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SubjectRe: [PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate code
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On Thu, 2011-06-16 at 14:40 -0700, Jeremy Fitzhardinge wrote:
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
>
> Hi all,
>
> I'm proposing this series for 3[.0].1.
>
> This is a repost of a series to clean up the x86 ticket lock
> code by converting it to a mostly C implementation and removing
> lots of duplicate code relating to the ticket size.
>
> The last time I posted this series, the only significant comments
> were from Nick Piggin, specifically relating to:
>
> 1. A wrongly placed barrier on unlock (which may have allowed the
> compiler to move things out of the locked region. I went
> belt-and-suspenders by having two barriers to prevent motion
> into or out of the locked region.
>
> 2. With NR_CPUS < 256 the ticket size is 8 bits. The compiler doesn't
> use the same trick as the hand-coded asm to directly compare the high
> and low bytes in the word, but does a bit of extra shuffling around.
> However, the Intel optimisation guide and several x86 experts have
> opined that its best to avoid the high-byte operations anyway, since
> they will cause a partial word stall, and the gcc-generated code should
> be better.
>
> Overall the compiler-generated code is very similar to the hand-coded
> versions, with the partial byte operations being the only significant
> difference. (Curiously, gcc does generate a high-byte compare for me
> in trylock, so it can if it wants to.)
>
> I've been running with this code in place for several months on 4 core
> systems without any problems.
>
> I couldn't measure a consistent performance difference between the two
> implemenations; there seemed to be +/- ~1% +/-, which is the level of
> variation I see from simply recompiling the kernel with slightly
> different code alignment.
>
> Overall, I think the large reduction in code size is a big win.

No complaints from me, I rather like the result.

One other thing you could contemplate is adding something like:

#define xadd(ptr, inc) \
do { \
switch(sizeof(*(ptr))) { \
case 1: \
asm volatile (LOCK_PREFIX "xaddb %0, %1\n" \
: "+r" (inc), "+m" (*(ptr)) \
: : "memory", "cc"); \
case 2:
... xaddw ...
case 4:
... xaddl ...
} while (0)

and a similar something for inc. For both there seem to be various asm
bits left (we could even consider adding xadd to
arch/x86/include/asm/cmpxchg*.h).

Also, you might have wanted to CC Linus on this, he's usually interested
in these bits.


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