Loadavg accounting error on arm64

[Mel Gorman]

On Mon, Nov 16, 2020 at 03:29:46PM +0000, Mel Gorman wrote:
> I did, it was the on_cpu ordering for the blocking case that had me
> looking at the smp_store_release and smp_cond_load_acquire in arm64 in
> the first place thinking that something in there must be breaking the
> on_cpu ordering. I'm re-reading it every so often while trying to figure
> out where the gap is or whether I'm imagining things.
> 
> Not fully tested but did not instantly break either
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d2003a7d5ab5..877eaeba45ac 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4459,14 +4459,26 @@ static void __sched notrace __schedule(bool preempt)
>  		if (signal_pending_state(prev_state, prev)) {
>  			prev->state = TASK_RUNNING;
>  		} else {
> -			prev->sched_contributes_to_load =
> +			int acct_load =
>  				(prev_state & TASK_UNINTERRUPTIBLE) &&
>  				!(prev_state & TASK_NOLOAD) &&
>  				!(prev->flags & PF_FROZEN);
>  
> -			if (prev->sched_contributes_to_load)
> +			prev->sched_contributes_to_load = acct_load;
> +			if (acct_load) {
>  				rq->nr_uninterruptible++;
>  
> +				/*
> +				 * Pairs with p->on_cpu ordering, either a
> +				 * smp_load_acquire or smp_cond_load_acquire
> +				 * in the ttwu path before ttwu_do_activate
> +				 * p->sched_contributes_to_load. It's only
> +				 * after the nr_interruptible update happens
> +				 * that the ordering is critical.
> +				 */
> +				smp_wmb();
> +			}
> +
>  			/*
>  			 * __schedule()			ttwu()
>  			 *   prev_state = prev->state;    if (p->on_rq && ...)
> 

This passed the test. Load averages taken once a minute after the test
completed showed

950.21 977.17 990.69 1/853 2117
349.00 799.32 928.69 1/859 2439
128.18 653.85 870.56 1/861 2736
47.08 534.84 816.08 1/860 3029
17.29 437.50 765.00 1/865 3357
6.35 357.87 717.13 1/865 3653
2.33 292.74 672.24 1/861 3709
0.85 239.46 630.17 1/859 3711
0.31 195.87 590.73 1/857 3713
0.11 160.22 553.76 1/853 3715

With 5.10-rc3, it got stuck with a load average of 244 after the test
completed even though the machine was idle.

-- 
Mel Gorman
SUSE Labs