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Message-ID: <1347647021.7172.74.camel@twins>
Subject: Re: [RFC] cgroup TODOs
From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>, Glauber Costa <glommer@parallels.com>,
        containers@lists.linux-foundation.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, Michal Hocko <mhocko@suse.cz>,
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        Thomas Graf <tgraf@suug.ch>, "Serge E. Hallyn" <serue@us.ibm.com>,
        Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Neil Horman <nhorman@tuxdriver.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        "Daniel P. Berrange" <berrange@redhat.com>,
        Lennart Poettering <lennart@poettering.net>,
        Kay Sievers <kay.sievers@vrfy.org>
Date: Fri, 14 Sep 2012 20:23:41 +0200
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On Fri, 2012-09-14 at 10:59 -0700, Tejun Heo wrote:
> Hello,
> 
> On Fri, Sep 14, 2012 at 05:12:31PM +0800, Li Zefan wrote:
> > Agreed. The biggest issue in cpuset is if hotplug makes a cpuset's cpulist
> > empty the tasks in it will be moved to an ancestor cgroup, which requires
> > holding cgroup lock. We have to either change cpuset's behavior or eliminate
> > the global lock.
> 
> Does that have to happen synchronously?  Can't we have a cgroup
> operation which asynchronously pushes all tasks in a cgroup to its
> parent from a work item?

Its hotplug, all hotplug stuff is synchronous, the last thing hotplug
needs is the added complexity of async callbacks. Also pushing stuff out
into worklets just to work around locking issues is vile.

<handwave as I never can remember all the cgroup stuff/>

Can't we play games by pinning both cgroups with a reference and playing
games with threadgroup_change / task_lock for the individual tasks being
moved about?