Re: [PATCH v2 6/8] sched/fair: use load instead of runnable load
From: Vincent Guittot
Date: Mon Aug 26 2019 - 11:46:09 EST
On Tue, 6 Aug 2019 at 18:07, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Aug 01, 2019 at 04:40:22PM +0200, Vincent Guittot wrote:
> > runnable load has been introduced to take into account the case
> > where blocked load biases the load balance decision which was selecting
> > underutilized group with huge blocked load whereas other groups were
> > overloaded.
> > The load is now only used when groups are overloaded. In this case,
> > it's worth being conservative and taking into account the sleeping
> > tasks that might wakeup on the cpu.
> This one scares me a little. I have the feeling I'm missing/forgetting
> Also; while the regular load-balance (find-busiest) stuff is now all
> aware of idle, this change also impacts wake_affine and find_idlest, and
> they've not changed.
Yes. I thought about this a bit before applying this changes to all
-For wake_affine, it starts by looking at an idle cpu with
wake_affine_idle which is similar in some way to the new load balance
approach even if it might need more changes to align more closely both
-For find_idlest, it looks at the group with most spare capacity in
priority and fall back to load. That being said I have overlooked that
both load and runnable load are already used and cpu.load is saved
twice instead of one.
I can't remember detailed results but this patch is responsible of
part of perf improvements described in the cover letter
Also, I haven't touch the numa stats which still use runnable_load.
But this should be addressed too