Messages in this thread | | | Date | Mon, 16 Nov 2020 15:52:32 +0000 | From | Mel Gorman <> | Subject | Re: Loadavg accounting error on arm64 |
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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.
> > > __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.
> 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.
-- Mel Gorman SUSE Labs
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