Re: [RFC PATCH 0/2] perf_events: add support for per-cpu per-cgroup monitoring (v3)

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2010-09-21 at 11:38 +0200, Peter Zijlstra wrote:
> On Thu, 2010-09-09 at 15:05 +0200, Stephane Eranian wrote:
> > The cgroup to monitor is designated by passing a file descriptor opened
> > on a new per-cgroup file in the cgroup filesystem (perf_event.perf). The
> > option must be activated by setting perf_event_attr.cgroup=1 and passing
> > a valid file descriptor in perf_event_attr.cgroup_fd. Those are the only
> > two ABI extensions. 
> 
> > +++ b/include/linux/perf_event.h
> > @@ -215,8 +215,9 @@ struct perf_event_attr {
> >                                  */
> >                                 precise_ip     :  2, /* skid constraint       */
> >                                 mmap_data      :  1, /* non-exec mmap data    */
> > +                               cgroup         :  1, /* cgroup aggregation    */
> >  
> > -                               __reserved_1   : 46;
> > +                               __reserved_1   : 45;
> >  
> >         union {
> >                 __u32           wakeup_events;    /* wakeup every n events */
> > @@ -226,6 +227,8 @@ struct perf_event_attr {
> >         __u32                   bp_type;
> >         __u64                   bp_addr;
> >         __u64                   bp_len;
> > +
> > +       int                     cgroup_fd;
> >  };
> >  
> >  /*
> 
> I'm not sure I like this much.. so we attach to {pid,cpu}, for nodes we
> can use cpu_to_node(cpu), which would suggest to use
> cgroup_of_task(pid), except that a task can be part of multiple cgroups,
> so its not unique.
> 
> One thing we could do is pass this cgroup identifier in the pid field
> and use PERF_FLAG_CGROUP or something. Currently the syscall signature
> uses pid_t, but I think we can safely change that to int.
> 
> You create a special new file in the cgroup stuff, I'm not sure about
> that either, but its not something I feel too strongly about, why
> wouldn't a fd of any file or even directory of that cgroup work? Do the
> cgroup people have an opinion?

Ahh, I just read more of the patch, and you create a full perf cgroup,
in which case cgroup_of_task(pid) will work, simply pick the perf
cgroup's tasks.

No need to actually create that file, open it and pass fds around, just
pick a task from that cgroup and attach to the cgroup through that.