[Patch (block.git) 2/2] Ensure single IPI generation for SMP call single

[Patch (block.git) 2/2] Ensure single IPI generation for SMP call single

[Alan D. Brunelle]

Signed-off-by: Alan D. Brunelle <alan.brunelle@hp.com>
---
 arch/ia64/kernel/smp.c   |    8 ++++++--
 arch/x86/kernel/smp_32.c |    8 ++++++--
 arch/x86/kernel/smp_64.c |    8 ++++++--
 include/linux/smp.h      |    1 +
 kernel/smp.c             |   34 ++++++++++++++++++++--------------
 5 files changed, 39 insertions(+), 20 deletions(-)

diff --git a/arch/ia64/kernel/smp.c b/arch/ia64/kernel/smp.c
index 04ba9f8..52225bb 100644
--- a/arch/ia64/kernel/smp.c
+++ b/arch/ia64/kernel/smp.c
@@ -364,7 +364,8 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
 {
 	struct call_single_queue *dst;
 	unsigned long flags;
-	int ipi, wait_done = data->flags & CSD_FLAG_WAIT;
+	int ipi = 0;
+	int wait_done = data->flags & CSD_FLAG_WAIT;
 
 	/* Can deadlock when called with interrupts disabled */
 	WARN_ON((data->flags & CSD_FLAG_WAIT) && irqs_disabled());
@@ -373,8 +374,11 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
 	dst = &per_cpu(call_single_queue, cpu);
 
 	spin_lock_irqsave(&dst->lock, flags);
-	ipi = list_empty(&dst->list);
 	list_add_tail(&data->list, &dst->list);
+	if (!dst->activated) {
+		dst->activated = 1;
+		ipi = 1;
+	}
 	spin_unlock_irqrestore(&dst->lock, flags);
 
 	if (ipi)
diff --git a/arch/x86/kernel/smp_32.c b/arch/x86/kernel/smp_32.c
index be0f6cb..d166f13 100644
--- a/arch/x86/kernel/smp_32.c
+++ b/arch/x86/kernel/smp_32.c
@@ -629,7 +629,8 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
 	cpumask_t mask = cpumask_of_cpu(cpu);
 	struct call_single_queue *dst;
 	unsigned long flags;
-	int ipi, wait_done = data->flags & CSD_FLAG_WAIT;
+	int ipi = 0;
+	int wait_done = data->flags & CSD_FLAG_WAIT;
 
 	/* Can deadlock when called with interrupts disabled */
 	WARN_ON((data->flags & CSD_FLAG_WAIT) && irqs_disabled());
@@ -638,8 +639,11 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
 	dst = &per_cpu(call_single_queue, cpu);
 
 	spin_lock_irqsave(&dst->lock, flags);
-	ipi = list_empty(&dst->list);
 	list_add_tail(&data->list, &dst->list);
+	if (!dst->activated) {
+		dst->activated = 1;
+		ipi = 1;
+	}
 	spin_unlock_irqrestore(&dst->lock, flags);
 
 	if (ipi)
diff --git a/arch/x86/kernel/smp_64.c b/arch/x86/kernel/smp_64.c
index 5bef2a6..fbfcdce 100644
--- a/arch/x86/kernel/smp_64.c
+++ b/arch/x86/kernel/smp_64.c
@@ -449,7 +449,8 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
 	cpumask_t mask = cpumask_of_cpu(cpu);
 	struct call_single_queue *dst;
 	unsigned long flags;
-	int ipi, wait_done = data->flags & CSD_FLAG_WAIT;
+	int ipi = 0;
+	int wait_done = data->flags & CSD_FLAG_WAIT;
 
 	/* Can deadlock when called with interrupts disabled */
 	WARN_ON((data->flags & CSD_FLAG_WAIT) && irqs_disabled());
@@ -458,8 +459,11 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
 	dst = &per_cpu(call_single_queue, cpu);
 
 	spin_lock_irqsave(&dst->lock, flags);
-	ipi = list_empty(&dst->list);
 	list_add_tail(&data->list, &dst->list);
+	if (!dst->activated) {
+		dst->activated = 1;
+		ipi = 1;
+	}
 	spin_unlock_irqrestore(&dst->lock, flags);
 
 	if (ipi)
diff --git a/include/linux/smp.h b/include/linux/smp.h
index c471d77..1e96f03 100644
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -65,6 +65,7 @@ extern void smp_cpus_done(unsigned int max_cpus);
 struct call_single_queue {
 	spinlock_t lock;
 	struct list_head list;
+	int activated;
 };
 
 /*
diff --git a/kernel/smp.c b/kernel/smp.c
index 7232e1c..f7ec401 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -23,28 +23,34 @@ void __cpuinit generic_init_call_single_data(void)
 
 void generic_smp_call_function_single_interrupt(void)
 {
-	struct call_single_queue *q;
 	unsigned long flags;
 	LIST_HEAD(list);
+	struct call_single_queue *q = &__get_cpu_var(call_single_queue);
 
-	q = &__get_cpu_var(call_single_queue);
 	spin_lock_irqsave(&q->lock, flags);
-	list_replace_init(&q->list, &list);
-	spin_unlock_irqrestore(&q->lock, flags);
+	while (!list_empty(&q->list)) {
+		list_replace_init(&q->list, &list);
+		spin_unlock_irqrestore(&q->lock, flags);
 
-	while (!list_empty(&list)) {
-		struct call_single_data *data;
+		do {
+			struct call_single_data *data;
 
-		data = list_entry(list.next, struct call_single_data, list);
-		list_del(&data->list);
+			data = list_entry(list.next, struct call_single_data,
+						list);
+			list_del(&data->list);
 
-		data->func(data->info);
-		if (data->flags & CSD_FLAG_WAIT) {
-			smp_wmb();
-			data->flags = 0;
-		} else if (data->flags & CSD_FLAG_ALLOC)
-			kfree(data);
+			data->func(data->info);
+			if (data->flags & CSD_FLAG_WAIT) {
+				smp_wmb();
+				data->flags = 0;
+			} else if (data->flags & CSD_FLAG_ALLOC)
+				kfree(data);
+		} while (!list_empty(&list));
+
+		spin_lock_irqsave(&q->lock, flags);
 	}
+	q->activated = 0;
+	spin_unlock_irqrestore(&q->lock, flags);
 }
 
 int generic_smp_call_function_single(int cpu, void (*func) (void *info),
-- 
1.5.2.5

Re: [Patch (block.git) 2/2] Ensure single IPI generation for SMP call single

[Jens Axboe]

On Mon, Mar 17 2008, Alan D. Brunelle wrote:
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 7232e1c..f7ec401 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -23,28 +23,34 @@ void __cpuinit generic_init_call_single_data(void)
>  
>  void generic_smp_call_function_single_interrupt(void)
>  {
> -	struct call_single_queue *q;
>  	unsigned long flags;
>  	LIST_HEAD(list);
> +	struct call_single_queue *q = &__get_cpu_var(call_single_queue);
>  
> -	q = &__get_cpu_var(call_single_queue);
>  	spin_lock_irqsave(&q->lock, flags);
> -	list_replace_init(&q->list, &list);
> -	spin_unlock_irqrestore(&q->lock, flags);
> +	while (!list_empty(&q->list)) {
> +		list_replace_init(&q->list, &list);
> +		spin_unlock_irqrestore(&q->lock, flags);
>  
> -	while (!list_empty(&list)) {
> -		struct call_single_data *data;
> +		do {
> +			struct call_single_data *data;
>  
> -		data = list_entry(list.next, struct call_single_data, list);
> -		list_del(&data->list);
> +			data = list_entry(list.next, struct call_single_data,
> +						list);
> +			list_del(&data->list);
>  
> -		data->func(data->info);
> -		if (data->flags & CSD_FLAG_WAIT) {
> -			smp_wmb();
> -			data->flags = 0;
> -		} else if (data->flags & CSD_FLAG_ALLOC)
> -			kfree(data);
> +			data->func(data->info);
> +			if (data->flags & CSD_FLAG_WAIT) {
> +				smp_wmb();
> +				data->flags = 0;
> +			} else if (data->flags & CSD_FLAG_ALLOC)
> +				kfree(data);
> +		} while (!list_empty(&list));
> +
> +		spin_lock_irqsave(&q->lock, flags);
>  	}
> +	q->activated = 0;
> +	spin_unlock_irqrestore(&q->lock, flags);
>  }

I agree with doing it this way, re-checking and doing another run (or
more). However I think we can improve it a bit so we don't always have
to grab the dst lock at least twice - it should be safe enough to
include the lock only inside the first loop, doing an smp_mb() before
the list_empty() check and again at the bottom before looping around and
doing the list_empty() check again.

I've rolled a new patch series here:

http://git.kernel.dk/?p=linux-2.6-block.git;a=shortlog;h=io-cpu-affinity

(or just pull the io-cpu-affinity branch), it also includes a bunch of
other cleanups like porting the faster smp_call_function() to ia64 and
powerpc as well. So the kernel/smp.c generic helpers have grown a bit,
while the arch bits are smaller.

I'm curious if it now boots on ia64, since I killed the hack to manually
call the __init manually there. If you could check, I would appreciate
it ;-)

I've built all 4 supported archs and they compile and link fine, but
nothing has been booted yet.

-- 
Jens Axboe

Re: [Patch (block.git) 2/2] Ensure single IPI generation for SMP call single

[Jens Axboe]

On Mon, Mar 17 2008, Jens Axboe wrote:
> On Mon, Mar 17 2008, Alan D. Brunelle wrote:
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 7232e1c..f7ec401 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -23,28 +23,34 @@ void __cpuinit generic_init_call_single_data(void)
> >  
> >  void generic_smp_call_function_single_interrupt(void)
> >  {
> > -	struct call_single_queue *q;
> >  	unsigned long flags;
> >  	LIST_HEAD(list);
> > +	struct call_single_queue *q = &__get_cpu_var(call_single_queue);
> >  
> > -	q = &__get_cpu_var(call_single_queue);
> >  	spin_lock_irqsave(&q->lock, flags);
> > -	list_replace_init(&q->list, &list);
> > -	spin_unlock_irqrestore(&q->lock, flags);
> > +	while (!list_empty(&q->list)) {
> > +		list_replace_init(&q->list, &list);
> > +		spin_unlock_irqrestore(&q->lock, flags);
> >  
> > -	while (!list_empty(&list)) {
> > -		struct call_single_data *data;
> > +		do {
> > +			struct call_single_data *data;
> >  
> > -		data = list_entry(list.next, struct call_single_data, list);
> > -		list_del(&data->list);
> > +			data = list_entry(list.next, struct call_single_data,
> > +						list);
> > +			list_del(&data->list);
> >  
> > -		data->func(data->info);
> > -		if (data->flags & CSD_FLAG_WAIT) {
> > -			smp_wmb();
> > -			data->flags = 0;
> > -		} else if (data->flags & CSD_FLAG_ALLOC)
> > -			kfree(data);
> > +			data->func(data->info);
> > +			if (data->flags & CSD_FLAG_WAIT) {
> > +				smp_wmb();
> > +				data->flags = 0;
> > +			} else if (data->flags & CSD_FLAG_ALLOC)
> > +				kfree(data);
> > +		} while (!list_empty(&list));
> > +
> > +		spin_lock_irqsave(&q->lock, flags);
> >  	}
> > +	q->activated = 0;
> > +	spin_unlock_irqrestore(&q->lock, flags);
> >  }
> 
> I agree with doing it this way, re-checking and doing another run (or
> more). However I think we can improve it a bit so we don't always have
> to grab the dst lock at least twice - it should be safe enough to
> include the lock only inside the first loop, doing an smp_mb() before
> the list_empty() check and again at the bottom before looping around and
> doing the list_empty() check again.
> 
> I've rolled a new patch series here:
> 
> http://git.kernel.dk/?p=linux-2.6-block.git;a=shortlog;h=io-cpu-affinity
> 
> (or just pull the io-cpu-affinity branch), it also includes a bunch of
> other cleanups like porting the faster smp_call_function() to ia64 and
> powerpc as well. So the kernel/smp.c generic helpers have grown a bit,
> while the arch bits are smaller.
> 
> I'm curious if it now boots on ia64, since I killed the hack to manually
> call the __init manually there. If you could check, I would appreciate
> it ;-)
> 
> I've built all 4 supported archs and they compile and link fine, but
> nothing has been booted yet.

x86 and x86-64 boot just fine, so powerpc should work as well (will do
performance testing on a 4-way ppc tomorrow). ia64 should also work, as
long as the init_call_single_data() gets called correctly.

If it doesn't work on tha ia64, try and change the core_initcall() to a
postcore_initcall() or even an arch_initcall(). It now resides in
kernel/smp.c.

-- 
Jens Axboe