Re: 回复: [PATCH] sched_ext: Fix NULL pointer dereferences in put_prev_task_scx

[Andrea Righi]

Hi liuwenfang,

On Tue, Jun 10, 2025 at 06:22:22AM +0000, liuwenfang wrote:
> Thanks for your feedback.
> This is triggered in kernel modules developed In the mobile scenario: 
> tasks on rq are migrated while the current cpu should be halted for power saving policy.
> Its migration logic:
> drain_rq_cpu_stop -- migrate_all_tasks :
> for (;;) {
> 	if (rq->nr_running == 1)
> 		break;
> 	for_each_class(class) {
> 		next = class->pick_next_task(rq);
> 		if (next) {
> 			next->sched_class->put_prev_task(rq, next, NULL);

Can you do something like this instead?

  next->sched_class->put_prev_task(rq, next, next);

this should give you the same behavior you're relying on, without requiring
the extra check in ext.c.

> 			break;
> 		}
> 	}
> 	if (is_idle_task(next))
> 		break;
> 	dest_cpu = select_task_rq(next...);
> 	move_queued_task(rq, rf, next, dest_cpu);
> 	...
> }
> 
> put_prev_task in this function is selected to update util and statistics info for each runnable tasks,
> here they are not dequeued yet.

I see, so it's an optimization/workaround to update utilization info
without fully dequeuing the tasks. Is this out-of-tree code, I guess?

When you say the CPU is halted, we're not talking about CPU hotplugging,
right? We're just migrating tasks off the CPU?

Also, if you're running sched_ext there are ways to exclude certain CPUs at
the scheduler's level via ops.select_cpu() / ops.dispatch(). Do you think
this could be a viable solution for your specific use case?

Thanks,
-Andrea

PS https://subspace.kernel.org/etiquette.html#do-not-top-post-when-replying

> 
> Best regards,
>  
> > On Mon, Jun 09, 2025 at 11:36:15AM +0000, liuwenfang wrote:
> > > As put_prev_task can be used in other kernel modules which can lead to
> > > a NULL pointer. Fix this by checking for a valid next.
> > 
> > Actually, put_prev_task() should be used only within kernel/sched/ and, in theory,
> > you should have done a dequeue_task() before put_prev_task() in this scenario,
> > so SCX_TASK_QUEUED shouldn't be set in p->scx.flags.
> > 
> > The change might still make sense, but can you clarify how you triggered the
> > NULL pointer dereference?
> > 
> > Thanks,
> > -Andrea
> > 
> > >
> > > Signed-off-by: l00013971 <l00013971@xxxxxxxxxxx>
> > > ---
> > >  kernel/sched/ext.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index
> > > f5133249f..6a579babd 100644
> > > --- a/kernel/sched/ext.c
> > > +++ b/kernel/sched/ext.c
> > > @@ -3262,7 +3262,7 @@ static void put_prev_task_scx(struct rq *rq, struct
> > task_struct *p,
> > >  		 * ops.enqueue() that @p is the only one available for this cpu,
> > >  		 * which should trigger an explicit follow-up scheduling event.
> > >  		 */
> > > -		if (sched_class_above(&ext_sched_class, next->sched_class)) {
> > > +		if (next && sched_class_above(&ext_sched_class, next->sched_class))
> > > +{
> > >  			WARN_ON_ONCE(!static_branch_unlikely(&scx_ops_enq_last));
> > >  			do_enqueue_task(rq, p, SCX_ENQ_LAST, -1);
> > >  		} else {
> > > --
> > > 2.17.1