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    SubjectRe: Multiple Tracers
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    Hi,

    On Thu, 2009-04-09 at 00:37 -0500, Tom Zanussi wrote:
    > Hi,
    >
    > On Wed, 2009-04-08 at 16:24 +0200, Ingo Molnar wrote:
    > > * 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.
    > >
    >
    > Yeah, the EVENT_TRACE() tracepoints can be mixed any way you want by
    > just enabling the ones you want to see:
    >
    [lots of useful explanation cut here]

    Thanks for the low down on this. Thats exactly what I need to know to
    answer my question. I will continue to work on trace events support for
    GFS2 and when I'm done with that, I may look at other things such as
    blktrace if nobody has beaten me to it in the mean time,

    Steve.





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