Messages in this thread Patch in this message | | | Date | Mon, 16 Nov 2020 17:54:15 +0100 | From | Peter Zijlstra <> | Subject | Re: Loadavg accounting error on arm64 |
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On Mon, Nov 16, 2020 at 03:52:32PM +0000, Mel Gorman wrote: > On Mon, Nov 16, 2020 at 03:20:05PM +0100, Peter Zijlstra wrote: > > > It used to be at least a WRITE_ONCE until 58877d347b58 ("sched: Better > > > document ttwu()") which changed it. Not sure why that is and didn't > > > think about it too deep as it didn't appear to be directly related to > > > the problem and didn't have ordering consequences. > > > > I'm confused; that commit didn't change deactivate_task(). Anyway, > > ->on_rq should be strictly under rq->lock. That said, since there is a > > READ_ONCE() consumer of ->on_rq it makes sense to have the stores as > > WRITE_ONCE(). > > > > It didn't change deactivate_task but it did this > > - WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING); > - dequeue_task(rq, p, DEQUEUE_NOCLOCK); > + deactivate_task(rq, p, DEQUEUE_NOCLOCK); > > which makes that write a > > p->on_rq = (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING; > > As activate_task is also a plain store and I didn't spot a relevant > ordering problem that would impact loadavg, I concluded it was not > immediately relevant, just a curiousity.
That's move_queued_task() case, which is irrelevant for the issue at hand.
> > > > __ttwu_queue_wakelist() we have: > > > > > > > > p->sched_remote_wakeup = !!(wake_flags & WF_MIGRATED); > > > > > > > > which can be invoked on the try_to_wake_up() path if p->on_rq is first read > > > > as zero and then p->on_cpu is read as 1. Perhaps these non-atomic bitfield > > > > updates can race and cause the flags to be corrupted? > > > > > > > > > > I think this is at least one possibility. I think at least that one > > > should only be explicitly set on WF_MIGRATED and explicitly cleared in > > > sched_ttwu_pending. While I haven't audited it fully, it might be enough > > > to avoid a double write outside of the rq lock on the bitfield but I > > > still need to think more about the ordering of sched_contributes_to_load > > > and whether it's ordered by p->on_cpu or not. > > > > The scenario you're worried about is something like: > > > > CPU0 CPU1 > > > > schedule() > > prev->sched_contributes_to_load = X; > > deactivate_task(prev); > > > > try_to_wake_up() > > if (p->on_rq &&) // false > > if (smp_load_acquire(&p->on_cpu) && // true > > ttwu_queue_wakelist()) > > p->sched_remote_wakeup = Y; > > > > smp_store_release(prev->on_cpu, 0); > > > > Yes, mostly because of what memory-barriers.txt warns about for bitfields > if they are not protected by the same lock.
I'm not sure memory-barriers.txt is relevant; that's simply two racing stores and 'obviously' buggered.
> > And then the stores of X and Y clobber one another.. Hummph, seems > > reasonable. One quick thing to test would be something like this: > > > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index 7abbdd7f3884..9844e541c94c 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -775,7 +775,9 @@ struct task_struct { > > unsigned sched_reset_on_fork:1; > > unsigned sched_contributes_to_load:1; > > unsigned sched_migrated:1; > > + unsigned :0; > > unsigned sched_remote_wakeup:1; > > + unsigned :0; > > #ifdef CONFIG_PSI > > unsigned sched_psi_wake_requeue:1; > > #endif > > I'll test this after the smp_wmb() test completes. While a clobbering may > be the issue, I also think the gap between the rq->nr_uninterruptible++ > and smp_store_release(prev->on_cpu, 0) is relevant and a better candidate.
I really don't understand what you wrote in that email...
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