Messages in this thread | | | Date | Tue, 25 Jan 2011 12:58:33 +1100 | Subject | Re: [PATCH 6/6] x86/ticketlock: make __ticket_spin_trylock common | From | Nick Piggin <> |
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On Tue, Jan 25, 2011 at 12:42 PM, Jeremy Fitzhardinge <jeremy@goop.org> wrote: > On 01/24/2011 05:16 PM, Nick Piggin wrote: >> On Tue, Jan 25, 2011 at 10:41 AM, Jeremy Fitzhardinge <jeremy@goop.org> wrote: >>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com> >>> >>> Make trylock code common regardless of ticket size. >> What's the asm for this look like? > > Asm: > > movzwl (%rdi), %eax # lock_1(D)->slock, tmp > cmpb %ah,%al # tmp > leal 0x100(%rax), %edx # tmp, new > jne 1f > lock; cmpxchgw %dx,(%rdi) # new, lock_1(D)->slock > 1: sete %dl # new > movzbl %dl,%eax # new, tmp > > > > C: > > movw (%rdi), %dx # lock_2(D)->D.5949.tickets, old > xorl %eax, %eax # D.13954 > movzbl %dh, %ecx # old, tmp70 > cmpb %dl, %cl # old, tmp70 > jne .L5 #, > leal 256(%rdx), %ecx #, D.13956 > movl %edx, %eax # old, __ret > lock; cmpxchgw %cx,(%rdi) # D.13956,* lock > cmpw %dx, %ax # old, __ret > sete %al #, D.13954 > movzbl %al, %eax # D.13954, D.13954 > .L5: > > > The C version can't take advantage of the fact that the cmpxchg directly > sets the flags, so it ends up re-comparing the old and swapped-out > values to set the return. And it doesn't re-use the same sete to set > the return value in the quick failed-to-acquire path.
Hm.
> It might be worth having a generic cmpxchg() variant which returns a > succeed/fail flag rather than the fetched value, to avoid comparison in > this case - since many (most?) cmpxchg() callers end up doing that > comparison. > > How performance critical is trylock? I guess the ones in fs/dcache.c > are the ones looming large in your mind.
Well they are on on the reclaim/free path rather than the _hottest_ paths, but yes they are performance critical. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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