Re: [PATCH v7 01/22] sched: Favour predetermined active CPU as migration destination
From: Peter Zijlstra <peterz@infradead.org>
Date: 2021-05-26 12:33:23
Also in:
linux-arch, lkml
On Wed, May 26, 2021 at 12:14:20PM +0100, Valentin Schneider wrote:
On 25/05/21 16:14, Will Deacon wrote:
quoted
@@ -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.
quoted hunk ↗ jump to hunk
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
quoted hunk ↗ jump to hunk
@@ -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);
quoted hunk ↗ jump to hunk
@@ -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. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel