Message-Id: <200810270940.21617.rusty@rustcorp.com.au>
Date:	Mon, 27 Oct 2008 09:40:21 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip/cpus4096-v2] cpumask: fix cpumask of call_function_data

On Saturday 25 October 2008 08:46:08 Hiroshi Shimamoto wrote:
> Rusty Russell wrote:
> > (Compiles, untested).
>
> comments below, not tested.

> > +	/* So, what's a CPU they want?  Ignoring this one. */
> > +	cpu = cpumask_first_and(mask, cpu_online_mask);
> > +	if (cpu == smp_processor_id())
> > +		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> > +	/* Nothing?  We're done. */
> > +	if (cpu >= nr_cpu_ids)
> > +		return;
> > +
> > +	/* Do we have another CPU which isn't us? */
> > +	next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
>
> I'm not sure,
> if next_cpu == smp_processor_id() &&
> 	cpumask_next_and(next_cpu, ...) >= nr_cpu_ids
>
> we can go fastpath, right?

Yes, that was intent, and after your fix below it should work.

> > +	if (cpu == smp_processor_id())
>
> so, next_cpu == smp_processor_id() ?

Oh, yes.  That is correct.

> > +		next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
> > +
> > +	/* Nope!  Fastpath: do that cpu by itself. */
> > +	if (next_cpu >= nr_cpu_ids)
> > +		smp_call_function_single(cpu, func, info, wait);
>
> first path, should return here?

First path is when no online CPUs are in mask at all.  This is when one
online CPU is in mask.

> >  		/* Slow path. */
> >  		for_each_online_cpu(cpu) {
> >  			if (cpumask_test_cpu(cpu, mask))
>
> I guess, another issue, should skip when cpu == smp_processor_id().

Yes, thanks!

> > +	data->csd.flags = CSD_FLAG_ALLOC;
> > +	if (wait)
> > +		data->csd.flags |= CSD_FLAG_WAIT;
> >  	data->csd.func = func;
> >  	data->csd.info = info;
> > -	data->refs = num_cpus;
> > -	data->cpumask = allbutself;
> > +	cpumask_and(to_cpumask(data->cpumask_bits), mask, cpu_online_mask);
>
> I guess, clear itself is needed.

Indeed, thanks.

Here is updated patch, also tested, and I forced the kmalloc failure path
to test that too...

From: Rusty Russell <rusty@...tcorp.com.au>
cpumask: smp_call_function_many()

Actually change smp_call_function_mask() to smp_call_function_many().

S390 has its own version, so we do trivial conversion on that too.

We have to do some dancing to figure out if 0 or 1 other cpus are in
the mask supplied and the online mask without allocating a tmp
cpumask.  It's still fairly cheap.

We allocate the cpumask at the end of the call_function_data
structure: if allocation fails we fallback to smp_call_function_single
rather than using the baroque quiescing code.

(Thanks to Hiroshi Shimamoto for spotting several bugs in previous versions!)

Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Signed-off-by: Mike Travis <travis@....com>
Cc: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
Cc: schwidefsky@...ibm.com
Cc: heiko.carstens@...ibm.com
---
 arch/s390/include/asm/smp.h |    3 
 arch/s390/kernel/smp.c      |    9 +-
 include/linux/smp.h         |   15 ++--
 kernel/smp.c                |  137 +++++++++++++++-----------------------------
 4 files changed, 60 insertions(+), 104 deletions(-)

diff -r f03d70de1da6 arch/s390/include/asm/smp.h
--- a/arch/s390/include/asm/smp.h	Sun Oct 26 19:17:26 2008 +1100
+++ b/arch/s390/include/asm/smp.h	Mon Oct 27 09:16:22 2008 +1100
@@ -90,9 +90,6 @@ extern int __cpu_up (unsigned int cpu);
 
 extern struct mutex smp_cpu_state_mutex;
 extern int smp_cpu_polarization[];
-
-extern int smp_call_function_mask(cpumask_t mask, void (*func)(void *),
-	void *info, int wait);
 #endif
 
 #ifndef CONFIG_SMP
diff -r f03d70de1da6 arch/s390/kernel/smp.c
--- a/arch/s390/kernel/smp.c	Sun Oct 26 19:17:26 2008 +1100
+++ b/arch/s390/kernel/smp.c	Mon Oct 27 09:16:22 2008 +1100
@@ -199,7 +199,7 @@ EXPORT_SYMBOL(smp_call_function_single);
 EXPORT_SYMBOL(smp_call_function_single);
 
 /**
- * smp_call_function_mask(): Run a function on a set of other CPUs.
+ * smp_call_function_many(): Run a function on a set of other CPUs.
  * @mask: The set of cpus to run on.  Must not include the current cpu.
  * @func: The function to run. This must be fast and non-blocking.
  * @info: An arbitrary pointer to pass to the function.
@@ -213,16 +213,17 @@ EXPORT_SYMBOL(smp_call_function_single);
  * You must not call this function with disabled interrupts or from a
  * hardware interrupt handler or from a bottom half handler.
  */
-int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,
-			   int wait)
+int smp_call_function_many(const struct cpumask *maskp,
+			   void (*func)(void *), void *info, bool wait)
 {
+	cpumask_t mask = *maskp;
 	spin_lock(&call_lock);
 	cpu_clear(smp_processor_id(), mask);
 	__smp_call_function_map(func, info, wait, mask);
 	spin_unlock(&call_lock);
 	return 0;
 }
-EXPORT_SYMBOL(smp_call_function_mask);
+EXPORT_SYMBOL(smp_call_function_many);
 
 void smp_send_stop(void)
 {
diff -r f03d70de1da6 include/linux/smp.h
--- a/include/linux/smp.h	Sun Oct 26 19:17:26 2008 +1100
+++ b/include/linux/smp.h	Mon Oct 27 09:16:22 2008 +1100
@@ -64,15 +64,16 @@ extern void smp_cpus_done(unsigned int m
  * Call a function on all other processors
  */
 int smp_call_function(void(*func)(void *info), void *info, int wait);
-/* Deprecated: use smp_call_function_many() which uses a cpumask ptr. */
-int smp_call_function_mask(cpumask_t mask, void(*func)(void *info), void *info,
-				int wait);
+void smp_call_function_many(const struct cpumask *mask,
+			    void (*func)(void *info), void *info, bool wait);
 
-static inline void smp_call_function_many(const struct cpumask *mask,
-					  void (*func)(void *info), void *info,
-					  int wait)
+/* Deprecated: Use smp_call_function_many which takes a pointer to the mask. */
+static inline int
+smp_call_function_mask(cpumask_t mask, void(*func)(void *info), void *info,
+		       int wait)
 {
-	smp_call_function_mask(*mask, func, info, wait);
+	smp_call_function_many(&mask, func, info, wait);
+	return 0;
 }
 
 int smp_call_function_single(int cpuid, void (*func) (void *info), void *info,
diff -r f03d70de1da6 kernel/smp.c
--- a/kernel/smp.c	Sun Oct 26 19:17:26 2008 +1100
+++ b/kernel/smp.c	Mon Oct 27 09:16:22 2008 +1100
@@ -24,8 +24,8 @@ struct call_function_data {
 	struct call_single_data csd;
 	spinlock_t lock;
 	unsigned int refs;
-	cpumask_t cpumask;
 	struct rcu_head rcu_head;
+	unsigned long cpumask_bits[];
 };
 
 struct call_single_queue {
@@ -109,13 +109,13 @@ void generic_smp_call_function_interrupt
 	list_for_each_entry_rcu(data, &call_function_queue, csd.list) {
 		int refs;
 
-		if (!cpu_isset(cpu, data->cpumask))
+		if (!cpumask_test_cpu(cpu, to_cpumask(data->cpumask_bits)))
 			continue;
 
 		data->csd.func(data->csd.info);
 
 		spin_lock(&data->lock);
-		cpu_clear(cpu, data->cpumask);
+		cpumask_clear_cpu(cpu, to_cpumask(data->cpumask_bits));
 		WARN_ON(data->refs == 0);
 		data->refs--;
 		refs = data->refs;
@@ -265,50 +265,12 @@ void __smp_call_function_single(int cpu,
 	generic_exec_single(cpu, data);
 }
 
-/* Dummy function */
-static void quiesce_dummy(void *unused)
-{
-}
-
-/*
- * Ensure stack based data used in call function mask is safe to free.
- *
- * This is needed by smp_call_function_mask when using on-stack data, because
- * a single call function queue is shared by all CPUs, and any CPU may pick up
- * the data item on the queue at any time before it is deleted. So we need to
- * ensure that all CPUs have transitioned through a quiescent state after
- * this call.
- *
- * This is a very slow function, implemented by sending synchronous IPIs to
- * all possible CPUs. For this reason, we have to alloc data rather than use
- * stack based data even in the case of synchronous calls. The stack based
- * data is then just used for deadlock/oom fallback which will be very rare.
- *
- * If a faster scheme can be made, we could go back to preferring stack based
- * data -- the data allocation/free is non-zero cost.
- */
-static void smp_call_function_mask_quiesce_stack(cpumask_t mask)
-{
-	struct call_single_data data;
-	int cpu;
-
-	data.func = quiesce_dummy;
-	data.info = NULL;
-
-	for_each_cpu_mask(cpu, mask) {
-		data.flags = CSD_FLAG_WAIT;
-		generic_exec_single(cpu, &data);
-	}
-}
-
 /**
- * smp_call_function_mask(): Run a function on a set of other CPUs.
- * @mask: The set of cpus to run on.
+ * smp_call_function_many(): Run a function on a set of other CPUs.
+ * @mask: The set of cpus to run on (only runs on online subset).
  * @func: The function to run. This must be fast and non-blocking.
  * @info: An arbitrary pointer to pass to the function.
  * @wait: If true, wait (atomically) until function has completed on other CPUs.
- *
- * Returns 0 on success, else a negative status code.
  *
  * If @wait is true, then returns once @func has returned. Note that @wait
  * will be implicitly turned on in case of allocation failures, since
@@ -318,71 +280,68 @@ static void smp_call_function_mask_quies
  * hardware interrupt handler or from a bottom half handler. Preemption
  * must be disabled when calling this function.
  */
-int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,
-			   int wait)
+void smp_call_function_many(const struct cpumask *mask,
+			    void (*func)(void *), void *info,
+			    bool wait)
 {
-	struct call_function_data d;
-	struct call_function_data *data = NULL;
-	cpumask_t allbutself;
+	struct call_function_data *data;
 	unsigned long flags;
-	int cpu, num_cpus;
-	int slowpath = 0;
+	int cpu, next_cpu;
 
 	/* Can deadlock when called with interrupts disabled */
 	WARN_ON(irqs_disabled());
 
-	cpu = smp_processor_id();
-	allbutself = cpu_online_map;
-	cpu_clear(cpu, allbutself);
-	cpus_and(mask, mask, allbutself);
-	num_cpus = cpus_weight(mask);
+	/* So, what's a CPU they want?  Ignoring this one. */
+	cpu = cpumask_first_and(mask, cpu_online_mask);
+	if (cpu == smp_processor_id())
+		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
+	/* No online cpus?  We're done. */
+	if (cpu >= nr_cpu_ids)
+		return;
 
-	/*
-	 * If zero CPUs, return. If just a single CPU, turn this request
-	 * into a targetted single call instead since it's faster.
-	 */
-	if (!num_cpus)
-		return 0;
-	else if (num_cpus == 1) {
-		cpu = first_cpu(mask);
-		return smp_call_function_single(cpu, func, info, wait);
-	}
+	/* Do we have another CPU which isn't us? */
+	next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
+	if (next_cpu == smp_processor_id())
+		next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
 
-	data = kmalloc(sizeof(*data), GFP_ATOMIC);
-	if (data) {
-		data->csd.flags = CSD_FLAG_ALLOC;
-		if (wait)
-			data->csd.flags |= CSD_FLAG_WAIT;
-	} else {
-		data = &d;
-		data->csd.flags = CSD_FLAG_WAIT;
-		wait = 1;
-		slowpath = 1;
+	/* Fastpath: do that cpu by itself. */
+	if (next_cpu >= nr_cpu_ids)
+		smp_call_function_single(cpu, func, info, wait);
+
+	data = kmalloc(sizeof(*data) + cpumask_size(), GFP_ATOMIC);
+	if (unlikely(!data)) {
+		/* Slow path. */
+		for_each_online_cpu(cpu) {
+			if (cpu == smp_processor_id())
+				continue;
+			if (cpumask_test_cpu(cpu, mask))
+				smp_call_function_single(cpu, func, info, wait);
+		}
+		return;
 	}
 
 	spin_lock_init(&data->lock);
+	data->csd.flags = CSD_FLAG_ALLOC;
+	if (wait)
+		data->csd.flags |= CSD_FLAG_WAIT;
 	data->csd.func = func;
 	data->csd.info = info;
-	data->refs = num_cpus;
-	data->cpumask = mask;
+	cpumask_and(to_cpumask(data->cpumask_bits), mask, cpu_online_mask);
+	cpumask_clear_cpu(smp_processor_id(), to_cpumask(data->cpumask_bits));
+	data->refs = cpumask_weight(to_cpumask(data->cpumask_bits));
 
 	spin_lock_irqsave(&call_function_lock, flags);
 	list_add_tail_rcu(&data->csd.list, &call_function_queue);
 	spin_unlock_irqrestore(&call_function_lock, flags);
 
 	/* Send a message to all CPUs in the map */
-	arch_send_call_function_ipi(mask);
+	arch_send_call_function_ipi(*to_cpumask(data->cpumask_bits));
 
 	/* optionally wait for the CPUs to complete */
-	if (wait) {
+	if (wait)
 		csd_flag_wait(&data->csd);
-		if (unlikely(slowpath))
-			smp_call_function_mask_quiesce_stack(mask);
-	}
-
-	return 0;
 }
-EXPORT_SYMBOL(smp_call_function_mask);
+EXPORT_SYMBOL(smp_call_function_many);
 
 /**
  * smp_call_function(): Run a function on all other CPUs.
@@ -390,7 +349,7 @@ EXPORT_SYMBOL(smp_call_function_mask);
  * @info: An arbitrary pointer to pass to the function.
  * @wait: If true, wait (atomically) until function has completed on other CPUs.
  *
- * Returns 0 on success, else a negative status code.
+ * Returns 0.
  *
  * If @wait is true, then returns once @func has returned; otherwise
  * it returns just before the target cpu calls @func. In case of allocation
@@ -401,12 +360,10 @@ EXPORT_SYMBOL(smp_call_function_mask);
  */
 int smp_call_function(void (*func)(void *), void *info, int wait)
 {
-	int ret;
-
 	preempt_disable();
-	ret = smp_call_function_mask(cpu_online_map, func, info, wait);
+	smp_call_function_many(cpu_online_mask, func, info, wait);
 	preempt_enable();
-	return ret;
+	return 0;
 }
 EXPORT_SYMBOL(smp_call_function);
 
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