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

Peter Zijlstra
2010-09-21T09:44:16Z

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.