linux-kernel - Re: Loadavg accounting error on arm64

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Date:   Mon, 16 Nov 2020 11:49:38 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>
Cc:     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 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.

(This is still at the chucking out ideas as I haven't context switched
back in all the memory barrier rules).

-- 
Mel Gorman
SUSE Labs