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

[Juri Lelli <juri.lelli@gmail.com>]

On 23/08/18 18:52, Dietmar Eggemann wrote:
> 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@google.com>
> > > 
> > > 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@google.com>
> > > Signed-off-by: Steve Muckle <smuckle@google.com>
> > > ---
> > >   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.
> > 
> > Signed-off-by: Miguel de Dios <migueldedios@google.com>
> > Tested-by: Miguel de Dios <migueldedios@google.com>
> 
> 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?

Maybe one could play with rt-app to recreate such specific use case?

https://github.com/scheduler-tools/rt-app/blob/master/doc/tutorial.txt#L459

Best,

- Juri