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authorPeter Zijlstra <peterz@infradead.org>2023-10-10 21:57:39 +0300
committerPeter Zijlstra <peterz@infradead.org>2023-10-13 10:56:29 +0300
commitf0498d2a54e7966ce23cd7c7ff42c64fa0059b07 (patch)
tree937edf90a458c4586d5164f739c00c9e34c55273 /kernel/sched/core.c
parent0c2924079f5a83ed715630680e338b3685a0bf7d (diff)
downloadlinux-f0498d2a54e7966ce23cd7c7ff42c64fa0059b07.tar.xz
sched: Fix stop_one_cpu_nowait() vs hotplug
Kuyo reported sporadic failures on a sched_setaffinity() vs CPU hotplug stress-test -- notably affine_move_task() remains stuck in wait_for_completion(), leading to a hung-task detector warning. Specifically, it was reported that stop_one_cpu_nowait(.fn = migration_cpu_stop) returns false -- this stopper is responsible for the matching complete(). The race scenario is: CPU0 CPU1 // doing _cpu_down() __set_cpus_allowed_ptr() task_rq_lock(); takedown_cpu() stop_machine_cpuslocked(take_cpu_down..) <PREEMPT: cpu_stopper_thread() MULTI_STOP_PREPARE ... __set_cpus_allowed_ptr_locked() affine_move_task() task_rq_unlock(); <PREEMPT: cpu_stopper_thread()\> ack_state() MULTI_STOP_RUN take_cpu_down() __cpu_disable(); stop_machine_park(); stopper->enabled = false; /> /> stop_one_cpu_nowait(.fn = migration_cpu_stop); if (stopper->enabled) // false!!! That is, by doing stop_one_cpu_nowait() after dropping rq-lock, the stopper thread gets a chance to preempt and allows the cpu-down for the target CPU to complete. OTOH, since stop_one_cpu_nowait() / cpu_stop_queue_work() needs to issue a wakeup, it must not be ran under the scheduler locks. Solve this apparent contradiction by keeping preemption disabled over the unlock + queue_stopper combination: preempt_disable(); task_rq_unlock(...); if (!stop_pending) stop_one_cpu_nowait(...) preempt_enable(); This respects the lock ordering contraints while still avoiding the above race. That is, if we find the CPU is online under rq-lock, the targeted stop_one_cpu_nowait() must succeed. Apply this pattern to all similar stop_one_cpu_nowait() invocations. Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()") Reported-by: "Kuyo Chang (張建文)" <Kuyo.Chang@mediatek.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: "Kuyo Chang (張建文)" <Kuyo.Chang@mediatek.com> Link: https://lkml.kernel.org/r/20231010200442.GA16515@noisy.programming.kicks-ass.net
Diffstat (limited to 'kernel/sched/core.c')
-rw-r--r--kernel/sched/core.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a3f9cd52eec5..264c2eb380d7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2645,9 +2645,11 @@ static int migration_cpu_stop(void *data)
* it.
*/
WARN_ON_ONCE(!pending->stop_pending);
+ preempt_disable();
task_rq_unlock(rq, p, &rf);
stop_one_cpu_nowait(task_cpu(p), migration_cpu_stop,
&pending->arg, &pending->stop_work);
+ preempt_enable();
return 0;
}
out:
@@ -2967,12 +2969,13 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
complete = true;
}
+ preempt_disable();
task_rq_unlock(rq, p, rf);
-
if (push_task) {
stop_one_cpu_nowait(rq->cpu, push_cpu_stop,
p, &rq->push_work);
}
+ preempt_enable();
if (complete)
complete_all(&pending->done);
@@ -3038,12 +3041,13 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
if (flags & SCA_MIGRATE_ENABLE)
p->migration_flags &= ~MDF_PUSH;
+ preempt_disable();
task_rq_unlock(rq, p, rf);
-
if (!stop_pending) {
stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
&pending->arg, &pending->stop_work);
}
+ preempt_enable();
if (flags & SCA_MIGRATE_ENABLE)
return 0;
@@ -9421,9 +9425,11 @@ static void balance_push(struct rq *rq)
* Temporarily drop rq->lock such that we can wake-up the stop task.
* Both preemption and IRQs are still disabled.
*/
+ preempt_disable();
raw_spin_rq_unlock(rq);
stop_one_cpu_nowait(rq->cpu, __balance_push_cpu_stop, push_task,
this_cpu_ptr(&push_work));
+ preempt_enable();
/*
* At this point need_resched() is true and we'll take the loop in
* schedule(). The next pick is obviously going to be the stop task