linux-kernel - Re: [RFC PATCH 2/3] x86,smp: proportional backoff for ticket spinlocks

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Date:	Fri, 21 Dec 2012 22:07:56 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, aquini@...hat.com, walken@...gle.com,
	lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [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@...hat.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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