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

[Josh Don <joshdon@google.com>]

On Thu, Apr 29, 2021 at 1:03 AM Aubrey Li <aubrey.intel@gmail.com> wrote:
>
> On Thu, Apr 22, 2021 at 8:39 PM Peter Zijlstra <peterz@infradead.org> wrote:
> ----snip----
> > @@ -199,6 +224,25 @@ void raw_spin_rq_unlock(struct rq *rq)
> >         raw_spin_unlock(rq_lockp(rq));
> >  }
> >
> > +#ifdef CONFIG_SMP
> > +/*
> > + * double_rq_lock - safely lock two runqueues
> > + */
> > +void double_rq_lock(struct rq *rq1, struct rq *rq2)
> > +{
> > +       lockdep_assert_irqs_disabled();
> > +
> > +       if (rq1->cpu > rq2->cpu)
>
> It's still a bit hard for me to digest this function, I guess using (rq->cpu)
> can't guarantee the sequence of locking when coresched is enabled.
>
> - cpu1 and cpu7 shares lockA
> - cpu2 and cpu8 shares lockB
>
> double_rq_lock(1,8) leads to lock(A) and lock(B)
> double_rq_lock(7,2) leads to lock(B) and lock(A)
>
> change to below to avoid ABBA?
> +       if (__rq_lockp(rq1) > __rq_lockp(rq2))
>
> Please correct me if I was wrong.

Great catch Aubrey. This is possibly what is causing the lockups that
Don is seeing.

The proposed usage of __rq_lockp() is prone to race with sched core
being enabled/disabled. It also won't order properly if we do
double_rq_lock(smt0, smt1) vs double_rq_lock(smt1, smt0), since these
would have equivalent __rq_lockp(). I'd propose an alternative but
similar idea: order by core, then break ties by ordering on cpu.

+#ifdef CONFIG_SCHED_CORE
+       if (rq1->core->cpu > rq2->core->cpu)
+               swap(rq1, rq2);
+       else if (rq1->core->cpu == rq2->core->cpu && rq1->cpu > rq2->cpu)
+               swap(rq1, rq2);
+#else
        if (rq1->cpu > rq2->cpu)
                swap(rq1, rq2);
+#endif