Messages in this thread | | | From | Aubrey Li <> | Date | Fri, 30 Apr 2021 22:15:37 +0800 | Subject | Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock |
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On Fri, Apr 30, 2021 at 4:48 PM Josh Don <joshdon@google.com> wrote: > > On Fri, Apr 30, 2021 at 1:20 AM Aubrey Li <aubrey.intel@gmail.com> wrote: > > > > 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? > > If there is a concurrent switch from sched_core enable <-> disable, > the value of __rq_lockp() will race. > > In the version you posted directly above, where we swap rq1 and rq2 if > __rq_lockp(rq1) > __rqlockp(rq2) rather than comparing the cpu, the > following can happen: > > cpu 1 and cpu 7 share a core lock when coresched is enabled > > - schedcore enabled > - double_lock(7, 1) > - __rq_lockp compares equal for 7 and 1; no swap is done > - schedcore disabled; now __rq_lockp returns the per-rq lock > - lock(__rq_lockp(7)) => lock(7) > - lock(__rq_lockp(1)) => lock(1) > > Then we can also have > > - schedcore disabled > - double_lock(1, 7) > - __rq_lock(1) < rq_lock(7), so no swap > - lock(__rqlockp(1)) => lock(1) > - lock(__rq_lockp(7)) => lock(7) > > So we have in the first 7->1 and in the second 1->7 > > > > > .> 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? > > Ensuring that core siblings always take their locks in the same order > if coresched is disabled. >
Both this and above make sense to me. Thanks for the great elaboration, Josh!
Thanks, -Aubrey
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