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

[Li Zefan <lizf@cn.fujitsu.com>]

02:56, stephane eranian wrote:
> On Wed, Oct 13, 2010 at 4:26 AM, Li Zefan <lizf@cn.fujitsu.com> wrote:
>> (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.
>>
> True.
> 
>> 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.
>>
> Something I may have missed since the beginning of our conversation
> is why do you think definition perf_cgroup subsys is wrong or useless.
> What kind of problem does it introduce. I think it is fine to reject cgroup
> mode if the perf cgroup is not mounted.
> 

Actually I don't have strong option over this perf_cgroup subsys. Anyway,
I already have cpuacct subsys.

For the disavantage I mentioned before:

- If one mounted cgroup fs without perf cgroup subsys, he can't monitor it.

This is not a problem if we can bind a subsys to a cgroup hierarchy via
remount. Currently we can do this only when the cgroupfs has root cgroup
only.

For the case that the cgroupfs has child cgroups, adding a subsys to it
should not be difficult, but seems removing is another story..