From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [RFC] Default child of a cgroup Date: Fri, 01 Feb 2008 08:58:32 +0100 Message-ID: <1201852712.32654.36.camel@lappy> References: <20080131024049.GA9544@linux.vnet.ibm.com> <6599ad830801311839x33cf2ebco9f501571fc129b11@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <6599ad830801311839x33cf2ebco9f501571fc129b11@mail.gmail.com> Sender: linux-kernel-owner@vger.kernel.org To: Paul Menage Cc: vatsa@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, Ingo Molnar , containers@lists.osdl.org, dhaval@linux.vnet.ibm.com, Balbir Singh , pj@sgi.com List-Id: containers.vger.kernel.org On Thu, 2008-01-31 at 18:39 -0800, Paul Menage wrote: > On Jan 30, 2008 6:40 PM, Srivatsa Vaddagiri wrote: > > > > Here are some questions that arise in this picture: > > > > 1. What is the relationship of the task-group in A/tasks with the > > task-group in A/a1/tasks? In otherwords do they form siblings > > of the same parent A? > > I'd argue the same as Balbir - tasks in A/tasks are are children of A > and are siblings of a1, a2, etc. > > > > > 2. Somewhat related to the above question, how much resource should the > > task-group A/a1/tasks get in relation to A/tasks? Is it 1/2 of parent > > A's share or 1/(1 + N) of parent A's share (where N = number of tasks > > in A/tasks)? > > Each process in A should have a scheduler weight that's derived from > its static_prio field. Similarly each subgroup of A will have a > scheduler weight that's determined by its cpu.shares value. So the cpu > share of any child (be it a task or a subgroup) would be equal to its > own weight divided by the sum of weights of all children. > > So yes, if a task in A forks lots of children, those children could > end up getting a disproportionate amount of the CPU compared to tasks > in A/a1 - but that's the same as the situation without cgroups. If you > want to control cpu usage between different sets of processes in A, > they should be in sibling cgroups, not directly in A. > > Is there a restriction in CFS that stops a given group from > simultaneously holding tasks and sub-groups? If so, couldn't we change > CFS to make it possible rather than enforcing awkward restructions on > cgroups? I think it is possible, just way more work than the proposed hack. > If we really can't change CFS in that way, then an alternative would > be similar to Peter's suggestion - make cpu_cgroup_can_attach() fail > if the cgroup has children, and make cpu_cgroup_create() fail if the > cgroup has any tasks - that way you limit the restriction to just the > hierarchy that has CFS attached to it, rather than generically for all > cgroups Agreed.