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SubjectRe: [RFC PATCH 1/2] perf_events: add support for per-cpu per-cgroup monitoring (v4)
>>>> +#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?

>>> - 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

They now have different hierarchy id, because they belong
to different cgroup hierarchy.

So pid + hierarchy_id locates the cgroup.

> When I compare with my approach, if perf_event is
> not mounted, then the file descriptor won't lead to the
> css, and therefore you will fail and that is fine because
> it means the perf_event subsystem is not instantiated
> therefore it cannot be used.
>
> In my patch, there was a missing check for a NULL
> css. I fixed that now, and it works fine.
>
> As for multiple mount points, it seems like the first
> mount determines the restrictions for all mounts.
> In other words, if you mount only cpuset, then no
> other mount can provide more than cpuset, and vice-versa.
>
> I have tried mounting cgroupfs in multiple places at the same
> time. Whatever directory I used, I got to the right css.
>
> Am I missing your point here?
>

I should use the words "cgroup hierarchies" instead of mount points..



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