Re: sched_ext: Fix SCX_KICK_WAIT to work reliably

[Tejun Heo <tj@kernel.org>]

Hello, Andrea.

On Wed, Oct 22, 2025 at 09:43:25AM +0200, Andrea Righi wrote:
> > @@ -5208,12 +5214,11 @@ static void kick_cpus_irq_workfn(struct
> >  
> >  		if (cpu != cpu_of(this_rq)) {
> 
> It's probably fine anyway, but should we check for cpu_online(cpu) here?

This block gets activated iff kick_one_cpu() returns true and that is gated
by the CPU being online && the current task being on SCX. For the CPU to go
offline, that task has to go off CPU and thus increment the sequence
counter.

> >  			while (smp_load_acquire(wait_pnt_seq) == pseqs[cpu])
> >  				cpu_relax();
> 
> I'm wondering if we can break the semantic if cpu_rq(cpu)->curr->scx.slice
> is refilled concurrently between kick_one_cpu() and this busy wait. In this
> case we return, because wait_pnt_seq is incremented, but we keep running
> the same task.
> 
> Should we introduce a flag (or something similar) to force the re-enqueue
> of the prev task in this case?

Ah, right, that's a hole. There's another hole. The BPF scheduler can choose
to run the same task and put_prev_task_scx() won't be called. I think we
need to bump the seq count on entry to pick_task_scx() too. That should
solve both problems. All that we're guaranteeing is that we wait until the
task enters scheduling path. If a higher class task gets picked,
put_prev_task_scx() will be called. Otherwise, we break the wait when
pick_task_scx() is entered.

Thanks.

-- 
tejun