linux-kernel - Re: Loadavg accounting error on arm64

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Message-ID: <20201116174119.GB30723@willie-the-truck>
Date:   Mon, 16 Nov 2020 17:41:20 +0000
From:   Will Deacon <will@...nel.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Loadavg accounting error on arm64

On Mon, Nov 16, 2020 at 05:24:44PM +0000, Mel Gorman wrote:
> 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.

Probably still worth trying Peter's hack to pad the bitfields though, as I
think that's still a plausible issue (and one which would appear to be
fixed by that smp_wmb() too).

Will