Messages in this thread | | | Date | Mon, 16 Nov 2020 13:58:03 +0100 | From | Peter Zijlstra <> | Subject | Re: Loadavg accounting error on arm64 |
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On Mon, Nov 16, 2020 at 01:53:55PM +0100, Peter Zijlstra wrote: > On Mon, Nov 16, 2020 at 11:49:38AM +0000, Mel Gorman wrote: > > On Mon, Nov 16, 2020 at 09:10:54AM +0000, Mel Gorman wrote: > > > I'll be looking again today to see can I find a mistake in the ordering for > > > how sched_contributes_to_load is handled but again, the lack of knowledge > > > on the arm64 memory model means I'm a bit stuck and a second set of eyes > > > would be nice :( > > > > > > > This morning, it's not particularly clear what orders the visibility of > > sched_contributes_to_load exactly like other task fields in the schedule > > vs try_to_wake_up paths. I thought the rq lock would have ordered them but > > something is clearly off or loadavg would not be getting screwed. It could > > be done with an rmb and wmb (testing and hasn't blown up so far) but that's > > far too heavy. smp_load_acquire/smp_store_release might be sufficient > > on it although less clear if the arm64 gives the necessary guarantees. > > > > (This is still at the chucking out ideas as I haven't context switched > > back in all the memory barrier rules). > > IIRC it should be so ordered by ->on_cpu. > > We have: > > schedule() > prev->sched_contributes_to_load = X; > smp_store_release(prev->on_cpu, 0); > > > on the one hand, and:
Ah, my bad, ttwu() itself will of course wait for !p->on_cpu before we even get here.
> sched_ttwu_pending() > if (WARN_ON_ONCE(p->on_cpu)) > smp_cond_load_acquire(&p->on_cpu) > > ttwu_do_activate() > if (p->sched_contributes_to_load) > ... > > on the other (for the remote case, which is the only 'interesting' one).
Also see the "Notes on Program-Order guarantees on SMP systems." comment.
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