On Wed, Feb 24, 2021 at 05:59:01PM +0000, Valentin Schneider wrote: > Your change reinstores the "triple SCA" pattern, where a stopper can run > with arg->pending && arg->pending != p->migration_pending, which I was > kinda happy to see go away...
Right, fair enough. Any workload that can tell the difference is doing it wrong anyway :-) OK, I've munged your two patches together into the below. --- Subject: sched: Simplify migration_cpu_stop() From: Valentin Schneider <valentin.schnei...@arm.com> Date: Thu Feb 25 10:22:30 CET 2021 Since, when ->stop_pending, only the stopper can uninstall p->migration_pending. This could simplify a few ifs, because: (pending != NULL) => (pending == p->migration_pending) Also, the fatty comment above affine_move_task() probably needs a bit of gardening. Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> --- kernel/sched/core.c | 27 ++++++++++++++++++--------- 1 file changed, 18 insertions(+), 9 deletions(-) --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1927,6 +1927,12 @@ static int migration_cpu_stop(void *data rq_lock(rq, &rf); /* + * If we were passed a pending, then ->stop_pending was set, thus + * p->migration_pending must have remained stable. + */ + WARN_ON_ONCE(pending && pending != p->migration_pending); + + /* * If task_rq(p) != rq, it cannot be migrated here, because we're * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because * we're holding p->pi_lock. @@ -1936,8 +1942,7 @@ static int migration_cpu_stop(void *data goto out; if (pending) { - if (p->migration_pending == pending) - p->migration_pending = NULL; + p->migration_pending = NULL; complete = true; } @@ -1976,8 +1981,7 @@ static int migration_cpu_stop(void *data * somewhere allowed, we're done. */ if (cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) { - if (p->migration_pending == pending) - p->migration_pending = NULL; + p->migration_pending = NULL; complete = true; goto out; } @@ -2165,16 +2169,21 @@ void do_set_cpus_allowed(struct task_str * * (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>