Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | Re: [PATCH 6/6] sched: Simplify set_affinity_pending refcounts | Date | Wed, 24 Feb 2021 15:34:49 +0000 |
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On 24/02/21 13:24, Peter Zijlstra wrote: > Now that we have set_affinity_pending::stop_pending to indicate if a > stopper is in progress, and we have the guarantee that if that stopper > exists, it will (eventually) complete our @pending we can simplify the > refcount scheme by no longer counting the stopper thread. > > Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()") > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > @@ -2199,12 +2199,16 @@ static int affine_move_task(struct rq *r > push_task = get_task_struct(p); > } > > + /* > + * If there are pending waiters, but no pending stop_work, > + * then complete now. > + */ > pending = p->migration_pending; > - if (pending) { > - refcount_inc(&pending->refs); > + if (pending && !pending->stop_pending) { > p->migration_pending = NULL; > complete = true; > } > + > task_rq_unlock(rq, p, rf); > > if (push_task) { > @@ -2213,7 +2217,7 @@ static int affine_move_task(struct rq *r > } > > if (complete) > - goto do_complete; > + complete_all(&pending->done);
We could've done this in the first place, right? I don't think this path actually needed to deal with the refcounts (at least not since we started counting the stoppers).
Musings aside, I believe the above means, for migration_cpu_stop():
(pending != NULL) => (pending == p->migration_pending)
Since, when ->stop_pending, only the stopper can uninstall p->migration_pending. This could simplify a few if's.
Also, the fatty comment above affine_move_task() probably needs a bit of gardening:
--- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 9492f8eb242a..6f649aa2795c 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2165,16 +2165,21 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask) * * (1) In the cases covered above. There is one more where the completion is * signaled within affine_move_task() itself: when a subsequent affinity request - * cancels the need for an active migration. Consider: + * occurs after the stopper bailed out due to the targeted task still being + * Migrate-Disable. Consider: * * Initial conditions: P0->cpus_mask = [0, 1] * - * P0@CPU0 P1 P2 - * - * migrate_disable(); - * <preempted> + * CPU0 P1 P2 + * <P0> + * migrate_disable(); + * <preempted> * set_cpus_allowed_ptr(P0, [1]); * <blocks> + * <migration/0> + * migration_cpu_stop() + * is_migration_disabled() + * <bails> * set_cpus_allowed_ptr(P0, [0, 1]); * <signal completion> * <awakes>
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