Re: [RFC PATCH 2/7] sched/fair: Handle throttle path for task based throttle

[K Prateek Nayak]

Hello Aaron,

On 3/14/2025 2:27 PM, Aaron Lu wrote:
> > > +static inline bool task_has_throttle_work(struct task_struct *p)
> > > +{
> > > +	return p->sched_throttle_work.next != &p->sched_throttle_work;
> > > +}
> > > +
> > > +static inline void task_throttle_setup_work(struct task_struct *p)
> > > +{
> > > +	/*
> > > +	 * Kthreads and exiting tasks don't return to userspace, so adding the
> > > +	 * work is pointless
> > > +	 */
> > > +	if ((p->flags & (PF_EXITING | PF_KTHREAD)))
> > > +		return;
> > > +
> > > +	if (task_has_throttle_work(p))
> > > +		return;
> > > +
> > > +	task_work_add(p, &p->sched_throttle_work, TWA_RESUME);
> > > +}
> > > +
> > >    static int tg_throttle_down(struct task_group *tg, void *data)
> > >    {
> > >    	struct rq *rq = data;
> > >    	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> > > +	struct task_struct *p;
> > > +	struct rb_node *node;
> > > +
> > > +	cfs_rq->throttle_count++;
> > > +	if (cfs_rq->throttle_count > 1)
> > > +		return 0;
> >
> > General question: Do we need the throttled_lb_pair() check in
> > can_migrate_task() with the per-task throttle? Moving a throttled task
> > to another CPU can ensures that the task can run quicker and exit to
> > user space as quickly as possible and once the task dequeues, it will
> > remove itself from the list of fair tasks making it unreachable for
> > the load balancer. Thoughts?
>
> That's a good point.
>
> The current approach dequeued the task and removed it from rq's
> cfs_tasks list, causing it lose the load balance opportunity. This is
> pretty sad.

That is fine. Today we have the throttled_lb_pair() check since tasks
on throttled hierarchy remain on the fair tasks list and the load
balancer should not move the around since they don't contribute to
current load in throttled state and moving them around will not change
anything since they'll still be waiting on another CPU for unthrottle.

With per-task throttle, we know that all the tasks on the fair task
list are runnable (except for the delayed ones but they contribute to
the load) - tasks on throttled hierarchy that exit to userspace will
dequeue themselves, removing them from the fair task list too.

Since a task that hasn't dequeued itself on a throttled hierarchy is
runnable, I'm suggesting we get rid of the throttled_lb_pair() check
in can_migrate_task() entirely.

> I'll need to think about this. I hope we can somehow keep the throttled
> tasks in cfs_tasks list, I'll see how to make this happen.
>
> Thanks,
> Aaron

[..snip..]

--
Thanks and Regards,
Prateek