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Date: Sun, 2 Nov 2008 17:52:02 +0100 From: "Frédéric Weisbecker" <fweisbec@...il.com> To: "Pekka Paalanen" <pq@....fi> Cc: "Steven Rostedt" <rostedt@...dmis.org>, linux-kernel <linux-kernel@...r.kernel.org>, "Ingo Molnar" <mingo@...e.hu> Subject: Re: Tracing framework regressions in 2.6.28-rc2 Hi Pekka! 2008/11/2 Pekka Paalanen <pq@....fi>: > Hi all, > > I finally had the time to check how mmiotrace works in 2.6.28-rc2. > On the first try I didn't find any mmiotrace problems, I traced the > Nouveau DRM driver just fine, but the file /debug/tracing/current_tracer > has started to misbehave. > > I think it is due to this patch: > > commit c2931e05ec5965597cbfb79ad332d4a29aeceb23 > Author: Frederic Weisbecker <fweisbec@...il.com> > Date: Sat Oct 4 22:04:44 2008 +0200 > > ftrace: return an error when setting a nonexistent tracer > > When one try to set a nonexistent tracer, no error is returned > as if the name of the tracer was correct. > We should return -EINVAL. > > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com> > Acked-by: Steven Rostedt <srostedt@...hat.com> > Signed-off-by: Ingo Molnar <mingo@...e.hu> > > Now, whatever I echo to current_tracer, I get EINVAL. > What's more confusing, if I do > echo mmiotrace > /debug/tracing/current_tracer > then I see the EINVAL error *and* mmiotrace is activated! > > The problem is with newlines, with 'echo -n' it works as it should. > > I'm starting to think the patch itself is fine, but it exposes an > assumption in the write fop: a single write call contains all the > data. I suspect there are actually two write calls in case of 'echo': > one with 'mmiotrace' and one with '\n'. I don't see how else I could > get the behaviour I get. A patch has been applied for this in the mainline, the issue will be fixed for rc3: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=60063a66236c15f5613f91390631e06718689782 > Another issue is the disappeared 'none' pseudo-tracer, which seems > to be replaced with the 'nop' real tracer. Is using 'nop' now equivalent > to the previous 'none' tracer? Yes. A nop tracer has been released to have a kind of pattern to write new tracers, and to have a tracer that does nothing but is able to show user marks for example. Since it does nothing, we replaced the none tracer by this one. > I mean, when the 'nop' tracer is current, are all tracing calls no-ops and > nothing will ever get into the ring buffer? > I have to update the mmiotrace documentation about disabling tracing. Hhm not really, you can receive pseudo entries like user marks. But you can disable all the tracing by setting 0 to tracing_enabled. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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