Re: [PATCH 03/21] perf_events: Add a helper to search for an event in a context

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Jul 01, 2010 at 06:24:24PM +0200, Borislav Petkov wrote:
> From: Frederic Weisbecker <fweisbec@gmail.com>
> Date: Thu, Jul 01, 2010 at 12:14:50PM -0400
> 
> Hi Frederic,
> 
> > On Thu, Jul 01, 2010 at 06:12:45PM +0200, Peter Zijlstra wrote:
> > > On Thu, 2010-07-01 at 18:11 +0200, Frederic Weisbecker wrote:
> > > > I suspect we need another syscall that can list all the persistent events
> > > > with a unique id and the attrs that follow.
> > > > 
> > > > So you get a unique id for all of them and you can create an fd on top
> > > > of this id by using a PERF_FLAG_REQUEST_PERSISTENT and this id put in
> > > > attr.config. 
> > > 
> > > Isn't that what filesystems were invented for?
> > 
> > 
> > The problem is when you create a persistent event, you lose the fd.
> > So you need to retrieve it somehow.
> 
> actually the idea is to decouple those from the fd alltogether and
> provide specific file_operations in debugfs and such, as Peter
> suggested. Which sounds much more sane to me especially since, at least
> in the MCE case, all the entities that register into that event need to
> see the same samples (and read the same buffers etc).



Sure the idea of putting that in fs is better.

Note the idea of a new syscall was quite close: it would have listed
unique ids of the persistent events, but not fds, you'd have yet to create
those fds on top of the ids.



> 
> And let's try not to read too much into those persistent events - it may
> just as well be that we need them only for MCEs and nothing else :)


Not really. It would be useful for boot tracing, amongst various other things
like flight recorder tracing, etc...