linux-kernel - Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HW Breakpoint interfaces

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Date:	Sun, 8 Mar 2009 11:09:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	prasad@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HW
	Breakpoint interfaces - v2


* KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> Hi
> 
> > This patch adds an ftrace plugin to detect and profile memory access over
> > kernel variables. It uses HW Breakpoint interfaces to 'watch memory
> > addresses.
> > 
> > Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com> 
> > ---
> >  kernel/trace/Kconfig          |    6 
> >  kernel/trace/Makefile         |    1 
> >  kernel/trace/trace.h          |   16 +
> >  kernel/trace/trace_ksym.c     |  448 ++++++++++++++++++++++++++++++++++++++++++
> >  kernel/trace/trace_selftest.c |   36 +++
> >  5 files changed, 507 insertions(+)
> 
> Could you please update Documentation/ftrace.txt?
> I guess many user interesting this patch. :)

Yeah, it has become a really nice feature this way. As i told it 
to K.Prasad before: we need this tracer because the data tracer 
will likely become the most common usecase of this facility. We 
will get the hw breakpoints facility tested and used.

And in fact we can go one step further: it would also be nice to 
wire it up with the ftrace histogram code: so that we can get 
usage histograms of kernel symbol read/write activities without 
the overhead of tracing. (The branch tracer already has this.)

Especially frequently used variables generate a _lot_ of events.

	Ingo
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