linux-kernel - Re: [PATCH 2/2] trace,x86: Add x86 irq vector entry/exit tracepoints

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Message-ID: <20111214163407.GE10791@somewhere.redhat.com>
Date:	Wed, 14 Dec 2011 17:34:09 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Seiji Aguchi <seiji.aguchi@....com>
Cc:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	Michael Rubin <mrubin@...gle.com>,
	David Sharp <dhsharp@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Satoru Moriya <satoru.moriya@....com>
Subject: Re: [PATCH 2/2] trace,x86: Add x86 irq vector entry/exit tracepoints

On Wed, Dec 14, 2011 at 10:09:11AM -0500, Seiji Aguchi wrote:
> Thank you for giving me a comment.
> Unfortunately, neither "perf record" nor "ftrace" works for me.
> 
> >What about using perf for that?
> >
> >Just run:
> >
> >	perf record -ag
> >	^C
> >	perf report
> >
> >And you should find in the callchains some informations about where your CPUs
> >are spending time.
> >
> >If you system is too slow for that 
> 
> When system is too slow, user command such as "perf record" may not work.
> 
> >but you're doing background tracing with
> >ftrace, you can use stacktrace with ftrace.
> 
> Actually, We're doing background tracing in our customer's system rather than kernel debugging. 
> Ftrace doesn't work for me because it checks the size of the stack at every function call.
> Our customers are seriously concerned about its overhead.
> 
> For reducing the overhead, I need tracepoints so we can hook minimal function calls.

Well ftrace is a whole subsystem that includes the function tracer and also an interface
for tracepoints in debugfs. I was rather suggesting the latter one. This is a good
choice for background tracing. And it supports stacktraces. If those generate too much
overhead perhaps you can tune the number of entries in the stacktrace, I don't remember
if we can do that currently but this can be an interesting feature.

What are you using currently for the background tracing?
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