: 1
Duration of test run: 1 seconds
Number of inversions per group: 1
     Admin thread SCHED_FIFO priority 4
1 groups of 3 threads will be created
      High thread SCHED_FIFO priority 3
       Med thread SCHED_FIFO priority 2
       Low thread SCHED_OTHER nice 0

# ./pip_stress

In both cases, the cfs task entering  rt_mutex_setprio() is queued, so 
dequeue_task_fair()->dequeue_entity(), which subtracts 
cfs_rq->min_vruntime from se->vruntime, is called on it before it gets 
the rt prio.

Maybe it requires a very specific use of the pthread library to provoke 
this issue by making sure that the cfs tasks really blocks/sleeps?