Re: sched/fair: Kernel panics in pick_next_entity

[Vishal Chourasia <vishalc@linux.ibm.com>]

On Tue, Oct 01, 2024 at 10:30:26AM +0200, Mike Galbraith wrote:
> 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>
Hello Mike, Thank you the patch!

With the below changes, I don't see any warnings been printed on the
console, along with it, there were no kernel panics

Tested-by: Vishal Chourasia <vishalc@linux.ibm.com>
> ---
>  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)))
>