Date:	Tue, 17 Feb 2009 13:30:27 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -v4] generic-ipi: remove kmalloc()

On Tue, Feb 17, 2009 at 08:29:18PM +0100, Peter Zijlstra wrote:
> On Tue, 2009-02-17 at 18:40 +0100, Peter Zijlstra wrote:
> > Let me spin a new patch and build a kernel with it ;-)
> 
> On top of Nick's patch.
> 
> My quad has been happily building kernels with this the past 30 minutes
> or so.
> 
> ---
> Subject: generic-ipi: remove kmalloc()
> 
> Remove the use of kmalloc() from the smp_call_function_*() calls.
> 
> Steven's generic-ipi patch (d7240b98: generic-ipi: use per cpu data for
> single cpu ipi calls) started the discussion on the use of kmalloc() in
> this code and fixed the smp_call_function_single(.wait=0) fallback case.
> 
> In this patch we complete this by also providing means for the _many()
> call, which fully removes the need for kmalloc() in this code.
> 
> The problem with the _many() call is that other cpus might still be
> observing our entry when we're done with it. It solved this by
> dynamically allocating data elements and RCU-freeing it.
> 
> We solve it by using a single per-cpu entry which provides static
> storage and solves one half of the problem (avoiding referencing freed
> data).
> 
> The other half, ensuring the queue iteration it still possible, is done
> by placing re-used entries at the head of the list. This means that if
> someone was still iterating that entry when it got moved will now
> re-visit the entries on the list it had already seen, but avoids
> skipping over entries like would have happened had we placed the new
> entry at the end.
> 
> Furthermore, visiting entries twice is not a problem, since we remove
> our cpu from the entry's cpumask once its called.
> 
> We also optimize both the _single() and _many() interrupt handler to
> copy the entry to their local stack and freeing it for re-use before we
> call the function.
> 
> Many thanks to Oleg for his suggestions and poking him holes in my
> earlier attempts.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  smp.c |  285 +++++++++++++++++++++++++++++++++++++++---------------------------
>  1 file changed, 171 insertions(+), 114 deletions(-)
> 
> Index: linux-2.6/kernel/smp.c
> ===================================================================
> --- linux-2.6.orig/kernel/smp.c
> +++ linux-2.6/kernel/smp.c
> @@ -10,23 +10,28 @@
>  #include <linux/rcupdate.h>
>  #include <linux/rculist.h>
>  #include <linux/smp.h>
> +#include <linux/cpu.h>
> 
>  static DEFINE_PER_CPU(struct call_single_queue, call_single_queue);
> -static LIST_HEAD(call_function_queue);
> -__cacheline_aligned_in_smp DEFINE_SPINLOCK(call_function_lock);
> +
> +static struct {
> +	struct list_head	queue;
> +	spinlock_t		lock;
> +} call_function __cacheline_aligned_in_smp = {
> +	.queue = LIST_HEAD_INIT(call_function.queue),
> +	.lock  = __SPIN_LOCK_UNLOCKED(call_function.lock),
> +};
> 
>  enum {
>  	CSD_FLAG_WAIT		= 0x01,
> -	CSD_FLAG_ALLOC		= 0x02,
> -	CSD_FLAG_LOCK		= 0x04,
> +	CSD_FLAG_LOCK		= 0x02,
>  };
> 
>  struct call_function_data {
>  	struct call_single_data csd;
>  	spinlock_t lock;
>  	unsigned int refs;
> -	struct rcu_head rcu_head;
> -	unsigned long cpumask_bits[];
> +	cpumask_var_t cpumask;
>  };
> 
>  struct call_single_queue {
> @@ -34,8 +39,45 @@ struct call_single_queue {
>  	spinlock_t lock;
>  };
> 
> +static DEFINE_PER_CPU(struct call_function_data, cfd_data) = {
> +	.lock = __SPIN_LOCK_UNLOCKED(cfd_data.lock),
> +};
> +
> +static int
> +hotplug_cfd(struct notifier_block *nfb, unsigned long action, void *hcpu)
> +{
> +	long cpu = (long)hcpu;
> +	struct call_function_data *cfd = &per_cpu(cfd_data, cpu);
> +
> +	switch (action) {
> +	case CPU_UP_PREPARE:
> +	case CPU_UP_PREPARE_FROZEN:
> +		if (!alloc_cpumask_var_node(&cfd->cpumask, GFP_KERNEL,
> +				cpu_to_node(cpu)))
> +			return NOTIFY_BAD;
> +		break;
> +
> +#ifdef CONFIG_CPU_HOTPLUG
> +	case CPU_UP_CANCELED:
> +	case CPU_UP_CANCELED_FROZEN:
> +
> +	case CPU_DEAD:
> +	case CPU_DEAD_FROZEN:
> +		free_cpumask_var(cfd->cpumask);
> +		break;
> +#endif
> +	};
> +
> +	return NOTIFY_OK;
> +}
> +
> +static struct notifier_block __cpuinitdata hotplug_cfd_notifier = {
> +	.notifier_call = hotplug_cfd,
> +};
> +
>  static int __cpuinit init_call_single_data(void)
>  {
> +	void *cpu = (void *)(long)smp_processor_id();
>  	int i;
> 
>  	for_each_possible_cpu(i) {
> @@ -44,18 +86,59 @@ static int __cpuinit init_call_single_da
>  		spin_lock_init(&q->lock);
>  		INIT_LIST_HEAD(&q->list);
>  	}
> +
> +	hotplug_cfd(&hotplug_cfd_notifier, CPU_UP_PREPARE, cpu);
> +	register_cpu_notifier(&hotplug_cfd_notifier);
> +
>  	return 0;
>  }
>  early_initcall(init_call_single_data);
> 
> -static void csd_flag_wait(struct call_single_data *data)
> +/*
> + * csd_wait/csd_complete are used for synchronous ipi calls
> + */
> +static void csd_wait_prepare(struct call_single_data *data)
> +{
> +	data->flags |= CSD_FLAG_WAIT;
> +}
> +
> +static void csd_complete(struct call_single_data *data)
> +{
> +	/*
> +	 * Serialize stores to data with the flag clear and wakeup.
> +	 */
> +	smp_wmb();

Shouldn't the above be an smp_mb()?  There are reads preceding the calls
to csd_complete() that look to me like they need to remain ordered
before the flag-clearing below -- just in case of a quick reuse of this
call_single_data structure.

> +	data->flags &= ~CSD_FLAG_WAIT;
> +}
> +
> +static void csd_wait(struct call_single_data *data)
> +{
> +	while (data->flags & CSD_FLAG_WAIT)
> +		cpu_relax();
> +}
> +
> +/*
> + * csd_lock/csd_unlock used to serialize access to per-cpu csd resources
> + *
> + * For non-synchronous ipi calls the csd can still be in use by the previous
> + * function call. For multi-cpu calls its even more interesting as we'll have
> + * to ensure no other cpu is observing our csd.
> + */
> +static void csd_lock(struct call_single_data *data)
>  {
> -	/* Wait for response */
> -	do {
> -		if (!(data->flags & CSD_FLAG_WAIT))
> -			break;
> +	while (data->flags & CSD_FLAG_LOCK)
>  		cpu_relax();
> -	} while (1);
> +	data->flags = CSD_FLAG_LOCK;

OK, I'll bite...  Why don't we need a memory barrier here?

> +}
> +
> +static void csd_unlock(struct call_single_data *data)
> +{
> +	WARN_ON(!(data->flags & CSD_FLAG_LOCK));
> +	/*
> +	 * Serialize stores to data with the flags clear.
> +	 */
> +	smp_wmb();

I am a bit worried about this being smp_wmb() rather than smp_mb(),
but don't have a smoking gun.

> +	data->flags &= ~CSD_FLAG_LOCK;
>  }
> 
>  /*
> @@ -89,16 +172,7 @@ static void generic_exec_single(int cpu,
>  		arch_send_call_function_single_ipi(cpu);
> 
>  	if (wait)
> -		csd_flag_wait(data);
> -}
> -
> -static void rcu_free_call_data(struct rcu_head *head)
> -{
> -	struct call_function_data *data;
> -
> -	data = container_of(head, struct call_function_data, rcu_head);
> -
> -	kfree(data);
> +		csd_wait(data);
>  }
> 
>  /*
> @@ -122,41 +196,36 @@ void generic_smp_call_function_interrupt
>  	 * It's ok to use list_for_each_rcu() here even though we may delete
>  	 * 'pos', since list_del_rcu() doesn't clear ->next
>  	 */
> -	rcu_read_lock();
> -	list_for_each_entry_rcu(data, &call_function_queue, csd.list) {
> -		int refs;
> -
> -		if (!cpumask_test_cpu(cpu, to_cpumask(data->cpumask_bits)))
> -			continue;
> -
> -		data->csd.func(data->csd.info);
> +	list_for_each_entry_rcu(data, &call_function.queue, csd.list) {
> +		void (*func)(void *);
> +		void *info;
> +		int refs, wait;
> 
>  		spin_lock(&data->lock);
> -		cpumask_clear_cpu(cpu, to_cpumask(data->cpumask_bits));
> +		if (!cpumask_test_cpu(cpu, data->cpumask)) {
> +			spin_unlock(&data->lock);
> +			continue;
> +		}
> +		cpumask_clear_cpu(cpu, data->cpumask);
>  		WARN_ON(data->refs == 0);
> -		data->refs--;
> -		refs = data->refs;
> +		refs = --data->refs;
> +		func = data->csd.func;
> +		info = data->csd.info;
> +		wait = (data->csd.flags & CSD_FLAG_WAIT);
>  		spin_unlock(&data->lock);
> 
> -		if (refs)
> -			continue;
> +		if (!refs) {
> +			spin_lock(&call_function.lock);
> +			list_del_rcu(&data->csd.list);
> +			spin_unlock(&call_function.lock);
> +			csd_unlock(&data->csd);
> +		}
> 
> -		spin_lock(&call_function_lock);
> -		list_del_rcu(&data->csd.list);
> -		spin_unlock(&call_function_lock);
> +		func(info);
> 
> -		if (data->csd.flags & CSD_FLAG_WAIT) {
> -			/*
> -			 * serialize stores to data with the flag clear
> -			 * and wakeup
> -			 */
> -			smp_wmb();
> -			data->csd.flags &= ~CSD_FLAG_WAIT;
> -		}
> -		if (data->csd.flags & CSD_FLAG_ALLOC)
> -			call_rcu(&data->rcu_head, rcu_free_call_data);
> +		if (!refs && wait)
> +			csd_complete(&data->csd);
>  	}
> -	rcu_read_unlock();
> 
>  	put_cpu();
>  }
> @@ -169,7 +238,6 @@ void generic_smp_call_function_single_in
>  {
>  	struct call_single_queue *q = &__get_cpu_var(call_single_queue);
>  	LIST_HEAD(list);
> -	unsigned int data_flags;
> 
>  	spin_lock(&q->lock);
>  	list_replace_init(&q->list, &list);
> @@ -177,29 +245,28 @@ void generic_smp_call_function_single_in
> 
>  	while (!list_empty(&list)) {
>  		struct call_single_data *data;
> +		void (*func)(void *);
> +		void *info;
> +		int wait;
> 
> -		data = list_entry(list.next, struct call_single_data,
> -					list);
> +		data = list_first_entry(&list, struct call_single_data, list);
>  		list_del(&data->list);
> +		func = data->func;
> +		info = data->info;
> 
>  		/*
> -		 * 'data' can be invalid after this call if
> -		 * flags == 0 (when called through
> -		 * generic_exec_single(), so save them away before
> -		 * making the call.
> +		 * 'data' can be invalid after this if flags == 0 
> +		 * when called through generic_exec_single()
>  		 */
> -		data_flags = data->flags;
> +		wait = (data->flags & CSD_FLAG_WAIT);
> 
> -		data->func(data->info);
> +		if (data->flags & CSD_FLAG_LOCK)
> +			csd_unlock(data);
> +
> +		func(info);
> 
> -		if (data_flags & CSD_FLAG_WAIT) {
> -			smp_wmb();
> -			data->flags &= ~CSD_FLAG_WAIT;
> -		} else if (data_flags & CSD_FLAG_LOCK) {
> -			smp_wmb();
> -			data->flags &= ~CSD_FLAG_LOCK;
> -		} else if (data_flags & CSD_FLAG_ALLOC)
> -			kfree(data);
> +		if (wait)
> +			csd_complete(data);
>  	}
>  }
> 
> @@ -218,7 +285,9 @@ static DEFINE_PER_CPU(struct call_single
>  int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
>  			     int wait)
>  {
> -	struct call_single_data d;
> +	struct call_single_data d = {
> +		.flags = 0,
> +	};

And about here I get lost -- trying to find what the heck this patch
applies to...  :-/

						Thanx, Paul

>  	unsigned long flags;
>  	/* prevent preemption and reschedule on another processor,
>  	   as well as CPU removal */
> @@ -239,13 +308,11 @@ int smp_call_function_single(int cpu, vo
>  			/*
>  			 * We are calling a function on a single CPU
>  			 * and we are not going to wait for it to finish.
> -			 * We first try to allocate the data, but if we
> -			 * fail, we fall back to use a per cpu data to pass
> -			 * the information to that CPU. Since all callers
> -			 * of this code will use the same data, we must
> -			 * synchronize the callers to prevent a new caller
> -			 * from corrupting the data before the callee
> -			 * can access it.
> +			 * We use a per cpu data to pass the information to
> +			 * that CPU. Since all callers of this code will
> +			 * use the same data, we must synchronize the
> +			 * callers to prevent a new caller from corrupting
> +			 * the data before the callee can access it.
>  			 *
>  			 * The CSD_FLAG_LOCK is used to let us know when
>  			 * the IPI handler is done with the data.
> @@ -255,18 +322,11 @@ int smp_call_function_single(int cpu, vo
>  			 * will make sure the callee is done with the
>  			 * data before a new caller will use it.
>  			 */
> -			data = kmalloc(sizeof(*data), GFP_ATOMIC);
> -			if (data)
> -				data->flags = CSD_FLAG_ALLOC;
> -			else {
> -				data = &per_cpu(csd_data, me);
> -				while (data->flags & CSD_FLAG_LOCK)
> -					cpu_relax();
> -				data->flags = CSD_FLAG_LOCK;
> -			}
> +			data = &per_cpu(csd_data, me);
> +			csd_lock(data);
>  		} else {
>  			data = &d;
> -			data->flags = CSD_FLAG_WAIT;
> +			csd_wait_prepare(data);
>  		}
> 
>  		data->func = func;
> @@ -326,14 +386,14 @@ void smp_call_function_many(const struct
>  {
>  	struct call_function_data *data;
>  	unsigned long flags;
> -	int cpu, next_cpu;
> +	int cpu, next_cpu, me = smp_processor_id();
> 
>  	/* Can deadlock when called with interrupts disabled */
>  	WARN_ON(irqs_disabled());
> 
>  	/* So, what's a CPU they want?  Ignoring this one. */
>  	cpu = cpumask_first_and(mask, cpu_online_mask);
> -	if (cpu == smp_processor_id())
> +	if (cpu == me)
>  		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
>  	/* No online cpus?  We're done. */
>  	if (cpu >= nr_cpu_ids)
> @@ -341,7 +401,7 @@ void smp_call_function_many(const struct
> 
>  	/* 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())
> +	if (next_cpu == me)
>  		next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
> 
>  	/* Fastpath: do that cpu by itself. */
> @@ -350,31 +410,28 @@ void smp_call_function_many(const struct
>  		return;
>  	}
> 
> -	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;
> -	}
> +	data = &per_cpu(cfd_data, me);
> +	csd_lock(&data->csd);
> 
> -	spin_lock_init(&data->lock);
> -	data->csd.flags = CSD_FLAG_ALLOC;
> +	spin_lock_irqsave(&data->lock, flags);
>  	if (wait)
> -		data->csd.flags |= CSD_FLAG_WAIT;
> +		csd_wait_prepare(&data->csd);
> +
>  	data->csd.func = func;
>  	data->csd.info = info;
> -	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);
> +	cpumask_and(data->cpumask, mask, cpu_online_mask);
> +	cpumask_clear_cpu(me, data->cpumask);
> +	data->refs = cpumask_weight(data->cpumask);
> +
> +	spin_lock(&call_function.lock);
> +	/*
> +	 * Place then entry at the _HEAD_ of the list, so that any cpu still
> +	 * observing the entry in generic_smp_call_function_interrupt() will
> +	 * not miss any other list entries (instead it can see some twice).
> +	 */
> +	list_add_rcu(&data->csd.list, &call_function.queue);
> +	spin_unlock(&call_function.lock);
> +	spin_unlock_irqrestore(&data->lock, flags);
> 
>  	/*
>  	 * Make the list addition visible before sending the ipi.
> @@ -384,11 +441,11 @@ void smp_call_function_many(const struct
>  	smp_mb();
> 
>  	/* Send a message to all CPUs in the map */
> -	arch_send_call_function_ipi_mask(to_cpumask(data->cpumask_bits));
> +	arch_send_call_function_ipi_mask(data->cpumask);
> 
>  	/* optionally wait for the CPUs to complete */
>  	if (wait)
> -		csd_flag_wait(&data->csd);
> +		csd_wait(&data->csd);
>  }
>  EXPORT_SYMBOL(smp_call_function_many);
> 
> @@ -418,20 +475,20 @@ EXPORT_SYMBOL(smp_call_function);
> 
>  void ipi_call_lock(void)
>  {
> -	spin_lock(&call_function_lock);
> +	spin_lock(&call_function.lock);
>  }
> 
>  void ipi_call_unlock(void)
>  {
> -	spin_unlock(&call_function_lock);
> +	spin_unlock(&call_function.lock);
>  }
> 
>  void ipi_call_lock_irq(void)
>  {
> -	spin_lock_irq(&call_function_lock);
> +	spin_lock_irq(&call_function.lock);
>  }
> 
>  void ipi_call_unlock_irq(void)
>  {
> -	spin_unlock_irq(&call_function_lock);
> +	spin_unlock_irq(&call_function.lock);
>  }
> 
> 
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