linux-kernel - Re: [RFC PATCH 1/2] perf_events: add support for per-cpu per-cgroup monitoring (v4)

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Date:	Wed, 13 Oct 2010 10:26:22 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Stephane Eranian <eranian@...gle.com>
CC:	eranian@...il.com, linux-kernel@...r.kernel.org,
	peterz@...radead.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, robert.richter@....com,
	acme@...hat.com
Subject: Re: [RFC PATCH 1/2] perf_events: add support for per-cpu per-cgroup
 monitoring (v4)

(Sorry for the late reply. I've been keeping busy..)

Stephane Eranian wrote:
> On Fri, Oct 8, 2010 at 2:46 AM, Li Zefan <lizf@...fujitsu.com> wrote:
>>>>>> +#ifdef CONFIG_CGROUPS
>>>>>> +struct perf_cgroup_time {
>>>>>> +     u64 time;
>>>>>> +     u64 timestamp;
>>>>>> +};
>>>>>> +
>>>>>> +struct perf_cgroup {
>>>>>> +     struct cgroup_subsys_state css;
>>>>>> +     struct perf_cgroup_time *time;
>>>>>> +};
>>>>> Can we avoid adding this perf cgroup subsystem? It has 2 disavantages:
>>>>>
>>>> Well, I need to maintain some timing information for each cgroup. This has
>>>> to be stored somewhere.
>>>>
>> Seems you can simply store it in struct perf_event?
>>
> No, timing has to be shared by events monitoring the same cgroup at
> the same time.
> Works like a timestamp. It needs to be centralized for all events
> attached to the same cgroup.
> 

I no little about internel perf code, so I don't know if we can store
this somewhere in perf. The last resort could be store it in struct cgroup.

>>>>> - If one mounted cgroup fs without perf cgroup subsys, he can't monitor it.
>>>> That's unfortunately true ;-)
>>>>
>>>>> - If there are several different cgroup mount points, only one can be
>>>>>  monitored.
>>>>>
>>>>> To choose which cgroup hierarchy to monitor, hierarchy id can be passed
>>>>> from userspace, which is the 2nd column below:
>>>>>
>>>> Ok, I will investigate this. As long as the hierarchy id is unique AND it can be
>>>> searched, then we can use it. Using /proc is fine with me.
>>>>
>>>>> $ cat /proc/cgroups
>>>>> #subsys_name    hierarchy       num_cgroups     enabled
>>>>> debug   0       1       1
>>>>> net_cls 0       1       1
>>>>>
>>> If I mount all subsystems:
>>> mount -t cgroup none /dev/cgroup
>>> Then, I get:
>>> #subsys_name  hierarchy       num_cgroups     enabled
>>> cpuset        1       1       1
>>> cpu           1       1       1
>>> perf_event    1       1       1
>>>
>>> In other words, the hierarchy id is not unique.
>>> If the perf_event is not mounted, then hierarchy id = 0.
>>>
>> Yes, it's unique. ;)
>>
>> You mounted them together, and that's a cgroup hierarchy, so
>> they have the same hierarchy id.
>>
>> If you mount them seperately:
>>
>> # mount -t cgroup -o debug xxx /cgroup1
>> # mount -t cgroup -o net_cls xxx /cgroup2/
>> # cat /proc/cgroups
>> #subsys_name    hierarchy       num_cgroups     enabled
>> debug   1       1       1
>> net_cls 2       1       1
>>
> Ok, but if you mount perf_event twice, you get the
> same hierarchy id for it:
> 
> # mount -t cgroup -operf_event none /cgroup
> # cat /proc/cgroups
> #subsys_name	hierarchy	num_cgroups	enabled
> cpuset	0	1	1
> cpu	        0	1	1
> perf_event	1	1	1
> 
> # mount -t cgroup -operf_event none /cgroup2
> # cat /proc/cgroups
> #subsys_name	hierarchy	num_cgroups	enabled
> cpuset	0	1	1
> cpu	        0	1	1
> perf_event	1	1	1
> 
> It does not seem like I can mount the same subsystem
> twice with difference hierarchies:
> 
> # umount /cgroup2
> # mount -t cgroup -operf_event,cpuset none /cgroup2
> mount: none already mounted or /cgroup2 busy
> # mount -t cgroup  none /cgroup2
> mount: none already mounted or /cgroup2 busy
> 
>> They now have different hierarchy id, because they belong
>> to different cgroup hierarchy.
>>
>> So pid + hierarchy_id locates the cgroup.
>>
> 
> I cannot do task's pid + cgroup hierarchy_id. It's one or the
> other.
> 

I've looked into the patch again, and I see you pass the fd from
userspace, so you don't need hierarchy_id.

And to get rid of perf_cgroup subsys, seems you just need to find
another place to store the time info, somewhere inside perf code
or in struct cgroup.
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