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Wysocki" , Dietmar Eggemann , Daniel Bristot de Oliveira , kernel-team@android.com Subject: Re: [PATCH v7 01/22] sched: Favour predetermined active CPU as migration destination Message-ID: References: <20210525151432.16875-1-will@kernel.org> <20210525151432.16875-2-will@kernel.org> <877djlhhmb.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <877djlhhmb.mognet@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Wed, May 26, 2021 at 12:14:20PM +0100, Valentin Schneider wrote: > On 25/05/21 16:14, Will Deacon wrote: > > @@ -1956,12 +1958,8 @@ static int migration_cpu_stop(void *data) > > complete = true; > > } > > > > - if (dest_cpu < 0) { > > - if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask)) > > - goto out; > > - > > - dest_cpu = cpumask_any_distribute(&p->cpus_mask); > > - } > > + if (dest_mask && (cpumask_test_cpu(task_cpu(p), dest_mask))) > > + goto out; > > > > IIRC the reason we deferred the pick to migration_cpu_stop() was because of > those insane races involving multiple SCA calls the likes of: > > p->cpus_mask = [0, 1]; p on CPU0 > > CPUx CPUy CPU0 > > SCA(p, [2]) > __do_set_cpus_allowed(); > queue migration_cpu_stop() > SCA(p, [3]) > __do_set_cpus_allowed(); > migration_cpu_stop() > > The stopper needs to use the latest cpumask set by the second SCA despite > having an arg->pending set up by the first SCA. Doesn't this break here? Yep. > I'm not sure I've paged back in all of the subtleties laying in ambush > here, but what about the below? > > --- > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 5226cc26a095..cd447c9db61d 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -1954,19 +1953,15 @@ static int migration_cpu_stop(void *data) > if (pending) { > p->migration_pending = NULL; > complete = true; > > if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask)) > goto out; > } > > if (task_on_rq_queued(p)) > + rq = __migrate_task(rq, &rf, p, arg->dest_cpu); > @@ -2249,7 +2244,7 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag > init_completion(&my_pending.done); > my_pending.arg = (struct migration_arg) { > .task = p, > + .dest_cpu = dest_cpu, > .pending = &my_pending, > }; > > @@ -2257,6 +2252,7 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag > } else { > pending = p->migration_pending; > refcount_inc(&pending->refs); > + pending->arg.dest_cpu = dest_cpu; > } > } Argh.. that might just work. But I'm thinking we wants comments this time around :-) This is even more subtle.