Date: Wed, 8 Apr 2009 16:24:13 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Steven Whitehouse <swhiteho@redhat.com>, Tom Zanussi <tzanussi@gmail.com>,
       =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>
Cc: rostedt@goodmis.org, linux-kernel@vger.kernel.org, mingo@redhat.com,
       axboe@kernel.dk
Subject: Re: Multiple Tracers
Message-ID: <20090408142413.GF12931@elte.hu>
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* Steven Whitehouse <swhiteho@redhat.com> wrote:

> Hi,
> 
> On Wed, 2009-04-08 at 15:53 +0200, Ingo Molnar wrote:
> > * Steven Whitehouse <swhiteho@redhat.com> wrote:
> > 
> > > Hi,
> > > 
> > > A little while back I posted a RFC patch which added gfs2 glock 
> > > tracing to blktrace. There was a suggestion that I should look 
> > > instead at the generic tracing code and add a new tracer, but I've 
> > > come up with a couple of issues along the way.
> > > 
> > > I think from what I can see that its only possible to run a single 
> > > tracer at once, so running blktrace (for example) would preclude 
> > > me from also tracing gfs2's glocks at the same time. Also, I can 
> > > see no mechanism by which I could ensure the sequencing between 
> > > the blktrace and glock traces other than exporting the blktrace 
> > > sequence number, even if running multiple tracers at the same time 
> > > was possible.
> > 
> > There's a recent addition: the EVENT_TRACE() facility. Would that 
> > suit your purposes?
> > 
> > 	Ingo
> 
> If I can get both blktrace and glock trace info from it, then yes. 
> I thought that the blktrace output went either via the original 
> relayfs path, or via its own tracer so that it wasn't possible to 
> use it and the event trace facility at the same time, unless I use 
> blktrace via relayfs. From what I've read I thought that probably 
> the relayfs interface for blktrace might eventually be removed in 
> favour of the generic tracing interface, but I'm not 100% sure of 
> that, so perhaps someone can confirm the plans in that area?

Tom Zanussi (Cc:-ed) recently converted the blktrace tracepoints to 
a tracing framework - and i think much of that could be reused to 
add EVENT_TRACE() tracepoints to blktrace too.

Tom, what's your take on this?

In general, this is the direction we want to take: EVENT_TRACE() 
tracepoints that can be used in a generic way, in any tracer.

	Ingo
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