Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 16 Nov 2020 12:00:22 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: Loadavg accounting error on arm64
Message-ID: <20201116120022.GO3371@techsingularity.net>
References: <20201116091054.GL3371@techsingularity.net>
 <20201116114938.GN3371@techsingularity.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-15
Content-Disposition: inline
In-Reply-To: <20201116114938.GN3371@techsingularity.net>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

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