Loadavg accounting error on arm64

[Mel Gorman]

On Mon, Nov 16, 2020 at 05:49:28PM +0100, Peter Zijlstra wrote:
> > So while we might be able to avoid a smp_rmb() before the read of
> > sched_contributes_to_load and rely on p->on_cpu ordering there,
> > we may still need a smp_wmb() after nr_interruptible() increments
> > instead of waiting until the smp_store_release() is hit while a task
> > is scheduling. That would be a real memory barrier on arm64 and a plain
> > compiler barrier on x86-64.
> 

Wish I read this before sending the changelog

> I'm mighty confused by your words here; and the patch below. What actual
> scenario are you worried about?
> 

The wrong one apparently. Even if the IRQ is released, the IPI would
deliver to the CPU that should observe the correct value or take the
other path when smp_cond_load_acquire(&p->on_cpu, !VAL) waits for the
schedule to finish so I'm now both confused and wondering why smp_wmb
made a difference at all.

-- 
Mel Gorman
SUSE Labs