From: Peter Zijlstra <peterz@infradead.org>
To: Dave Jones <davej@codemonkey.org.uk>,
Mel Gorman <mgorman@techsingularity.net>,
Linux Kernel <linux-kernel@vger.kernel.org>,
mingo@kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
paul.gortmaker@windriver.com, valentin.schneider@arm.com
Subject: Re: weird loadavg on idle machine post 5.7
Date: Tue, 7 Jul 2020 09:48:57 +0200 [thread overview]
Message-ID: <20200707074857.GJ4800@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200706212057.GA18637@codemonkey.org.uk>
On Mon, Jul 06, 2020 at 05:20:57PM -0400, Dave Jones wrote:
> 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
Heh, that one made me nearly spill my tea, much funnies :-)
But yes, Dave Chinner also complained about this for the previous fix.
I've written this:
https://lore.kernel.org/lkml/20200703133259.GE4781@hirez.programming.kicks-ass.net/
to help with that. But clearly I'll need to update that patch again
after this little adventure.
> > 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.
\o/ hooray! Thanks for testing Dave!
next prev parent reply other threads:[~2020-07-07 7:49 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-02 17:15 weird loadavg on idle machine post 5.7 Dave Jones
2020-07-02 19:46 ` Dave Jones
2020-07-02 21:15 ` Paul Gortmaker
2020-07-03 13:23 ` Paul Gortmaker
2020-07-02 21:36 ` Mel Gorman
2020-07-02 23:11 ` Michal Kubecek
2020-07-02 23:24 ` Dave Jones
2020-07-03 9:02 ` Peter Zijlstra
2020-07-03 10:40 ` Peter Zijlstra
2020-07-03 20:51 ` Dave Jones
2020-07-06 14:59 ` Peter Zijlstra
2020-07-06 21:20 ` Dave Jones
2020-07-07 7:48 ` Peter Zijlstra [this message]
2020-07-06 23:56 ` Valentin Schneider
2020-07-07 8:17 ` Peter Zijlstra
2020-07-07 10:20 ` Valentin Schneider
2020-07-07 10:29 ` Peter Zijlstra
2020-07-08 9:46 ` [tip: sched/urgent] sched: Fix loadavg accounting race tip-bot2 for Peter Zijlstra
2020-07-07 9:20 ` weird loadavg on idle machine post 5.7 Qais Yousef
2020-07-07 9:47 ` Peter Zijlstra
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