Re: [External] Re: [RFC PATCH 6/7] sched/fair: fix tasks_rcu with task based throttle

[Aaron Lu]

On Fri, Mar 14, 2025 at 09:44:45AM +0530, K Prateek Nayak wrote:
> Hello Aaron,
> 
> On 3/13/2025 12:52 PM, Aaron Lu wrote:
> > Taskes throttled on exit to user path are scheduled by cond_resched() in
> > task_work_run() but that is a preempt schedule and doesn't mark a task
> > rcu quiescent state.
> > 
> > Fix this by directly calling schedule() in throttle_cfs_rq_work().
> 
> Perhaps that can be gotten around by just using set_ti_thread_flag()
> resched_curr() will also call set_preempt_need_resched() which allows
> cond_resched() to resched the task.
> 
> Since exit_to_user_mode_loop() will run once again seeing that
> TIF_NEED_RESCHED is set, schedule() should follow soon. Thoughts?

Might also work, will give it a shot, thanks for the suggestion.

Best regards,
Aaron

> > 
> > Signed-off-by: Aaron Lu <ziqianlu@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index f26d53ac143fe..be96f7d32998c 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5847,6 +5847,7 @@ static void throttle_cfs_rq_work(struct
> > callback_head *work)
> >   	struct cfs_rq *cfs_rq;
> >   	struct rq *rq;
> >   	struct rq_flags rf;
> > +	bool sched = false;
> > 
> >   	WARN_ON_ONCE(p != current);
> >   	p->sched_throttle_work.next = &p->sched_throttle_work;
> > @@ -5879,9 +5880,13 @@ static void throttle_cfs_rq_work(struct
> > callback_head *work)
> >   	dequeue_task_fair(rq, p, DEQUEUE_SLEEP | DEQUEUE_SPECIAL);
> >   	list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
> >   	resched_curr(rq);
> > +	sched = true;
> > 
> >   out_unlock:
> >   	task_rq_unlock(rq, p, &rf);
> > +
> > +	if (sched)
> > +		schedule();
> >   }
> > 
> >   void init_cfs_throttle_work(struct task_struct *p)
> 
> -- 
> Thanks and Regards,
> Prateek
>