Messages in this thread | | | From | Aubrey Li <> | Date | Fri, 30 Apr 2021 16:20:21 +0800 | Subject | Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock |
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On Fri, Apr 30, 2021 at 4:40 AM Josh Don <joshdon@google.com> wrote: > > 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()
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
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