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SubjectRe: [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@cn.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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