Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

Jens Axboe
2008-04-22T16:50:09Z

On Tue, Apr 22 2008, Linus Torvalds wrote:
>
>
> On Tue, 22 Apr 2008, Jens Axboe wrote:
> > >
> > > You forgot to free the "data" here? The waiter must also free the object,
> > > since now the callee does not.
> >
> > The ipi interrupt handler does that, see kfree() in
> > generic_smp_call_function_single_interrupt() or call_func_data_free() in
> > generic_smp_call_function_interrupt().
>
> Hell no, it does *not*.
>
> Doing that for the waiting case would be a *huge* bug, since the waiter
> needs to wait until the flag is clear - and if the waitee free's the
> allocation, that will never happen.
>
> So the rule *must* be:
> - waiter frees
> - ipi interrupt frees non-waiting ones.
> because anything else cannot work.
>
> And you must have known that, because the code you pointed me to does
> *not* free the data at all. It just clears the FLAG_WAIT flag:
>
> + if (data->csd.flags & CSD_FLAG_WAIT) {
> + smp_wmb();
> + data->csd.flags &= ~CSD_FLAG_WAIT;
> + } else
> + call_func_data_free(data);
>
> So please think about this some more.

Yep, sorry, pre-dinner sugar low. So how about this, it implements what
I/you detailed:

- If wait == 1, then we always use on-stack allocation
- If wait == 0 and alloc fails, set wait = 1 and repeat.
- For wait == 1, there's never anything to free.
- For wait == 0, the ipi interrupt handler frees the structure if
CSD_FLAG_ALLOC is set. This COULD be an else without checking
CSD_FLAG_ALLOC for smp_call_function_mask(), only
smp_call_function_single() really needs the check. But I left it there
in case we allow pre-allocated insert for that path as well in the
future.

OK?

>From 94bbe959d98add7fa23c270afbedea08917c5fd6 Mon Sep 17 00:00:00 2001
From: Jens Axboe <jens.axboe@oracle.com>
Date: Tue, 22 Apr 2008 18:42:25 +0200
Subject: [PATCH] Add generic helpers for arch IPI function calls

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@hp.com> has
contributed lots of fixes and suggestions as well.

Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
arch/Kconfig | 3 +
include/linux/smp.h | 27 ++++-
init/main.c | 3 +
kernel/Makefile | 1 +
kernel/smp.c | 343 +++++++++++++++++++++++++++++++++++++++++++++++++++
5 files changed, 375 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

/*
* Call a function on all processors
@@ -112,7 +136,6 @@ static inline void smp_send_reschedule(int cpu) { }
})
#define smp_call_function_mask(mask, func, info, wait) \
(up_smp_call_function(func, info))
-
#endif /* !SMP */

/*
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
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..7b44820
--- /dev/null
+++ b/kernel/smp.c
@@ -0,0 +1,343 @@
+/*
+ * Generic helpers for smp ipi calls
+ *
+ * (C) Jens Axboe <jens.axboe@oracle.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,
+};
+
+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;
+};
+
+void __cpuinit init_call_single_data(void)
+{
+ int i;
+
+ for_each_cpu_mask(i, cpu_possible_map) {
+ 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);
+}
+
+/*
+ * 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);
+}
+
+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);
+}
+
+/*
+ * Invoked by arch to handle an IPI for call function. Must be called with
+ * interrupts disabled.
+ */
+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();
+ 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));
+ 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();
+ data->csd.flags &= ~CSD_FLAG_WAIT;
+ } else if (data->csd.flags & CSD_FLAG_ALLOC)
+ call_rcu(&data->rcu_head, rcu_free_call_data);
+ }
+ rcu_read_unlock();
+
+ put_cpu();
+}
+
+/*
+ * Invoked by arch to handle an IPI for call function single. Must be called
+ * from the arch with interrupts disabled.
+ */
+void generic_smp_call_function_single_interrupt(void)
+{
+ struct call_single_queue *q = &__get_cpu_var(call_single_queue);
+ LIST_HEAD(list);
+
+ smp_mb();
+ while (!list_empty(&q->list)) {
+ 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);
+ }
+ smp_mb();
+ }
+}
+
+/*
+ * 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.
+ *
+ */
+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());
+
+ 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;
+do_wait:
+ data = &d;
+ data->flags = CSD_FLAG_WAIT;
+ } else {
+ data = kmalloc(sizeof(*data), GFP_ATOMIC);
+ if (unlikely(!data)) {
+ wait = 1;
+ goto do_wait;
+ }
+ data->flags = CSD_FLAG_ALLOC;
+ }
+
+ data->func = func;
+ data->info = info;
+ generic_exec_single(cpu, data);
+ }
+
+ put_cpu();
+ return 0;
+}
+EXPORT_SYMBOL(smp_call_function_single);
+
+/**
+ * __smp_call_function_single(): Run a function on another CPU
+ * @cpu: The CPU to run on.
+ * @data: Pre-allocated and setup data structure
+ *
+ * Like smp_call_function_single(), but allow caller to pass in a pre-allocated
+ * data structure. Useful for embedding @data inside other structures, for
+ * instance.
+ *
+ */
+void __smp_call_function_single(int cpu, struct call_single_data *data)
+{
+ 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.
+ * @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.
+ *
+ * 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)
+{
+ struct call_function_data *data;
+ cpumask_t allbutself;
+ unsigned long flags;
+ int num_cpus;
+
+ /* Can deadlock when called with interrupts disabled */
+ WARN_ON(wait && irqs_disabled());
+
+ allbutself = cpu_online_map;
+ cpu_clear(smp_processor_id(), allbutself);
+ cpus_and(mask, mask, allbutself);
+ num_cpus = cpus_weight(mask);
+
+ if (!num_cpus)
+ return 0;
+
+ if (wait) {
+ struct call_function_data d;
+do_wait:
+ data = &d;
+ data->csd.flags = CSD_FLAG_WAIT;
+ } else {
+ data = kmalloc(sizeof(*data), GFP_ATOMIC);
+ if (!unlikely(!data)) {
+ wait = 1;
+ goto do_wait;
+ }
+ data->csd.flags = CSD_FLAG_ALLOC;
+ }
+
+ spin_lock_init(&data->lock);
+ data->csd.func = func;
+ data->csd.info = info;
+ data->refs = num_cpus;
+ data->cpumask = mask;
+
+ 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);
+
+ /* optionally wait for the CPUs to complete */
+ if (wait)
+ csd_flag_wait(&data->csd);
+
+ return 0;
+}
+EXPORT_SYMBOL(smp_call_function_mask);
+
+/**
+ * smp_call_function(): Run a function on all other CPUs.
+ * @func: The function to run. This must be fast and non-blocking.
+ * @info: An arbitrary pointer to pass to the function.
+ * @natomic: Unused
+ * @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; otherwise
+ * it returns just before the target cpu calls @func.
+ *
+ * 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(void (*func)(void *), void *info, int natomic, int wait)
+{
+ int ret;
+
+ preempt_disable();
+ ret = smp_call_function_mask(cpu_online_map, func, info, wait);
+ preempt_enable();
+ return ret;
+}
+EXPORT_SYMBOL(smp_call_function);
--
1.5.5.1.57.g5909c

--
Jens Axboe