Messages in this thread Patch in this message | | | Date | Mon, 20 Jul 2020 16:21:05 +0200 | From | Peter Zijlstra <> | Subject | Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917 |
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On Mon, Jul 20, 2020 at 04:02:24PM +0200, Oleg Nesterov wrote: > I have to admit, I do not understand the usage of prev_state in schedule(), > it looks really, really subtle...
Right, so commit dbfb089d360 solved a problem where schedule() re-read prev->state vs prev->on_rq = 0. That is, schedule()'s dequeue and ttwu()'s enqueue disagreed over sched_contributes_to_load. and as a result load-accounting went wobbly.
Now, looking at that commit again, I might've solved the problem twice :-P
So on the one hand, I provider ordering:
LOAD p->state LOAD-ACQUIRE p->on_rq == 0 MB STORE p->on_rq, 0 STORE p->state, TASK_WAKING
such that ttwu() will only change p->state, after on_rq==0, which is after loading p->state in schedule().
At the same time, I also had schedule() set p->sched_contributes_to_load once, and then consistently used that value throughout, without ever looking at p->state again, which too makes it much harder to mess load-avg up.
Now, the ordering in schedule(), relies on doing the p->state load before:
spin_lock(rq->lock) smp_mb__after_spinlock();
and doing a re-load check after, with the assumption that if the reload is different, it will not block.
That said, in a crossed email, I just proposed we could simplify all this like so.. but now I need to go ask people to re-validate that loadavg muck again :-/
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a2a244af9a53..437fc3b241f2 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4193,9 +4193,6 @@ static void __sched notrace __schedule(bool preempt) local_irq_disable(); rcu_note_context_switch(preempt); - /* See deactivate_task() below. */ - prev_state = prev->state; - /* * Make sure that signal_pending_state()->signal_pending() below * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE) @@ -4223,7 +4220,8 @@ static void __sched notrace __schedule(bool preempt) * We must re-load prev->state in case ttwu_remote() changed it * before we acquired rq->lock. */ - if (!preempt && prev_state && prev_state == prev->state) { + prev_state = prev->state; + if (!preempt && prev_state) { if (signal_pending_state(prev_state, prev)) { prev->state = TASK_RUNNING; } else { @@ -4237,10 +4235,12 @@ static void __sched notrace __schedule(bool preempt) /* * __schedule() ttwu() - * prev_state = prev->state; if (READ_ONCE(p->on_rq) && ...) - * LOCK rq->lock goto out; - * smp_mb__after_spinlock(); smp_acquire__after_ctrl_dep(); - * p->on_rq = 0; p->state = TASK_WAKING; + * if (prev_state) if (p->on_rq && ...) + * p->on_rq = 0; goto out; + * smp_acquire__after_ctrl_dep(); + * p->state = TASK_WAKING + * + * Where __schedule() and ttwu() have matching control dependencies. * * After this, schedule() must not care about p->state any more. */
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