Date: Thu, 7 Apr 2016 12:42:59 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>, torvalds@linux-foundation.org,
        akpm@linux-foundation.org, mingo@redhat.com, lizefan@huawei.com,
        pjt@google.com, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-api@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCHSET RFC cgroup/for-4.6] cgroup, sched: implement resource
 group and PRIO_RGRP
Message-ID: <20160407104259.GQ3430@twins.programming.kicks-ass.net>
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 <20160407073547.GA12560@cmpxchg.org>
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On Thu, Apr 07, 2016 at 05:28:24AM -0400, Johannes Weiner wrote:
> Hm? The root group can always contain tasks. It's not the only thing
> the root is exempt from, it can't control any resources either:

it does in fact control resouces; the hierarchy directly affects the
proportional distribution of time.

> sched_group_set_shares():
> 
> 	/*
> 	 * We can't change the weight of the root cgroup.
> 	 */
> 	if (!tg->se[0])
> 		return -EINVAL;

The root has, per definition, no siblings, so setting a weight is
entirely pointless.

> tg_set_cfs_bandwidth():
> 
> 	if (tg == &root_task_group)
> 		return -EINVAL;
> 

We have had patches to implement this, but have so far held off because
they add a bunch of cycles to some really hot paths and we'd rather not
do that. Its not impossible, or unthinkable to do this otherwise.