Re: [PATCH 19/17] sched_ext: Fix the stall watchdog blaming a bypassing owner for a stuck task

[Andrea Righi]

Hi Tejun,

On Fri, Oct 23, 2026 at 10:06:40AM -1000, Tejun Heo wrote:
> check_rq_for_timeouts() blames a stall on the scheduler whose DSQ the task
> sits on, and on the task's owner when the DSQ has none. A bypassed task can
> sit on the rescue DSQ or the local DSQ, and the blame then lands on its
> owner. The owner is bypassing and places nothing. The host, the nearest
> ancestor that is not bypassing, runs the bypassed tasks.
> 
> That matters when the owner is already exiting, during its own disable for
> one. __scx_exit() on it is a no-op, the watchdog still ends its pass, and
> nothing is ejected. If the host is not running the bypassed tasks, the
> owner's disable, which waits on them, may never finish.
> 
> Blame the host for a stuck task whose owner is bypassing. Ejecting the host
> re-hosts the bypassed subtree one level up. A task whose owner is not
> bypassing keeps the blame on the owner, and with every ancestor bypassing,
> scx_exec_sched() gives the root. A task on the host's bypass DSQ already led
> to the host through dsq->sched.
> 
> The rescue overload accounting keeps charging the owner. Overload going
> unanswered while the owner exits is acceptable.
> 
> Fixes: 2c91169377f4 ("sched_ext: Blame the DSQ's owning scheduler for a runnable stall")
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
>  kernel/sched/ext/ext.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 8241cf374ef9..ae5e8861ca0a 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -4376,9 +4376,16 @@ static bool check_rq_for_timeouts(struct rq *rq)
>  			 * ancestor's bypass DSQ while the owner is bypassing.
>  			 * Blame the drainer. The local DSQ is consumed by the
>  			 * cpu itself and keeps blame on the owner.
> +			 *
> +			 * A bypassing owner places nothing, and its tasks run
> +			 * through the host even from the rescue DSQ or the
> +			 * local DSQ. Blame the host. Blaming an owner that is
> +			 * already exiting would eject nothing.
>  			 */
>  			if (dsq && dsq->sched && dsq->id != SCX_DSQ_LOCAL)
>  				sch = dsq->sched;
> +			else if (scx_bypassing(sch, cpu_of(rq)))
> +				sch = scx_exec_sched(sch, cpu_of(rq));

Should we recheck watchdog_timeout after changing sch?

The stall test above uses the task owner's timeout. So, a bypassing
child with a short timeout could potentially cause its host to be
ejected before the host's own timeout expires.

Thanks,
-Andrea

>  
>  			__scx_exit(sch, SCX_EXIT_ERROR_STALL, 0, cpu_of(rq),
>  				   "%s[%d] failed to run for %u.%03us",
> -- 
> 2.56.0
>