Messages in this thread | | | Date | Tue, 10 May 2022 18:37:07 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 17/22] sched/core: fix opencoded cpumask_any_but() |
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On Tue, May 10, 2022 at 08:47:45AM -0700, Yury Norov wrote: > sched_core_cpu_starting() and sched_core_cpu_deactivate() implement > opencoded cpumask_any_but(). Fix it. > > CC: Ben Segall <bsegall@google.com> > CC: Daniel Bristot de Oliveira <bristot@redhat.com> > CC: Dietmar Eggemann <dietmar.eggemann@arm.com> > CC: Ingo Molnar <mingo@redhat.com> > CC: Juri Lelli <juri.lelli@redhat.com> > CC: Mel Gorman <mgorman@suse.de> > CC: Peter Zijlstra <peterz@infradead.org> > CC: Steven Rostedt <rostedt@goodmis.org> > CC: Valentin Schneider <vschneid@redhat.com> > CC: Vincent Guittot <vincent.guittot@linaro.org> > CC: linux-kernel@vger.kernel.org > Signed-off-by: Yury Norov <yury.norov@gmail.com> > --- > kernel/sched/core.c | 33 +++++++++++++-------------------- > 1 file changed, 13 insertions(+), 20 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index f5ebc392493d..9700001948d0 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -6125,7 +6125,7 @@ static void queue_core_balance(struct rq *rq) > static void sched_core_cpu_starting(unsigned int cpu) > { > const struct cpumask *smt_mask = cpu_smt_mask(cpu); > - struct rq *rq = cpu_rq(cpu), *core_rq = NULL; > + struct rq *rq = cpu_rq(cpu), *core_rq; > unsigned long flags; > int t; > > @@ -6138,19 +6138,16 @@ static void sched_core_cpu_starting(unsigned int cpu) > goto unlock; > > /* find the leader */ > - for_each_cpu(t, smt_mask) { > - if (t == cpu) > - continue; > - rq = cpu_rq(t); > - if (rq->core == rq) { > - core_rq = rq; > - break; > - } > - } > + t = cpumask_any_but(smt_mask, cpu); > + if (t >= nr_cpu_ids) > + goto unlock; > > - if (WARN_ON_ONCE(!core_rq)) /* whoopsie */ > + rq = cpu_rq(t); > + if (WARN_ON_ONCE(rq->core != rq)) /* whoopsie */ > goto unlock; > > + core_rq = rq; > + > /* install and validate core_rq */ > for_each_cpu(t, smt_mask) { > rq = cpu_rq(t);
I don't think this is equivalent. Imagine SMT4, with:
rqN->core_rq = rq0
Now, further suppose smt0-2 are online and we're about to online smt3. Then t above is free to be smt2, which then results in insta triggering:
+ if (WARN_ON_ONCE(rq->core != rq)) /* whoopsie */
You seem to have lost how the first loop searches for rq->core.
Please, be more careful. Also, all of this is super cold path don't bother with optimizations. Much of the patches you have in this series fall under that.
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