Re: weird loadavg on idle machine post 5.7

Dave Jones
2020-07-06T21:21:07Z

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