linux-kernel - Re: [PATCH 1/3] sched/fair: Peter's shares

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Date:   Thu, 11 May 2017 08:59:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
        Chris Mason <clm@...com>, kernel-team@...com
Subject: Re: [PATCH 1/3] sched/fair: Peter's shares_type patch

On Wed, May 10, 2017 at 06:07:14PM +0200, Vincent Guittot wrote:
> On 10 May 2017 at 17:09, Tejun Heo <tj@...nel.org> wrote:
> > Hello, Vincent.
> >
> > On Fri, May 05, 2017 at 11:30:31AM -0400, Tejun Heo wrote:
> >> > For shares_runnable, it should be
> >> >
> >> > group_entity->runnable_load_avg = cfs_rq->runnable_load_avg *
> >> > group_entity->avg.load_avg / cfs_rq->avg.load_avg
> >>
> >> Yeah, that could be one way to calculate the value while avoiding the
> >> artifacts.  Hmmm... IIUC, replacing the local contribution with the
> >> current one is to ensure that we at least calculate with the current
> >> term on the local queue.  This makes sense for weight and shares but
> >> as you pointed out it doesn't make sense to replace local base with
> >> runnable when the base is expected to be sum of load_avgs.  How about
> >> something like the following?
> >
> > Vincent, have you given this patch a try?
> 
> No I haven't.
> My understand of Peter's feedback is that calc_cfs_shares should not
> be the place where to implement calculation of
> group_entity->runnable_load_avg and group_entity->avg.load_avg when
> propagating

Right, so I have a pile of patches that implement all that my longish
email outlined. I'm just chasing some strange behaviour; in particular
I'm having runnable_load_avg > load_avg, which is something that should
not happen.

It _looks_ like the add/sub cycle leaks a little and a lot of such
cycles then push runnable_load_avg out. I've not managed to pin it down.

If I don't find it, I'll send it out regardless as an RFC so that others
can 'enjoy'.

One request for Chris / Tejun, could you guys pretty please make a
reproducible benchmark? Relying on some ill specified background noise
just doesn't work, as this thread has clearly illustrated, nobody can
reproduce your issue.

And although I think the specific issue has been fairly well explained,
it would be good to have a working benchmark to prove the point.