Re: [RFC PATCH -v2 0/4] Persistent events

Peter Zijlstra
2012-08-21T10:31:09Z

On Thu, 2012-08-16 at 19:45 +0200, Borislav Petkov wrote:
>
> off and on I get some free time to work on that, here's the latest
> incarnation. It contains review feedback from the earlier round.
>
> Patch 1/4 adds a trace_add_file() interface which adds an additional
> file to debugfs, in this case the "persistent" file which contains the
> normal perf file descriptor sys_perf_event_open gives to the perf tool.
>
> IOW, one gets:
>
> /mnt/dbg/tracing/events/mce/mce_record/
> |-- enable
> |-- filter
> |-- format
> |-- id
> `-- persistent1
>
> 0 directories, 5 files
>
> [ 1 is the CPU number so sticking all per-CPU descriptors in this
> directory could get a little cluttered and ugly so I'll have to think
> about that a bit more. ]
>
> 3/4 is the meat which adds <kernel/events/persistent.c> and 4/4 shows
> how one can init a persistent event on a CPU.
>
> What remains is adding code which can enable events on boot from the
> kernel cmdline and more testing.
>
> As always, comments and suggestions are appreciated.

Good progress there, there's still a few things though:

- the point also raised by Steven, I'm pretty sure that the placing of
the debugfs files unfortunate. I would much rather see something
like /debug/perf/persistent/$foo, also dropping your
perf_event_desc::dir_name.

- I would make perf_add_persistent_on_cpu() static and create something
like perf_add_persistent() which iterates all CPUs and creates:
"%s-%04d", perf_event_desc::fname, cpu. This needs a little extra for
cpu-hotplug, not sure what to do there.

- related to the first point, by not tying them to actual events you
can create a persistent 'event' that contains multiple events. Its
quite possible to create multiple kernel events and use the
equivalent of PERF_EVENT_IOC_SET_OUTPUT on them to the exposed FD.

- It might be good to provide means of changing the persistent event's
buffer size, or maybe even 'destroy' persistent buffers.