linux-kernel - Re: [PATCH v0 00/71] perf: Add support for Intel Processor Trace

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Message-ID: <20131216155730.GB26686@localhost.localdomain>
Date:	Mon, 16 Dec 2013 16:57:32 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH v0 00/71] perf: Add support for Intel Processor Trace

On Mon, Dec 16, 2013 at 07:45:27AM -0800, Andi Kleen wrote:
> > You're right it's extremely slow. But it can still be relevant for debugging,
> > at least for apps that don't do too much CPU bound stuffs.
> 
> There are patches from Markus already for gdb to use it (using the old
> BTS perf interface). I'm not sure they have been merged into gdb
> mainline yet though.

Ok.

> 
> > My hope has always been that we can make a userspace function graph tracer
> > out of its dumps. And I think we can, I'm pretty sure that would be a useful tool.
> 
> I wrote one, based on the __fentry__, like the kernel:
> http://github.com/andikleen/ftracer

Sounds like nice stuff, but that implies building with the gcc option I think.

> 
> BTS has no timing information, so you could at best do a function tracer
> without timing.

Right, now although the function timing was the initial purpose of the function graph
tracer, the graph itself proved to be much more useful :)

But yeah the timing is nice too when we chase hotspot though probably perf report deprecated it.

> 
> -Andi
> -- 
> ak@...ux.intel.com -- Speaking for myself only
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