[PATCH 0/6] Clean up ticketlock implementation

From: Jeremy Fitzhardinge
Date: Mon Jan 24 2011 - 18:42:23 EST

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

Hi all,

This series cleans up the x86 ticketlock implementation by converting
a large proportion of it to C. This eliminates the need for having
separate implementations for "large" (NR_CPUS >= 256) and "small"
(NR_CPUS < 256) ticket locks.

This also lays the groundwork for future changes to the ticketlock

Of course, the big question when converting from assembler to C is
what the compiler will do to the code. In general, the results are
very similar.

For example, the original hand-coded small-ticket ticket_lock is:
movl $256, %eax
lock xadd %ax,(%rdi)
1: cmp %ah,%al
je 2f
mov (%rdi),%al
jmp 1b

The C version, compiled by gcc 4.5.1 is:
movl $256, %eax
lock; xaddw %ax, (%rdi)
movzbl %ah, %edx
.L3: cmpb %dl, %al
je .L2
rep; nop
movb (%rdi), %al # lock_1(D)->D.5949.tickets.head, inc$head
jmp .L3 #

So very similar, except the compiler misses directly comparing
%ah to %al.

With big tickets, which is what distros are typically compiled with,
the results are:

movl $65536, %eax #, inc
lock; xaddl %eax, (%rdi) # inc, lock_2(D)->slock
movzwl %ax, %edx # inc, tmp
shrl $16, %eax # inc
1: cmpl %eax, %edx # inc, tmp
je 2f
rep ; nop
movzwl (%rdi), %edx # lock_2(D)->slock, tmp
jmp 1b

Compiled C:
movl $65536, %eax #, tickets
lock; xaddl %eax, (%rdi) # tickets, lock_1(D)->D.5952.tickets
movl %eax, %edx # tickets,
shrl $16, %edx #,
.L3: cmpw %dx, %ax # tickets$tail, inc$head
je .L2 #,
rep; nop
movw (%rdi), %ax # lock_1(D)->D.5952.tickets.head, inc$head
jmp .L3 #

In this case the code is pretty much identical except for slight
variations in where the 32-bit values are truncated to 16.

So overall, I think this change will have negligable performance


Jeremy Fitzhardinge (6):
x86/ticketlock: clean up types and accessors
x86/ticketlock: convert spin loop to C
x86/ticketlock: Use C for __ticket_spin_unlock
x86/ticketlock: make large and small ticket versions of spin_lock the
x86/ticketlock: make __ticket_spin_lock common
x86/ticketlock: make __ticket_spin_trylock common

arch/x86/include/asm/spinlock.h | 146 ++++++++++++---------------------
arch/x86/include/asm/spinlock_types.h | 22 +++++-
2 files changed, 73 insertions(+), 95 deletions(-)


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