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

[Peter Zijlstra]

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.


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