Messages in this thread | | | Date | Mon, 16 Nov 2020 13:53:55 +0100 | From | Peter Zijlstra <> | Subject | Re: Loadavg accounting error on arm64 |
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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:
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).
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