Re: [PATCH 2/5] x86,smp: proportional backoff for ticket spinlocks

[Rik van Riel]

On 01/08/2013 05:50 PM, Eric Dumazet wrote:
> On Tue, 2013-01-08 at 17:32 -0500, Rik van Riel wrote:
> > Subject: x86,smp: proportional backoff for ticket spinlocks
> >
> > Simple fixed value proportional backoff for ticket spinlocks.
> > By pounding on the cacheline with the spin lock less often,
> > bus traffic is reduced. In cases of a data structure with
> > embedded spinlock, the lock holder has a better chance of
> > making progress.
> >
> > If we are next in line behind the current holder of the
> > lock, we do a fast spin, so as not to waste any time when
> > the lock is released.
> >
> > The number 50 is likely to be wrong for many setups, and
> > this patch is mostly to illustrate the concept of proportional
> > backup. The next patch automatically tunes the delay value.
> >
> > Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> > Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
> > ---
> >   arch/x86/kernel/smp.c |   23 ++++++++++++++++++++---
> >   1 files changed, 20 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> > index 20da354..aa743e9 100644
> > --- a/arch/x86/kernel/smp.c
> > +++ b/arch/x86/kernel/smp.c
> > @@ -117,11 +117,28 @@ static bool smp_no_nmi_ipi = false;
> >    */
> >   void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
> >   {
> > +	__ticket_t head = inc.head, ticket = inc.tail;
> > +	__ticket_t waiters_ahead;
> > +	unsigned loops;
> > +
> >   	for (;;) {
> > -		cpu_relax();
> > -		inc.head = ACCESS_ONCE(lock->tickets.head);
> > +		waiters_ahead = ticket - head - 1;
> > +		/*
> > +		 * We are next after the current lock holder. Check often
> > +		 * to avoid wasting time when the lock is released.
> > +		 */
> > +		if (!waiters_ahead) {
> > +			do {
> > +				cpu_relax();
> > +			} while (ACCESS_ONCE(lock->tickets.head) != ticket);
> > +			break;
> > +		}
> > +		loops = 50 * waiters_ahead;
> > +		while (loops--)
> > +			cpu_relax();
> >
> > -		if (inc.head == inc.tail)
> > +		head = ACCESS_ONCE(lock->tickets.head);
> > +		if (head == ticket)
> >   			break;
> >   	}
> >   }
>
> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> In my tests, I used the following formula :
>
> loops = 50 * ((ticket - head) - 1/2);
>
> or :
>
> delta = ticket - head;
> loops = delay * delta - (delay >> 1);

I suppose that rounding down the delta might result
in more stable results, due to undersleeping less
often.

> (And I didnt use the special :
>
> 	if (!waiters_ahead) {
> 		do {
> 			cpu_relax();
> 		} while (ACCESS_ONCE(lock->tickets.head) != ticket);
> 		break;
> 	}
>
> Because it means this wont help machines with 2 cpus.
>
> (or more generally if there _is_ contention, but with
> one lock holder and one lock waiter)

Machines with 2 CPUs should not need help, because the
cpu_relax() alone gives enough of a pause that the lock
holder can make progress.

It may be interesting to try out your rounding-down of
delta, to see if that makes things better.

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