On Fri, Apr 13, 2012 at 11:25 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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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