linux-kernel - Re: [PATCHSET RFC cgroup/for-4.6] cgroup, sched: implement resource group and PRIO

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Date:	Thu, 7 Apr 2016 12:42:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Tejun Heo <tj@...nel.org>, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, mingo@...hat.com, lizefan@...wei.com,
	pjt@...gle.com, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, linux-api@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCHSET RFC cgroup/for-4.6] cgroup, sched: implement resource
 group and PRIO_RGRP

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.