linux-kernel - Re: [PATCH] perf annotate: Numeric assembly labels cause incorrect annotation

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Date:	Tue, 7 Feb 2012 09:25:21 -0500
From:	Eric B Munson <emunson@...bm.net>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Frederic Weisbecker <fweisbec@...il.com>, imunsie@....ibm.com,
	eranian@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf annotate: Numeric assembly labels cause incorrect
 annotation

On Tue, 07 Feb 2012, Anton Blanchard wrote:

> 
> A perf annotate of a kernel function written in assembly shows
> very strange percentages:
> 
>          :      _GLOBAL(__copy_tofrom_user_base)
> 
> ...
> 
>          :              addi    r3,r3,1
>    99.67 :      c00000000004d78c:       addi    r3,r3,1
>          :
>     0.00 :      1:      bf      cr7*4+2,2f
>     0.07 :      c00000000004d790:       bne-    cr7,c00000000004d7a4
>          :      err1;   lhz     r0,0(r4)
>     0.00 :      c00000000004d794:       lhz     r0,0(r4)
>          :              addi    r4,r4,2
>     0.00 :      c00000000004d798:       addi    r4,r4,2
>          :      err1;   sth     r0,0(r3)
>     0.00 :      c00000000004d79c:       sth     r0,0(r3)
>          :              addi    r3,r3,2
>    99.60 :      c00000000004d7a0:       addi    r3,r3,2
>          :
>     0.00 :      2:      bf      cr7*4+1,3f
>     0.12 :      c00000000004d7a4:       ble-    cr7,c00000000004d7b8
>          :      err1;   lwz     r0,0(r4)
>     0.00 :      c00000000004d7a8:       lwz     r0,0(r4)
>          :              addi    r4,r4,4
>     0.00 :      c00000000004d7ac:       addi    r4,r4,4
>          :      err1;   stw     r0,0(r3)
>     0.00 :      c00000000004d7b0:       stw     r0,0(r3)
>          :              addi    r3,r3,4
>    99.48 :      c00000000004d7b4:       addi    r3,r3,4
> 
> ~300% in one function. Urgh.
> 
> This is caused by the way we parse objdump -S output, eg:
> 
>         addi    r3,r3,1
> c00000000004d78c:       addi    r3,r3,1
> 
> 1:      bf      cr7*4+2,2f
> c00000000004d790:       bne-    cr7,c00000000004d7a4 
> 
> We assume the asm label (1:) is an address, compute a bogus offset
> into the function and then screw up the matching of samples to lines.
> I notice this also fails with c inline assembly in a similar
> manner.
> 
> We already have a sanity check that the address is not beyond the end
> of the function, so add a check against the start too.
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>

Acked-by: Eric B Munson <emunson@...bm.net>

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