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

From: Peter Zijlstra
Date: Wed May 03 2017 - 05:37:44 EST

On Wed, May 03, 2017 at 09:34:51AM +0200, Vincent Guittot wrote:

> We use load_avg for calculating a stable share and we want to use it
> more and more. So breaking it because it's easier doesn't seems to be
> the right way to do IMHO

So afaict we calculate group se->load.weight (aka shares, see
calc_cfs_shares), using cfs_rq->avg.load_avg, which feeds into
tg->load_avg through cfs_rq->tg_load_avg_contrib and

And cfs_rq->avg.load_avg is computed from cfs_rq->load.weight, which
is \Sum se->load.weight.

OTOH group se->avg.load_avg isn't used much, which is TJ's point.

The only cases where group se->avg.load_avg are relevant to
cfs_rq->avg.load are the cases I listed yesterday, group creation and
group destruction. There we use the group se->avg.load_avg to manage the
boundary conditions.

So with the proposed change to se->avg.load_avg we can have some
(temporary) boundary effect when you destroy a lot of (previously
active) cgroups.

Of course, it could be I overlooked something, in which case, please
tell :-)

That said, I agree it would be nice to entirely get rid of runnable_avg,
but that is a much larger change and would require a lot more work. I
don't immediately see why we can't fix the thing now and then work on
removing runnable_load_avg later.

Of course, we should not regress either, I'll go read up on that part.