linux-kernel - Re: weird loadavg on idle machine post 5.7

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Message-ID: <20200706212057.GA18637@codemonkey.org.uk>
Date:   Mon, 6 Jul 2020 17:20:57 -0400
From:   Dave Jones <davej@...emonkey.org.uk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Linux Kernel <linux-kernel@...r.kernel.org>, mingo@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        paul.gortmaker@...driver.com, valentin.schneider@....com
Subject: Re: weird loadavg on idle machine post 5.7

On Mon, Jul 06, 2020 at 04:59:52PM +0200, Peter Zijlstra wrote:
 > On Fri, Jul 03, 2020 at 04:51:53PM -0400, Dave Jones wrote:
 > > On Fri, Jul 03, 2020 at 12:40:33PM +0200, Peter Zijlstra wrote:
 > >  
 > > looked promising the first few hours, but as soon as it hit four hours
 > > of uptime, loadavg spiked and is now pinned to at least 1.00
 > 
 > OK, lots of cursing later, I now have the below...
 > 
 > The TL;DR is that while schedule() doesn't change p->state once it
 > starts, it does read it quite a bit, and ttwu() will actually change it
 > to TASK_WAKING. So if ttwu() changes it to WAKING before schedule()
 > reads it to do loadavg accounting, things go sideways.
 > 
 > The below is extra complicated by the fact that I've had to scrounge up
 > a bunch of load-store ordering without actually adding barriers. It adds
 > yet another control dependency to ttwu(), so take that C standard :-)

Man this stuff is subtle. I could've read this a hundred times and not
even come close to approaching this.

Basically me reading scheduler code:
http://www.quickmeme.com/img/96/9642ed212bbced00885592b39880ec55218e922245e0637cf94db2e41857d558.jpg

 > I've booted it, and build a few kernels with it and checked loadavg
 > drops to 0 after each build, so from that pov all is well, but since
 > I'm not confident I can reproduce the issue, I can't tell this actually
 > fixes anything, except maybe phantoms of my imagination.

Five hours in, looking good so far.  I think you nailed it.

	Dave