Re: [PATCH] sched/pi: Reweight fair_policy() tasks when inheriting prio

[Qais Yousef]

On 04/03/24 09:42, Steven Rostedt wrote:
> On Wed, 3 Apr 2024 15:11:06 +0200
> Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> 
> > On Wed, 3 Apr 2024 at 02:59, Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
> > >
> > > For fair tasks inheriting the priority (nice) without reweighting is
> > > a NOP as the task's share won't change.  
> > 
> > AFAICT, there is no nice priority inheritance with rt_mutex; All nice
> > tasks are sorted with the same "default prio" in the rb waiter tree.
> > This means that the rt top waiter is not the cfs with highest prio but
> > the 1st cfs waiting for the mutex.
> 
> I think the issue here is that the running process doesn't update its
> weight and if there are other tasks that are not contending on this mutex,
> they can still starve the lock owner.
> 
> IIUC (it's been ages since I looked at the code), high nice values (low
> priority) turn to at lease nice 0 when they are "boosted". It doesn't
> improve their chances of getting the lock though.

The main issue is that if this low nice value is holding the lock and the cpus
are busy, it can take a long time to release the lock.

In today's systems the amount of waiting time that use cases 'permitted' (for
lack of better word) keeps shrinking.

And the way userspace is coded these days with all these layers, app writers
might not be aware there's this dependency where a low priority task might
contend for a lock required by something important. Being able to rely on
PTHREAD_PRIO_INHERIT to ensure they get boosted is useful to protect against
a thread that has a low priority inadvertently holds this lock from causing
massive delays to waiters as it might not get enough RUNNING time to release
it.

To my knowledge windows has some sort of boost mechanism (that maybe Joel knows
more about than me) for the lock holder to help release the lock faster that
app developers rely on to fix similar issues.

> 
> > 
> > >
> > > This is visible when running with PTHREAD_PRIO_INHERIT where fair tasks
> > > with low priority values are susceptible to starvation leading to PI
> > > like impact on lock contention.
> > >
> > > The logic in rt_mutex will reset these low priority fair tasks into nice
> > > 0, but without the additional reweight operation to actually update the
> > > weights, it doesn't have the desired impact of boosting them to allow
> > > them to run sooner/longer to release the lock.
> > >
> > > Apply the reweight for fair_policy() tasks to achieve the desired boost
> > > for those low nice values tasks. Note that boost here means resetting
> > > their nice to 0; as this is what the current logic does for fair tasks.  
> > 
> > But you can at the opposite decrease the cfs prio of a task
> > and even worse with the comment :
> > /* XXX used to be waiter->prio, not waiter->task->prio */
> > 
> > we use the prio of the top cfs waiter (ie the one waiting for the
> > lock) not the default 0 so it can be anything in the range [-20:19]
> > 
> > Then, a task with low prio (i.e. nice > 0) can get a prio boost even
> > if this task and the waiter are low priority tasks
> 
> 
> Yeah, I'm all confused to exactly how the inheritance works with
> SCHED_OTHER. I know John Stultz worked on this for a bit recently. He's
> Cc'ed. But may not be paying attention ;-)
> 
> -- Steve