Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock

[Don Hiatt]

On Wed, Apr 28, 2021 at 3:57 AM Aubrey Li <aubrey.intel@xxxxxxxxx> wrote:
>
> On Wed, Apr 28, 2021 at 2:05 PM Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx> wrote:
> >
> > On 4/28/21 9:03 AM, Aubrey Li wrote:
> > > On Wed, Apr 28, 2021 at 7:36 AM Josh Don <joshdon@xxxxxxxxxx> wrote:
> > >>
> > >> On Tue, Apr 27, 2021 at 10:10 AM Don Hiatt <dhiatt@xxxxxxxxxxxxxxxx> wrote:
> > >>> Hi Josh and Peter,
> > >>>
> > >>> I've been running into soft lookups and hard lockups when running a script
> > >>> that just cycles setting the cookie of a group of processes over and over again.
> > >>>
> > >>> Unfortunately the only way I can reproduce this is by setting the cookies
> > >>> on qemu. I've tried sysbench, stress-ng but those seem to work just fine.
> > >>>
> > >>> I'm running Peter's branch and even tried the suggested changes here but
> > >>> still see the same behavior. I enabled panic on hard lockup and here below
> > >>> is a snippet of the log.
> > >>>
> > >>> Is there anything you'd like me to try or have any debugging you'd like me to
> > >>> do? I'd certainly like to get to the bottom of this.
> > >>
> > >> Hi Don,
> > >>
> > >> I tried to repro using qemu, but did not generate a lockup. Could you
> > >> provide more details on what your script is doing (or better yet,
> > >> share the script directly)? I would have expected you to potentially
> > >> hit a lockup if you were cycling sched_core being enabled and
> > >> disabled, but it sounds like you are just recreating the cookie for a
> > >> process group over and over?
> > >>
> > >
> > > I saw something similar on a bare metal hardware. Also tried the suggested
> > > patch here and no luck. Panic stack attached with
> > > softlockup_all_cpu_backtrace=1.
> > > (sorry, my system has 192 cpus and somehow putting 184 cpus offline causes
> > > system hang without any message...)
> >
> > Can you please try the following change to see if the problem is gone on your side?
> >
>
> Please ignore this patch, as the change of double_rq_unlock() in
> Peter's last patch
> fixed the problem.
>
> Thanks,
> -Aubrey
>
I'm still seeing hard lockups while repeatedly setting cookies on qemu
processes even with
the updated patch. If there is any debug you'd like me to turn on,
just let me know.

Thanks!

don



> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index f732642e3e09..1ef13b50dfcd 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -493,14 +493,17 @@ void double_rq_lock(struct rq *rq1, struct rq *rq2)
> >  {
> >         lockdep_assert_irqs_disabled();
> >
> > -       if (rq1->cpu > rq2->cpu)
> > -               swap(rq1, rq2);
> > -
> > -       raw_spin_rq_lock(rq1);
> > -       if (__rq_lockp(rq1) == __rq_lockp(rq2))
> > -               return;
> > -
> > -       raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
> > +       if (__rq_lockp(rq1) == __rq_lockp(rq2)) {
> > +               raw_spin_rq_lock(rq1);
> > +       } else {
> > +               if (__rq_lockp(rq1) < __rq_lockp(rq2)) {
> > +                       raw_spin_rq_lock(rq1);
> > +                       raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
> > +               } else {
> > +                       raw_spin_rq_lock(rq2);
> > +                       raw_spin_rq_lock_nested(rq1, SINGLE_DEPTH_NESTING);
> > +               }
> > +       }
> >  }
> >  #endif
> >