linux-kernel - Re: [tip:perf/core] perf ui annotate browser: Allow toggling addr offset view

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Date:	Fri, 13 Apr 2012 11:30:52 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	mingo@...nel.org, hpa@...or.com, acme@...hat.com, paulus@...ba.org,
	eranian@...gle.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, efault@....de, peterz@...radead.org,
	namhyung@...il.com, fweisbec@...il.com, dsahern@...il.com,
	tglx@...utronix.de
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf ui annotate browser: Allow toggling addr
 offset view

On Fri, Apr 13, 2012 at 11:25 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> <kmem_cache_free>:
>    1.91 :          push   %rbp

Oh, btw, talking about kmem_cache_free: that one uses altinstructions,
and so perf report shows the hottest instruction wrong (and I'm not
talking about "ugly"):

   12.38 :        ffffffff810d7ee5:       lea    (%r8),%rsi
    0.71 :        ffffffff810d7ee8:       callq  ffffffff812d3df0
<this_cpu_cmpxchg16b_emu>

that "lea" really isn't very expensive. In reality, it's not
"lea+call", it's a "lock ; cmpxchg16b + setz" instruction. But "perf"
doesn't know about alternative instructions, and if somebody were to
try to teach it, that would be lovely.

Happily, x86-64 doesn't have quite as many of them as x86-32 does. But
they are there, sometimes in interesting functions.

                 Linus
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