| Message ID | 20231010200442.GA16515@noisy.programming.kicks-ass.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | sched: Fix stop_one_cpu_nowait() vs hotplug | expand |
On Tue, 2023-10-10 at 22:04 +0200, Peter Zijlstra wrote: > > External email : Please do not click links or open attachments until > you have verified the sender or the content. > 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... ? > Thanks for illustrate the race scenario. It looks good to me. But how about RT? Does RT also need this invocations as below? --- kernel/sched/rt.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index e93b69ef919b..6aaf0a3d6081 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -2063,9 +2063,11 @@ static int push_rt_task(struct rq *rq, bool pull) */ push_task = get_push_task(rq); if (push_task) { + preempt_disable(); raw_spin_rq_unlock(rq); stop_one_cpu_nowait(rq->cpu, push_cpu_stop, push_task, &rq->push_work); + preempt_enable(); raw_spin_rq_lock(rq); } @@ -2402,9 +2404,11 @@ static void pull_rt_task(struct rq *this_rq) 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); } } > --- > 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: > > CPU0CPU1 > > // 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;
On Wed, Oct 11, 2023 at 03:24:19AM +0000, Kuyo Chang (張建文) wrote: > On Tue, 2023-10-10 at 22:04 +0200, Peter Zijlstra wrote: > > > > External email : Please do not click links or open attachments until > > you have verified the sender or the content. > > 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... ? > > > Thanks for illustrate the race scenario. It looks good to me. > But how about RT? Does RT also need this invocations as below? > > --- > kernel/sched/rt.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index e93b69ef919b..6aaf0a3d6081 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -2063,9 +2063,11 @@ static int push_rt_task(struct rq *rq, bool > pull) > */ > push_task = get_push_task(rq); > if (push_task) { > + preempt_disable(); > raw_spin_rq_unlock(rq); > stop_one_cpu_nowait(rq->cpu, push_cpu_stop, > push_task, &rq->push_work); > + preempt_enable(); > raw_spin_rq_lock(rq); > } > > @@ -2402,9 +2404,11 @@ static void pull_rt_task(struct rq *this_rq) > 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); > } > } bah, clearly git-grep didn't work for me last night, I'll go fix up.
--- 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;