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Subject: Re: [PATCH 0/7] cgroups: Allow to bind/unbind subsystems to/from
 non-trival hierarchy
From: Peter Zijlstra <peterz@infradead.org>
To: Li Zefan <lizf@cn.fujitsu.com>
Cc: "akpm >> Andrew Morton" <akpm@linux-foundation.org>,
        Paul Menage <menage@google.com>, Stephane Eranian <eranian@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
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Date: Fri, 22 Oct 2010 14:50:01 +0200
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On Fri, 2010-10-22 at 16:09 +0800, Li Zefan wrote:
> Stephane posted a patchset to add perf_cgroup subsystem, so perf can
> be used to monitor all threads belonging to a cgroup.
> 
> But if you already mounted a cgroup hierarchy but without perf_cgroup
> and the hierarchy has sub-cgroups, you can't bind perf_cgroup to it,
> and thus you're not able to use per-cgroup perf feature.
> 
> This patchset alleviates the pain, and then a subsytem can be
> bound/unbound to/from a hierarchy which has sub-cgroups in it.
> 
> Some subsystems still can't take advantage of this patchset, memcgroup
> and cpuset in specific.
> 
> For cpuset, if a hierarchy has a sub-cgroup and the cgroup has tasks,
> we can't decide sub-cgroup's cpuset.mems and cpuset.cpus automatically
> if we try to bind cpuset to this hierarchy.
> 
> For memcgroup, memcgroup uses css_get/put(), and due to some complexity,
> for now bindable subsystems should not use css_get/put().
> 
> Usage:
> 
> # mount -t cgroup -o cpuset xxx /mnt
> # mkdir /mnt/tmp
> # echo $$ > /mnt/tmp/tasks
> 
> (add cpuacct to the hierarchy)
> # mount -o remount,cpuset,cpuacct xxx /mnt
> 
> (remove it from the hierarchy)
> # mount -o remount,cpuset xxx /mnt
> 
> There's another limitation, cpuacct should not be bound to any mounted
> hierarchy before the above operation. But that's not a problem, as you
> can remove it from a hierarchy and bind it to another one.

Right, so the only remaining problem I see with this approach is that
you cannot profile two different hierarchies at the same time, but I
can't really think of a solution to that problem (nor do I care very
much).

Seems like a nice approach, Thanks Li!