linux-kernel - Re: [RFC PATCH 1/3] x86,smp: move waiting on contended lock out of line

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Message-ID: <20121223225212.GA4186@x61.redhat.com>
Date:	Sun, 23 Dec 2012 20:52:13 -0200
From:	Rafael Aquini <aquini@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, walken@...gle.com,
	lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 1/3] x86,smp: move waiting on contended lock out of
 line

On Fri, Dec 21, 2012 at 06:50:38PM -0500, Rik van Riel wrote:
> Subject: x86,smp: move waiting on contended ticket lock out of line
> 
> Moving the wait loop for congested loops to its own function allows
> us to add things to that wait loop, without growing the size of the
> kernel text appreciably.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---

Reviewed-by: Rafael Aquini <aquini@...hat.com>


>  arch/x86/include/asm/spinlock.h |   13 +++++++------
>  arch/x86/kernel/smp.c           |   14 ++++++++++++++
>  2 files changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index 33692ea..2a45eb0 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -34,6 +34,8 @@
>  # define UNLOCK_LOCK_PREFIX
>  #endif
>  
> +extern void ticket_spin_lock_wait(arch_spinlock_t *, struct __raw_tickets);
> +
>  /*
>   * Ticket locks are conceptually two parts, one indicating the current head of
>   * the queue, and the other indicating the current tail. The lock is acquired
> @@ -53,12 +55,11 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>  
>  	inc = xadd(&lock->tickets, inc);
>  
> -	for (;;) {
> -		if (inc.head == inc.tail)
> -			break;
> -		cpu_relax();
> -		inc.head = ACCESS_ONCE(lock->tickets.head);
> -	}
> +	if (inc.head == inc.tail)
> +		goto out;
> +
> +	ticket_spin_lock_wait(lock, inc);
> + out:
>  	barrier();		/* make sure nothing creeps before the lock is taken */
>  }
>  
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 48d2b7d..20da354 100644
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -113,6 +113,20 @@ static atomic_t stopping_cpu = ATOMIC_INIT(-1);
>  static bool smp_no_nmi_ipi = false;
>  
>  /*
> + * Wait on a congested ticket spinlock.
> + */
> +void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
> +{
> +	for (;;) {
> +		cpu_relax();
> +		inc.head = ACCESS_ONCE(lock->tickets.head);
> +
> +		if (inc.head == inc.tail)
> +			break;
> +	}
> +}
> +
> +/*
>   * this function sends a 'reschedule' IPI to another CPU.
>   * it goes straight through and wastes no time serializing
>   * anything. Worst case is that we lose a reschedule ...
--
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