linux-kernel - Re: Loadavg accounting error on arm64

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Message-ID: <20201116120022.GO3371@techsingularity.net>
Date:   Mon, 16 Nov 2020 12:00:22 +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 11:49:38AM +0000, Mel Gorman wrote:
> 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.
> 

And smp_* can't be used anyway because sched_contributes_to_load is a
bit field that is not protected with a specific lock so it's "special".

-- 
Mel Gorman
SUSE Labs