From 741ba80f6f9a4702089c122129f22df9774b3e64 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Sat, 16 Jan 2021 11:56:37 +0100 Subject: [PATCH] sched: Relax the set_cpus_allowed_ptr() semantics Now that we have KTHREAD_IS_PER_CPU to denote the critical per-cpu tasks to retain during CPU offline, we can relax the warning in set_cpus_allowed_ptr(). Any spurious kthread that wants to get on at the last minute will get pushed off before it can run. While during CPU online there is no harm, and actual benefit, to allowing kthreads back on early, it simplifies hotplug code and fixes a number of outstanding races. Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Lai jiangshan Reviewed-by: Valentin Schneider Tested-by: Valentin Schneider Link: https://lkml.kernel.org/r/20210121103507.240724591@infradead.org --- kernel/sched/core.c | 21 ++++++++++----------- 1 file changed, 10 insertions(+), 11 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 56b09628692a..ff74fca39ed2 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2342,7 +2342,9 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, if (p->flags & PF_KTHREAD || is_migration_disabled(p)) { /* - * Kernel threads are allowed on online && !active CPUs. + * Kernel threads are allowed on online && !active CPUs, + * however, during cpu-hot-unplug, even these might get pushed + * away if not KTHREAD_IS_PER_CPU. * * Specifically, migration_disabled() tasks must not fail the * cpumask_any_and_distribute() pick below, esp. so on @@ -2386,16 +2388,6 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, __do_set_cpus_allowed(p, new_mask, flags); - if (p->flags & PF_KTHREAD) { - /* - * For kernel threads that do indeed end up on online && - * !active we want to ensure they are strict per-CPU threads. - */ - WARN_ON(cpumask_intersects(new_mask, cpu_online_mask) && - !cpumask_intersects(new_mask, cpu_active_mask) && - p->nr_cpus_allowed != 1); - } - return affine_move_task(rq, p, &rf, dest_cpu, flags); out: @@ -7518,6 +7510,13 @@ int sched_cpu_deactivate(unsigned int cpu) int ret; set_cpu_active(cpu, false); + + /* + * From this point forward, this CPU will refuse to run any task that + * is not: migrate_disable() or KTHREAD_IS_PER_CPU, and will actively + * push those tasks away until this gets cleared, see + * sched_cpu_dying(). + */ balance_push_set(cpu, true); /*