Re: sched/fair: Kernel panics in pick_next_entity

[Benjamin Segall <bsegall@google.com>]

Mike Galbraith <efault@gmx.de> writes:

> On Tue, 2024-10-01 at 00:45 +0530, Vishal Chourasia wrote:
>> >
>> for sanity, I ran the workload (kernel compilation) on the base commit
>> where the kernel panic was initially observed, which resulted in a
>> kernel panic, along with it couple of warnings where also printed on the
>> console, and a circular locking dependency warning with it.
>>
>> Kernel 6.11.0-kp-base-10547-g684a64bf32b6 on an ppc64le
>>
>> ------------[ cut here ]------------
>>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 6.11.0-kp-base-10547-g684a64bf32b6 #69 Not tainted
>> ------------------------------------------------------
>
> ...
>
>> --- interrupt: 900
>> se->sched_delayed
>> WARNING: CPU: 1 PID: 27867 at kernel/sched/fair.c:6062 unthrottle_cfs_rq+0x644/0x660
>
> ...that warning also spells eventual doom for the box, here it does
> anyway, running LTPs cfs_bandwidth01 testcase and hackbench together,
> box grinds to a halt in pretty short order.
>
> With the patchlet below (submitted), I can beat on box to my hearts
> content without meeting throttle/unthrottle woes.
>
> sched: Fix sched_delayed vs cfs_bandwidth
>
> Meeting an unfinished DELAY_DEQUEUE treated entity in unthrottle_cfs_rq()
> leads to a couple terminal scenarios.  Finish it first, so ENQUEUE_WAKEUP
> can proceed as it would have sans DELAY_DEQUEUE treatment.
>
> Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
> Reported-by: Venkat Rao Bagalkote <venkat88@linux.vnet.ibm.com>
> Tested-by: Venkat Rao Bagalkote <venkat88@linux.vnet.ibm.com>
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> ---
>  kernel/sched/fair.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6058,10 +6058,13 @@ void unthrottle_cfs_rq(struct cfs_rq *cf
>  	for_each_sched_entity(se) {
>  		struct cfs_rq *qcfs_rq = cfs_rq_of(se);
>
> -		if (se->on_rq) {
> -			SCHED_WARN_ON(se->sched_delayed);
> +		/* Handle any unfinished DELAY_DEQUEUE business first. */
> +		if (se->sched_delayed) {
> +			int flags = DEQUEUE_SLEEP | DEQUEUE_DELAYED;
> +
> +			dequeue_entity(qcfs_rq, se, flags);
> +		} else if (se->on_rq)
>  			break;
> -		}
>  		enqueue_entity(qcfs_rq, se, ENQUEUE_WAKEUP);
>
>  		if (cfs_rq_is_idle(group_cfs_rq(se)))

Yeah, if we can wind up here to hit that warning, then we need to get it
out of delay state, not just leave it. Whether dequeue_entity +
enqueue_entity is better or worse than requeue_delayed_entity (+ break), I really
don't know.

This did prompt me to try and figure out if we could limit delay to the
EEVDF parts. I think ideally, we'd only have delayed tasks accounted for
in min_vruntime/avg_vruntime/avg_load, but trying to keep them out of
nr_running/load.weight seems like it runs into too many issues, while
having them still counted there means skipping dequeue_entity like the
current version, which means it wouldn't have saved us this.

I think we could still probably remove the current core.c touchpoints in
favor of just having migrate_task_rq_fair take a parameter for "do we
already hold rq_lock rather than just p->pi_lock" (and it and
switched_from/changed_group handling things)? We'd probably need to do
something very careful to avoid migrate_task_rq_fair unconditionally
needing rq_lock though, which would be a pain, so it might not be worth
it even if I'm not forgetting some other reason for pushing the delay
knowledge all the way out to p->on_rq.