Messages in this thread | | | Date | Mon, 16 Nov 2020 19:31:49 +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: > > 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.
> 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
And this works.
986.01 1008.17 1013.15 2/855 1212 362.19 824.70 949.75 1/856 1564 133.19 674.65 890.32 1/864 1958 49.04 551.89 834.61 2/871 2339 18.33 451.54 782.41 1/867 2686 6.77 369.37 733.45 1/866 2929 2.55 302.16 687.55 1/864 2931 0.97 247.18 644.52 1/860 2933 0.48 202.23 604.20 1/849 2935
I should have gone with this after rereading the warning about bit fields having to be protected by the same lock in the "anti-guarantees" section of memory-barriers.txt :(
sched_psi_wake_requeue can probably stay with the other three fields given they are under the rq lock but sched_remote_wakeup needs to move out.
-- Mel Gorman SUSE Labs
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