Re: [PATCH 2/2] sched/fair: Always propagate runnable_load_avg

From: Tejun Heo
Date: Tue May 02 2017 - 18:02:03 EST


On Mon, May 01, 2017 at 05:56:13PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 28, 2017 at 04:33:47PM -0400, Tejun Heo wrote:
> > I'm attaching the debug patch. With your change (avg instead of
> > runnable_avg), the following trace shows why it's wrong.
> Ah, OK. So you really want runnable_avg (and I understand why), which is
> rather unfortunate, since we have everything on load_avg.
> So for shares, load_avg gives a more stable number. This is important
> since tg->load_avg is a global number, so computing it is expensive (and
> slow). Therefore more stable numbers are good.
> > The thing with cfs_rq se's load_avg is that, it isn't really used
> > anywhere else AFAICS, so overriding it to the cfs_rq's
> > runnable_load_avg isn't prettiest but doesn't really change anything.
> You mean, consistently expressing a group's se->load.weight in terms of
> runnable_load_avg? See the above.

I think you got this on the other thread but for clarity:

cfs_rq->avg.load_avg is used for share calculation and we want to keep
it that way as the calcluation is expensive and rather decoupled
across CPUs (the deviation can be quite a bit without the stability).
But the group *se*->avg.load_avg is a separate thing which isn't
really used anywhere except for as a propagation channel from group
cfs_rq to its parent cfs_rq.