linux-kernel - Re: [RFC PATCH 00/11] mcount tracing utility

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Date:	Thu, 3 Jan 2008 12:42:17 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"William L. Irwin" <sparclinux@...r.kernel.org>,
	Tim Bird <tim.bird@...sony.com>
Subject: Re: [RFC PATCH 00/11] mcount tracing utility


On Thu, 3 Jan 2008, Mathieu Desnoyers wrote:

> Hi Steven,
>
> Great work!

Thanks!

>
> (added Tim Bird, author of KFT/KFI to the CC list)

I'm currently investigating using -finstrument-functions instead of -pg,
but if the overhead is too much, I may try to incorporate both.

>
> One interesting aspect of LTTng is that is would be very lightweight.
> You seem to use interrupt disabling with your simple tracer and do a
> _lot_ of cacheline bouncing (trace_idx[NR_CPUS] is a very good exemple).

Please note that this tracer is more of a "simple example". There's lots
of improvements that can be made. It was meant more of to show what mcount
can bring than to push the tracer itself.

I want to stress that the tracer in this patch set is a *much* simplified
version of the latency_tracer in the RT patch. I want to start out simple,
complexity can come later ;-)

>
> LTTng would write the information to a per-cpu memory buffer in binary
> format. I see that it would be especially useful in flight recorder
> mode, where we overwrite the buffers without writing them to disk : when
> a problematic condition is reached, (a kernel oops would be a good one),
> then we just stop tracing and dump the last buffers to disk. In this
> case, we would have the last function calls that led to an OOPS.

This sounds great. My hope is that we can get the mcount (or cyg_profile)
functionality in the kernel that many different users can deploy.

-- Steve

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