linux-kernel - Re: [PATCH 3.18-rc3 v9 1/5] irqchip: gic: Finer grain locking for gic_raise

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Message-ID: <5474BF19.3040707@arm.com>
Date:	Tue, 25 Nov 2014 17:40:41 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Daniel Thompson <daniel.thompson@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>
CC:	Russell King <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Daniel Drake <drake@...lessm.com>,
	Dmitry Pervushin <dpervushin@...il.com>,
	Tim Sander <tim@...eglstein.org>
Subject: Re: [PATCH 3.18-rc3 v9 1/5] irqchip: gic: Finer grain locking for
 gic_raise_softirq

Hi Daniel,

On 25/11/14 17:26, Daniel Thompson wrote:
> irq_controller_lock is used for multiple purposes within the gic driver.
> Primarily it is used to make register read-modify-write sequences atomic.
> It is also used by gic_raise_softirq() in order that the big.LITTLE
> migration logic can figure out when it is safe to migrate interrupts
> between physical cores.
> 
> The second usage of irq_controller_lock is difficult to discern when
> reviewing the code because the migration itself takes place outside
> the lock.
> 
> This patch makes the second usage more explicit by splitting it out into
> a separate lock and providing better comments.

While we're at it, how about an additional patch that would make this
lock disappear entirely when the big-little stuff is not compiled in,
which is likely to be the case on a lot of (dare I say most?) systems?
That will save expensive barriers that we definitely could do without.

It otherwise looks good to me.

Thanks,

	M.

> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Jason Cooper <jason@...edaemon.net>
> Cc: Russell King <linux@....linux.org.uk>
> Cc: Marc Zyngier <marc.zyngier@....com>
> ---
>  drivers/irqchip/irq-gic.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
> index 38493ff28fa5..bb4bc20573ea 100644
> --- a/drivers/irqchip/irq-gic.c
> +++ b/drivers/irqchip/irq-gic.c
> @@ -73,6 +73,12 @@ struct gic_chip_data {
>  static DEFINE_RAW_SPINLOCK(irq_controller_lock);
>  
>  /*
> + * This lock is used by the big.LITTLE migration code to ensure no
> + * IPIs can be pended on the old core after the map has been updated.
> + */
> +static DEFINE_RAW_SPINLOCK(cpu_map_migration_lock);
> +
> +/*
>   * The GIC mapping of CPU interfaces does not necessarily match
>   * the logical CPU numbering.  Let's use a mapping as returned
>   * by the GIC itself.
> @@ -624,7 +630,7 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>  	int cpu;
>  	unsigned long flags, map = 0;
>  
> -	raw_spin_lock_irqsave(&irq_controller_lock, flags);
> +	raw_spin_lock_irqsave(&cpu_map_migration_lock, flags);
>  
>  	/* Convert our logical CPU mask into a physical one. */
>  	for_each_cpu(cpu, mask)
> @@ -639,7 +645,7 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>  	/* this always happens on GIC0 */
>  	writel_relaxed(map << 16 | irq, gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
>  
> -	raw_spin_unlock_irqrestore(&irq_controller_lock, flags);
> +	raw_spin_unlock_irqrestore(&cpu_map_migration_lock, flags);
>  }
>  #endif
>  
> @@ -710,8 +716,17 @@ void gic_migrate_target(unsigned int new_cpu_id)
>  
>  	raw_spin_lock(&irq_controller_lock);
>  
> -	/* Update the target interface for this logical CPU */
> +	/*
> +	 * Update the target interface for this logical CPU
> +	 *
> +	 * From the point we release the cpu_map_migration_lock any new
> +	 * SGIs will be pended on the new cpu which makes the set of SGIs
> +	 * pending on the old cpu static. That means we can defer the
> +	 * migration until after we have released the irq_controller_lock.
> +	 */
> +	raw_spin_lock(&cpu_map_migration_lock);
>  	gic_cpu_map[cpu] = 1 << new_cpu_id;
> +	raw_spin_unlock(&cpu_map_migration_lock);
>  
>  	/*
>  	 * Find all the peripheral interrupts targetting the current
> 


-- 
Jazz is not dead. It just smells funny...
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