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. >
And smp_* can't be used anyway because sched_contributes_to_load is a bit field that is not protected with a specific lock so it's "special". -- Mel Gorman SUSE Labs