linux-kernel - Re: [PATCH 03/15] sched,fair: redefine runnable_load_avg as the sum of task_h

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Date:   Wed, 28 Aug 2019 10:47:48 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>, Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 03/15] sched,fair: redefine runnable_load_avg as the sum
 of task_h_load

On Wed, 2019-08-28 at 15:50 +0200, Vincent Guittot wrote:
> Hi Rik,
> 
> On Thu, 22 Aug 2019 at 04:18, Rik van Riel <riel@...riel.com> wrote:
> > The runnable_load magic is used to quickly propagate information
> > about
> > runnable tasks up the hierarchy of runqueues. The runnable_load_avg
> > is
> > mostly used for the load balancing code, which only examines the
> > value at
> > the root cfs_rq.
> > 
> > Redefine the root cfs_rq runnable_load_avg to be the sum of
> > task_h_loads
> > of the runnable tasks. This works because the hierarchical
> > runnable_load of
> > a task is already equal to the task_se_h_load today. This provides
> > enough
> > information to the load balancer.
> > 
> > The runnable_load_avg of the cgroup cfs_rqs does not appear to be
> > used for anything, so don't bother calculating those.
> > 
> > This removes one of the things that the code currently traverses
> > the
> > cgroup hierarchy for, and getting rid of it brings us one step
> > closer
> > to a flat runqueue for the CPU controller.
> 
> I like your proposal but just wanted to clarify one thing with this
> patch.
> Although you removed the computation of runnable_load_avg of the
> cgroup cfs_rq, we are still traversing the hierarchy to update the
> root cfs_rq runnable_load_avg because we are traversing the hierarchy
> for computing the task_h_loads

The task_h_load hierarchy traversal in update_cfs_rq_h_load
is rate limited to once a jiffy, though.  Rate limiting the
hierarchy traversal significantly reduces overhead.

> That being said, if we manage to remove the need on using
> runnable_load_avg we will completely skip this traversal. I have a
> proposal to remove it from load balance and wake up path but i
> haven't
> look at numa stats which also use it

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