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

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Date:	Thu, 3 Jan 2013 17:05:44 +0530
From:	Raghavendra KT <raghavendra.kt.linux@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, aquini@...hat.com, walken@...gle.com,
	eric.dumazet@...il.com, lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>, knoel@...hat.com,
	Raghavendra KT <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 2/5] x86,smp: proportional backoff for ticket spinlocks

[Ccing IBM id]
On Thu, Jan 3, 2013 at 10:52 AM, Rik van Riel <riel@...hat.com> wrote:
> 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@...hat.com>
> Signed-off-by: Michel Lespinasse <walken@...gle.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..9c56fe3 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;
                                             ^^^^^^^^^^^^^^
Just wondering,
Does wraparound affects this?
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