linux-kernel - Re: Tracing configuration review

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Date:	Thu, 27 May 2010 16:50:44 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Chase Douglas <chase.douglas@...onical.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	Soeren Sandmann <sandmann@...mi.au.dk>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Subject: Re: Tracing configuration review

On Wed, May 26, 2010 at 01:06:57AM +0200, Frederic Weisbecker wrote:
> On Tue, May 25, 2010 at 05:09:59PM -0400, Chase Douglas wrote:
> > On Tue, 2010-05-25 at 22:13 +0200, Frederic Weisbecker wrote:
> > > On Tue, May 25, 2010 at 03:31:46PM -0400, Chase Douglas wrote:

<snipped>

> > > IMO, it is deprecated. The perf interface is much more powerful and flexible.
> > > Prasad, do you agree if I remove this ftrace plugin?

Sure, go ahead.

> > 
> > If there isn't any use in enabling it due to perf's features, then we
> > can turn it off. However, if there's any use to be gained by this over
> > perf's features, then I'd prefer to leave it on. Thoughts?
> 
> 
> 
> No, perf does much more:
> 
> - stacktraces recording
> - "top" alike view with perf top
> - stat with perf stat, etc...
> - userspace memory accesses
> 
> 
> Here is a quick example:
> 
> $ cat test.c
> int var;
> 
> void func_c(void)
> {
> 	var++;
> }
> 
> void func_b(void)
> {
> 	func_c();
> }
> 
> 
> void func_a(void)
> {
> 	func_c();
> }
> 
> 
> int main(int argc, char **argv)
> {
> 	int i;
> 
> 	for (i = 0; i < 1000; i++)
> 		if (i % 2)
> 			func_a();
> 		else
> 			func_b();
> 
> 	return 0;
> }
> //end test.c
> 
> $ gcc test.c -fno-omit-frame-pointer -o test
> 
> $ readelf -s test | grep var
>     74: 000000000060102c     4 OBJECT  GLOBAL DEFAULT   25 var
> 
> $ perf record -g -c 1 -e mem:0x000000000060102c:w ./test
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.069 MB perf.data (~3020 samples) ]
> 
> $ perf report
> 
> # Events: 1K cycles
> #
> # Overhead  Command      Shared Object  Symbol
> # ........  .......  .................  ......
> #
>     99.90%     test  test               [.] func_c
>                |
>                --- func_c
>                   |          
>                   |--49.95%-- func_a
>                   |          |          
>                   |          |--99.60%-- main
>                   |          |          __libc_start_main
>                   |           --0.40%-- [...]
>                   |          
>                   |--49.85%-- func_b
>                   |          main
>                   |          |          
>                   |          |--99.60%-- __libc_start_main
>                   |           --0.40%-- [...]
>                    --0.20%-- [...]
> 
> 
> To sum up, there is nothing the ksym tracer does that perf can't.
> 

I second Frederic's opinion on this. 

Thanks,
K.Prasad

> Well, may be perf doesn't offer the time ordered view of memory
> accesses, I must confess. Although this is still something we can
> easily provide if people want it.
> 
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