linux-kernel - Re: [PATCH 6/6] x86/ticketlock: make __ticket_spin

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Date:	Tue, 25 Jan 2011 12:58:33 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 6/6] x86/ticketlock: make __ticket_spin_trylock common

On Tue, Jan 25, 2011 at 12:42 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> On 01/24/2011 05:16 PM, Nick Piggin wrote:
>> On Tue, Jan 25, 2011 at 10:41 AM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.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.
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