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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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