On Tue, Jan 08, 2013 at 05:32:41PM -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 <r...@redhat.com>
> Signed-off-by: Michel Lespinasse <wal...@google.com>
> ---
Acked-by: Rafael Aquini <aqu...@redhat.com>
> 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;
> }
> }
>
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