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SubjectRe: [RFC PATCH 2/3] x86,smp: proportional backoff for ticket spinlocks
On Fri, Dec 21, 2012 at 06:51:15PM -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.
>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
> arch/x86/kernel/smp.c | 6 ++++--
> 1 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 20da354..4e44840 100644
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -118,9 +118,11 @@ static bool smp_no_nmi_ipi = false;
> void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
> {
> for (;;) {
> - cpu_relax();
> - inc.head = ACCESS_ONCE(lock->tickets.head);
> + int loops = 50 * (__ticket_t)(inc.tail - inc.head);
> + while (loops--)
> + cpu_relax();

-ENOCOMMENT

Please add a comment above to explain what it's doing. Don't expect
people to check change logs. Also, explain why you picked 50.

-- Steve

>
> + inc.head = ACCESS_ONCE(lock->tickets.head);
> if (inc.head == inc.tail)
> break;
> }
>
> --
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