linux-kernel - Re: [perf tool] cgroup support broken on Debian?

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Date:	Wed, 17 Dec 2014 11:02:36 +0800
From:	Zefan Li <lizefan@...wei.com>
To:	<eranian@...il.com>
CC:	Stephane Eranian <eranian@...glemail.com>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [perf tool] cgroup support broken on Debian?

On 2014/12/17 10:29, Stephane Eranian wrote:
> On Wed, Dec 17, 2014 at 3:20 AM, Zefan Li <lizefan@...wei.com> wrote:
>> On 2014/12/17 1:17, Vince Weaver wrote:
>>> On Mon, 15 Dec 2014, Stephane Eranian wrote:
>>>> On Mon, Dec 15, 2014 at 11:01 PM, Arnaldo Carvalho de Melo <arnaldo.melo@...il.com> wrote:
>>>
>>>> fs is visible. The cgroup file system type is not there anymore. They are using
>>>> tmpfs which is not ideal to detect just cgroup. Looks like now, we have to look
>>>> at the mount point which is flaky.
>>>
>>> The trivial fix is to just always assume things will be under
>>>       /sys/fs/cgroup
>>> which looks like to be the new official mount point.
>>>
>>> This will break on older systems though, or systems that mount cgroupfs in
>>> multiple locations.
>>>
>>> Another alternative is to change the interface to require the full
>>> cgroupfs pathname as an argument to -G
>>>
>>
>> What's the problem here?
>>
>> none /sys/fs/cgroup tmpfs rw,relatime,size=4k,mode=755 0 0
>> systemd /sys/fs/cgroup/systemd cgroup rw,nosuid,nodev,noexec,relatime,perf_event,name=systemd 0 0
>>
>> cgroup is mounted in /sys/fs/cgroup/systemd, so you should pass '/' to the -G argument:
>>
> Is that the only mountpoint possible?
> The tool needs to detect a valid mount point to locate the named cgroup.
> That's assuming that if I create cgroup foo, then it appears under
> //sys/fs/cgroup/systemd/foo
> 

There can be only one cgroupfs mountpoint which has perf_event subsystem
attached to it.

So for this setup:

mount -t tmpfs /sys/fs/cgroup
mkdir /sys/fs/cgroup/memory
mkdir /sys/fs/cgroup/perf
mount -t cgroup -o memory memcg /sys/fs/cgroup/memory
mount -t cgroup -o perf_event perf /sys/fs/cgroup/perf

The perf tool will locate the mountpoint as /sys/fs/cgroup/perf.

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