Messages in this thread Patch in this message | | | Date | Tue, 10 Oct 2023 22:04:42 +0200 | From | Peter Zijlstra <> | Subject | [PATCH] sched: Fix stop_one_cpu_nowait() vs hotplug |
| |
On Tue, Oct 10, 2023 at 04:57:47PM +0200, Peter Zijlstra wrote: > On Tue, Oct 10, 2023 at 02:40:22PM +0000, Kuyo Chang (張建文) wrote:
> > It is running good so far(more than a week)on hotplug/set affinity > > stress test. I will keep it testing and report back if it happens > > again. > > OK, I suppose I should look at writing a coherent Changelog for this > then...
Something like the below... ?
--- Subject: sched: Fix stop_one_cpu_nowait() vs hotplug From: Peter Zijlstra <peterz@infradead.org> Date: Tue Oct 10 20:57:39 CEST 2023
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> --- kernel/sched/core.c | 10 ++++++++-- kernel/sched/deadline.c | 2 ++ kernel/sched/fair.c | 4 +++- 3 files changed, 13 insertions(+), 3 deletions(-)
--- 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 *r 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 *r 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; @@ -9459,9 +9463,11 @@ 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 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -2420,9 +2420,11 @@ static void pull_dl_task(struct rq *this double_unlock_balance(this_rq, src_rq); if (push_task) { + preempt_disable(); raw_spin_rq_unlock(this_rq); stop_one_cpu_nowait(src_rq->cpu, push_cpu_stop, push_task, &src_rq->push_work); + preempt_enable(); raw_spin_rq_lock(this_rq); } } --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -11299,13 +11299,15 @@ static int load_balance(int this_cpu, st busiest->push_cpu = this_cpu; active_balance = 1; } - raw_spin_rq_unlock_irqrestore(busiest, flags); + preempt_disable(); + raw_spin_rq_unlock_irqrestore(busiest, flags); if (active_balance) { stop_one_cpu_nowait(cpu_of(busiest), active_load_balance_cpu_stop, busiest, &busiest->active_balance_work); } + preempt_enable(); } } else { sd->nr_balance_failed = 0;
| |