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

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Message-ID: <20100701162742.GI10616@nowhere>
Date:	Thu, 1 Jul 2010 18:27:47 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Borislav Petkov <bp@...64.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/21] perf_events: Add a helper to search for an event
	in a context

On Thu, Jul 01, 2010 at 06:24:24PM +0200, Borislav Petkov wrote:
> From: Frederic Weisbecker <fweisbec@...il.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...

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