linux-kernel - Re: [PATCH v7 2/5] cpuset: Add cpuset.sched_load

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Message-ID: <20180502134225.GR12217@hirez.programming.kicks-ass.net>
Date:   Wed, 2 May 2018 15:42:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v7 2/5] cpuset: Add cpuset.sched_load_balance to v2

On Wed, May 02, 2018 at 09:29:54AM -0400, Waiman Long wrote:
> On 05/02/2018 06:24 AM, Peter Zijlstra wrote:
> > On Thu, Apr 19, 2018 at 09:47:01AM -0400, Waiman Long wrote:
> >> +  cpuset.sched_load_balance
> >> +	A read-write single value file which exists on non-root cgroups.
> > Uhhm.. it should very much exist in the root group too. Otherwise you
> > cannot disable it there, which is required to allow smaller groups to
> > load-balance between themselves.
> >
> >> +	The default is "1" (on), and the other possible value is "0"
> >> +	(off).
> >> +
> >> +	When it is on, tasks within this cpuset will be load-balanced
> >> +	by the kernel scheduler.  Tasks will be moved from CPUs with
> >> +	high load to other CPUs within the same cpuset with less load
> >> +	periodically.
> >> +
> >> +	When it is off, there will be no load balancing among CPUs on
> >> +	this cgroup.  Tasks will stay in the CPUs they are running on
> >> +	and will not be moved to other CPUs.
> >> +
> >> +	This flag is hierarchical and is inherited by child cpusets. It
> >> +	can be turned off only when the CPUs in this cpuset aren't
> >> +	listed in the cpuset.cpus of other sibling cgroups, and all
> >> +	the child cpusets, if present, have this flag turned off.
> >> +
> >> +	Once it is off, it cannot be turned back on as long as the
> >> +	parent cgroup still has this flag in the off state.
> > That too is wrong and broken. You explicitly want to turn it on for
> > children.
> >
> > So the idea is that you can have:
> >
> > 		R
> > 	      /   \
> >             A       B
> >
> > With:
> >
> > 	R cpus=0-3, load_balance=0
> > 	A cpus=0-1, load_balance=1
> > 	B cpus=2-3, load_balance=1
> >
> > Which will allow all tasks in A,B (and its children) to load-balance
> > across 0-1 or 2-3 resp.
> >
> > If you don't allow the root group to disable load_balance, it will
> > always be the largest group and load-balancing will always happen system
> > wide.
> 
> If you look at the remaining patches in the series, I was proposing a
> different way to support isolcpus and separate sched domains with
> turning off load balancing in the root cgroup.
> 
> For me, it doesn't feel right to have load balancing disabled in the
> root cgroup as we probably cannot move all the tasks away from the root
> cgroup anyway. I am going to update the current patchset to incorporate
> suggestion from Tejun. It will probably be ready sometime next week.
> 

I've read half of the next patch that adds the isolation thing. And
while that kludges around the whole root cgorup is magic thing, it
doesn't help if you move the above scenario on level down:


	R
     /    \
   A        B
          /   \
        C       D


R: cpus=0-7, load_balance=0
A: cpus=0-1, load_balance=1
B: cpus=2-7, load_balance=0
C: cpus=2-3, load_balance=1
D: cpus=4-7, load_balance=1


Also, I feel we should strive to have a minimal amount of tasks that
cannot be moved out of the root group; the current set is far too large.