Re: [PATCH 00/13] [PATCH RFC] Paravirtualized ticketlocks

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 09/02/2011 08:38 AM, Linus Torvalds wrote:
> On Thu, Sep 1, 2011 at 5:54 PM, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>> The inner part of ticket lock code becomes:
>>        inc = xadd(&lock->tickets, inc);
>>        inc.tail &= ~TICKET_SLOWPATH_FLAG;
>>
>>        for (;;) {
>>                unsigned count = SPIN_THRESHOLD;
>>
>>                do {
>>                        if (inc.head == inc.tail)
>>                                goto out;
>>                        cpu_relax();
>>                        inc.head = ACCESS_ONCE(lock->tickets.head);
>>                } while (--count);
>>                __ticket_lock_spinning(lock, inc.tail);
>>        }
> Hmm. It strikes me that I don't think you should touch the
> TICKET_SLOWPATH_FLAG in the fastpath at all.
>
> Can't you just do this:
>
>    inc = xadd(&lock->tickets, inc);
>    if (likely(inc.head == inc.tail))
>      goto out;
>
>    ### SLOWPATH ###
>    inc.tail &= ~TICKET_SLOWPATH_FLAG;
>    for (;;) {
>       .. as before ..
>
> which might alleviate the problem with the fastpath being polluted by
> all those silly slowpath things.  Hmm?
>
> (This assumes that TICKET_SLOWPATH_FLAG is never set in inc.head, so
> if it's set that equality check will fail. I didn't actually check if
> that assumption was correct)

Yes, nice idea.  That ends up making the overall code slightly longer,
but the fastpath becomes identical to the non-pv case:

	mov    $512,%ecx
	lock xadd %cx,(%rdi)
	movzbl %ch,%edx
	cmp    %cl,%dl
	je     2f

	### SLOWPATH START
	and    $-2,%edx
	mov    $8192,%eax
	movzbl %dl,%esi
1:	cmp    %dl,%cl
	je     2f
	pause  
	dec    %eax
	mov    (%rdi),%cl
	jne    1b
	callq  __ticket_lock_spinning
	mov    $8192,%eax
	jmp    1b
	### SLOWPATH ENDS

2:


It's especially nice that it also moves the spin counter and arg setup
into the slowpath code.

And that entire piece of slowpath code can be moved out into its own
function, so the fastpath becomes:

	mov    $512,%eax
	lock xadd %ax,(%rdi)
	movzbl %ah,%esi
	cmp    %al,%sil
	je     1f

	movzbl %sil,%esi
	callq  __ticket_lock_slow
1:

I don't know whether that fastpath code is small enough to consider
inlining everywhere?

    J