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

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Message-ID: <CABPqkBRnrEwcKgbfXroHm6q1+dho9h7MnfqamyL7SPxsrjnTqA@mail.gmail.com>
Date:	Wed, 17 Dec 2014 11:36:33 -0500
From:	Stephane Eranian <eranian@...gle.com>
To:	Zefan Li <lizefan@...wei.com>
Cc:	Stephane Eranian <eranian@...il.com>,
	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>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [perf tool] cgroup support broken on Debian?

On Tue, Dec 16, 2014 at 10:02 PM, Zefan Li <lizefan@...wei.com> wrote:
> 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.
>
You can mount the cgroup fs for perf_event multiple times. I
tried and it works, though it is useless.

Vince, I think the correct way to detect which entry is for perf_event
is to look for filesystem type cgroup and option perf_event. It cannot
be anything else. First match is good enough.
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