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

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Message-ID: <5475B40F.5030209@linaro.org>
Date:	Wed, 26 Nov 2014 11:05:51 +0000
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>,
	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

On 25/11/14 17:40, Marc Zyngier wrote:
> 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.

Will do.


> 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
>>
> 
> 

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