Message-ID: <20080427103017.GT12774@kernel.dk>
Date:	Sun, 27 Apr 2008 12:30:17 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	npiggin@...e.de, torvalds@...ux-foundation.org,
	peterz@...radead.org, sam@...nborg.org,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

On Fri, Apr 25 2008, Andrew Morton wrote:
> > On Tue, 22 Apr 2008 20:50:17 +0200 Jens Axboe <jens.axboe@...cle.com> wrote:
> > This adds kernel/smp.c which contains helpers for IPI function calls. In
> > addition to supporting the existing smp_call_function() in a more efficient
> > manner, it also adds a more scalable variant called smp_call_function_single()
> > for calling a given function on a single CPU only.
> > 
> > The core of this is based on the x86-64 patch from Nick Piggin, lots of
> > changes since then. "Alan D. Brunelle" <Alan.Brunelle@...com> has
> > contributed lots of fixes and suggestions as well.
> > 
> > Acked-by: Ingo Molnar <mingo@...e.hu>
> > Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> > ---
> >  arch/Kconfig        |    3 +
> >  include/linux/smp.h |   27 ++++-
> >  init/main.c         |    3 +
> >  kernel/Makefile     |    1 +
> >  kernel/smp.c        |  366 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  5 files changed, 398 insertions(+), 2 deletions(-)
> >  create mode 100644 kernel/smp.c
> > 
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 694c9af..a5a0184 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -36,3 +36,6 @@ config HAVE_KPROBES
> >  
> >  config HAVE_KRETPROBES
> >  	def_bool n
> > +
> > +config USE_GENERIC_SMP_HELPERS
> > +	def_bool n
> > diff --git a/include/linux/smp.h b/include/linux/smp.h
> > index 55232cc..4a5418b 100644
> > --- a/include/linux/smp.h
> > +++ b/include/linux/smp.h
> > @@ -7,9 +7,19 @@
> >   */
> >  
> >  #include <linux/errno.h>
> > +#include <linux/list.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/cpumask.h>
> >  
> >  extern void cpu_idle(void);
> >  
> > +struct call_single_data {
> > +	struct list_head list;
> > +	void (*func) (void *info);
> > +	void *info;
> > +	unsigned int flags;
> > +};
> > +
> >  #ifdef CONFIG_SMP
> >  
> >  #include <linux/preempt.h>
> > @@ -53,9 +63,23 @@ extern void smp_cpus_done(unsigned int max_cpus);
> >   * Call a function on all other processors
> >   */
> >  int smp_call_function(void(*func)(void *info), void *info, int retry, int wait);
> > -
> > +int smp_call_function_mask(cpumask_t mask, void(*func)(void *info), void *info,
> > +				int wait);
> >  int smp_call_function_single(int cpuid, void (*func) (void *info), void *info,
> >  				int retry, int wait);
> > +void __smp_call_function_single(int cpuid, struct call_single_data *data);
> > +
> > +/*
> > + * Generic and arch helpers
> > + */
> > +#ifdef CONFIG_USE_GENERIC_SMP_HELPERS
> > +void generic_smp_call_function_single_interrupt(void);
> > +void generic_smp_call_function_interrupt(void);
> > +void init_call_single_data(void);
> > +void arch_send_call_function_single_ipi(int cpu);
> > +void arch_send_call_function_ipi(cpumask_t mask);
> > +extern spinlock_t call_function_lock;
> > +#endif
> 
> Add a #else here ...
> 
> > diff --git a/init/main.c b/init/main.c
> > index 833a67d..0b7578c 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -773,6 +773,9 @@ static void __init do_pre_smp_initcalls(void)
> >  {
> >  	extern int spawn_ksoftirqd(void);
> >  
> > +#ifdef CONFIG_USE_GENERIC_SMP_HELPERS
> > +	init_call_single_data();
> > +#endif
> 
> and we can lose this ifdef here.
> 
> Also, init_call_single_data() is __cpuinit but isn't declared that way. 
> There have been rare occasions (FRV was one) where this matters - iirc the
> compiler was emitting a short-relative-addressing-form instruction which
> which wasn't able to branch far enough once things were linked.  We hae
> this problem in eight zillion other places, of course.  And it's a pita to
> go adding __cpunit etc to the declaration because the compiler usually
> won't tell us when it gets out of sync with reality.  So we could leave he
> code as-is and wait for stuff to break :(

I'll get rid of the ifdef.

> >  	migration_init();
> >  	spawn_ksoftirqd();
> >  	if (!nosoftlockup)
> > diff --git a/kernel/Makefile b/kernel/Makefile
> > index 6c5f081..7e275d4 100644
> > --- a/kernel/Makefile
> > +++ b/kernel/Makefile
> > @@ -28,6 +28,7 @@ obj-$(CONFIG_DEBUG_RT_MUTEXES) += rtmutex-debug.o
> >  obj-$(CONFIG_RT_MUTEX_TESTER) += rtmutex-tester.o
> >  obj-$(CONFIG_GENERIC_ISA_DMA) += dma.o
> >  obj-$(CONFIG_SMP) += cpu.o spinlock.o
> > +obj-$(CONFIG_USE_GENERIC_SMP_HELPERS) += smp.o
> >  obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
> >  obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
> >  obj-$(CONFIG_UID16) += uid16.o
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > new file mode 100644
> > index 0000000..a177a0d
> > --- /dev/null
> > +++ b/kernel/smp.c
> > @@ -0,0 +1,366 @@
> > +/*
> > + * Generic helpers for smp ipi calls
> > + *
> > + * (C) Jens Axboe <jens.axboe@...cle.com> 2008
> > + *
> > + */
> > +#include <linux/init.h>
> > +#include <linux/module.h>
> > +#include <linux/percpu.h>
> > +#include <linux/rcupdate.h>
> > +#include <linux/smp.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);
> > +
> > +enum {
> > +	CSD_FLAG_WAIT		= 0x01,
> > +	CSD_FLAG_ALLOC		= 0x02,
> > +	CSD_FLAG_FALLBACK	= 0x04,
> > +};
> > +
> > +struct call_function_data {
> > +	struct call_single_data csd;
> > +	spinlock_t lock;
> > +	unsigned int refs;
> > +	cpumask_t cpumask;
> > +	struct rcu_head rcu_head;
> > +};
> > +
> > +struct call_single_queue {
> > +	struct list_head list;
> > +	spinlock_t lock;
> > +};
> > +
> > +static struct call_function_data cfd_fallback;
> > +static unsigned long cfd_fallback_used;
> > +
> > +void __cpuinit init_call_single_data(void)
> > +{
> > +	int i;
> > +
> > +	for_each_cpu_mask(i, cpu_possible_map) {
> 
> for_each_possible_cpu()?

Sure

> Do we _have_ to consider all possible CPUs here?  That can be much larger
> than num_online_cpus.

We could do online CPUs and use a notifier for hotplug, but does it
really matter?

> > +		struct call_single_queue *q = &per_cpu(call_single_queue, i);
> > +
> > +		spin_lock_init(&q->lock);
> > +		INIT_LIST_HEAD(&q->list);
> > +	}
> > +}
> > +
> > +static inline void csd_flag_wait(struct call_single_data *data)
> > +{
> > +	/* Wait for response */
> > +	do {
> > +		/*
> > +		 * We need to see the flags store in the IPI handler
> > +		 */
> > +		smp_mb();
> > +		if (!(data->flags & CSD_FLAG_WAIT))
> > +			break;
> > +		cpu_relax();
> > +	} while (1);
> > +}
> 
> -ETOOLAGETOINLINE?

If you think that is too large to inline, then can we inline anything?
It's a barrier and a andl basically, a handful of instructions perhaps?
That said, I don't care much either way. It's a sync wait, so it need
not be inline.

> > + * Insert a previously allocated call_single_data element for execution
> > + * on the given CPU. data must already have ->func, ->info, and ->flags set.
> > + */
> > +static void generic_exec_single(int cpu, struct call_single_data *data)
> > +{
> > +	struct call_single_queue *dst = &per_cpu(call_single_queue, cpu);
> > +	int wait = data->flags & CSD_FLAG_WAIT, ipi;
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&dst->lock, flags);
> > +	ipi = list_empty(&dst->list);
> > +	list_add_tail(&data->list, &dst->list);
> > +	spin_unlock_irqrestore(&dst->lock, flags);
> > +
> > +	if (ipi)
> > +		arch_send_call_function_single_ipi(cpu);
> > +
> > +	if (wait)
> > +		csd_flag_wait(data);
> > +}
> 
> Is this a must-be-called-with-local-interrupts-enable function?  (It
> doesn't say so in the covering comment) If so, a runtime check for that
> would be needed - we screw that up regularly.  Ditto any other places where
> this applies.

See below, the smp_call_* restrictions apply.

> > +static void rcu_free_call_data(struct rcu_head *head)
> > +{
> > +	struct call_function_data *cfd;
> > +
> > +	cfd = container_of(head, struct call_function_data, rcu_head);
> > +	kfree(cfd);
> > +}
> > +
> > +static void call_func_data_free(struct call_function_data *data)
> > +{
> > +	if (data->csd.flags & CSD_FLAG_ALLOC)
> > +		call_rcu(&data->rcu_head, rcu_free_call_data);
> > +	else
> > +		clear_bit_unlock(0, &cfd_fallback_used);
> > +}
> 
> Let's cc Mr RCU - he reviews well and often finds problems.

Sure, though this is pretty straightforward usage of RCU, it cannot hurt
:)

> > +/*
> > + * Invoked by arch to handle an IPI for call function. Must be called with
> > + * interrupts disabled.
> 
> A runtime check would be nice.  The get_cpu() will give us partial coverage
> but won't detect irqs-on-inside-spinlock state.

Noted, I'll make sure all paths are covered.

> > + */
> > +void generic_smp_call_function_interrupt(void)
> > +{
> > +	struct list_head *pos;
> > +	int cpu = get_cpu();
> > +
> > +	/*
> > +	 * 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();
> 
> Sigh.  irqs are disabled, so this is a waste of CPU cycles.  With some of
> our many RCU flavours, at least.

Yes I know, I can add it to the comment instead. And even if it wasn't
with irqs disabled, the get_cpu() has already disabled preemption.

> > +	list_for_each_rcu(pos, &call_function_queue) {
> > +		struct call_function_data *data;
> > +		int refs;
> > +
> > +		data = list_entry(pos, struct call_function_data, csd.list);
> > +		if (!cpu_isset(cpu, data->cpumask))
> > +			continue;
> > +
> > +		data->csd.func(data->csd.info);
> > +
> > +		spin_lock(&data->lock);
> > +		cpu_clear(cpu, data->cpumask);
> > +		WARN_ON(data->refs == 0);
> > +		data->refs--;
> > +		refs = data->refs;
> > +		spin_unlock(&data->lock);
> > +
> > +		if (refs)
> > +			continue;
> > +
> > +		WARN_ON(cpus_weight(data->cpumask));
> 
> !cpus_empty()

Thanks, I'll use that.

> > +		spin_lock(&call_function_lock);
> > +		list_del_rcu(&data->csd.list);
> > +		spin_unlock(&call_function_lock);
> > +
> > +		if (data->csd.flags & CSD_FLAG_WAIT) {
> > +			smp_wmb();
> 
> It's nice to comment open-coded barriers.
> 
> > +			data->csd.flags &= ~CSD_FLAG_WAIT;

OK, will do. This one is to serialize data/csd stores so they are seen
before the flag wake up.

> > + */
> > +void generic_smp_call_function_single_interrupt(void)
> > +{
> > +	struct call_single_queue *q = &__get_cpu_var(call_single_queue);
> > +	LIST_HEAD(list);
> > +
> > +	smp_mb();
> 
> Unclear why this is here - comment?
> 
> > +	while (!list_empty(&q->list)) {

To avoid taking the lock before the list empty check. Will comment.

> > +		unsigned int data_flags;
> > +
> > +		spin_lock(&q->lock);
> > +		list_replace_init(&q->list, &list);
> > +		spin_unlock(&q->lock);
> > +
> > +		while (!list_empty(&list)) {
> > +			struct call_single_data *data;
> > +
> > +			data = list_entry(list.next, struct call_single_data,
> > +						list);
> > +			list_del(&data->list);
> > +
> > +			/*
> > +			 * '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_flags = data->flags;
> > +
> > +			data->func(data->info);
> > +
> > +			if (data_flags & CSD_FLAG_WAIT) {
> > +				smp_wmb();
> > +				data->flags &= ~CSD_FLAG_WAIT;
> > +			} else if (data_flags & CSD_FLAG_ALLOC)
> > +				kfree(data);
> > +			else if (data_flags & CSD_FLAG_FALLBACK)
> > +				clear_bit_unlock(0, &cfd_fallback_used);
> > +		}
> > +		smp_mb();

Ditto for that one, when we go around the loop again.

> > + * smp_call_function_single - Run a function on a specific CPU
> > + * @func: The function to run. This must be fast and non-blocking.
> > + * @info: An arbitrary pointer to pass to the function.
> > + * @retry: Unused
> > + * @wait: If true, wait until function has completed on other CPUs.
> > + *
> > + * Returns 0 on success, else a negative status code.
> > + *
> 
> stray line here.
> 
> > + */
> 
> Some of the exported functions have kerneldoc, others do not.
> 
> > +int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
> > +			     int retry, int wait)
> > +{
> > +	unsigned long flags;
> > +	/* prevent preemption and reschedule on another processor */
> > +	int me = get_cpu();
> > +
> > +	/* Can deadlock when called with interrupts disabled */
> > +	WARN_ON(wait && irqs_disabled());
> 
> Perhaps that addresses above comments?

Probably not for __smp_call_function_single(), but the others should be
covered.

> > +	if (cpu == me) {
> > +		local_irq_save(flags);
> > +		func(info);
> > +		local_irq_restore(flags);
> > +	} else {
> > +		struct call_single_data *data;
> > +
> > +		if (wait) {
> > +			struct call_single_data d;
> > +
> > +			data = &d;
> > +			data->flags = CSD_FLAG_WAIT;
> > +		} else {
> > +			data = kmalloc(sizeof(*data), GFP_ATOMIC);
> > +			if (data)
> > +				data->flags = CSD_FLAG_ALLOC;
> > +			else {
> > +				while (test_and_set_bit_lock(0,
> > +							&cfd_fallback_used))
> 
> cfd_fallback_used is rather mysterious - it could do with a comment at its
> definition site.  I'm wondering if we should/could be doing bit_spin_lock
> on it.  But if we did that, it could just be a spinlock.

The bit lock is smaller, but apart from that there should be zero
functional difference.

-- 
Jens Axboe

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