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

[Aubrey Li]

On Fri, Apr 30, 2021 at 4:40 AM Josh Don <joshdon@xxxxxxxxxx> wrote:
>
> On Thu, Apr 29, 2021 at 1:03 AM Aubrey Li <aubrey.intel@xxxxxxxxx> wrote:
> >
> > On Thu, Apr 22, 2021 at 8:39 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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()

If __rq_lockp(smt0) == __rq_lockp(smt1), rq0 and rq1 won't swap,
Later only one rq is locked and just returns. I'm not sure how does it not
order properly?

.> 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);

That is, why the "else if" branch is needed?

> +#else
>         if (rq1->cpu > rq2->cpu)
>                 swap(rq1, rq2);
> +#endif