Re: Linux 2.6.32-rc1

Ingo Molnar
2009-09-30T18:53:30Z

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, 30 Sep 2009, Eric Dumazet wrote:
> >
> > lock; cmpxchg8b (%%esi)
> >
> > gives 4 bytes opcode : f0 0f c7 0e
> > Because alternative (call cmpxchg8b_emu) uses 5 bytes, a nop will be added.
> >
> > Choosing ".byte 0xf0, 0x0f, 0xc7, 0x4e, 0x00" aka "lock cmpxchg8b 0x0(%esi)" is a litle bit better ?
>
> And if you want to be really clever, you would want to handle the
> non-SMP case too, where you have just "cmpxchgb (%%esi)" (three bytes)
> without the lock prefix.
>
> However, at this point I think Arjan's patch is already way superior
> to what we have now (feel free to take a look at what we generate on
> 32-bit without PAE today - just have a barf-bag handy), so all I'd
> really want is a few "tested-by"s to make me feel happier about it,
> and a few more people looking at the emulation routine to all say "ok,
> looks sane, ACK".
>
> And at that point we can then either make "cmpxchg()" just do the
> 8-byte case natively, or just take your patch to change sched_clock.c
> to now use the no-longer-entirely-disgusting cmpxchg64().
>
> Ingo - I suspect both those patches should just go through you. You do
> both x86 and scheduler, so I'll happily pull the end result.

Yeah - working on it - just got back from a trip. It's two risky patches
and if that place breaks everyone will be affected so i'll probably send
the pull request tomorrow.

Ingo