cpumask: smp_call_function_many()

Impact: Implementation change to remove cpumask_t from stack.

Actually change smp_call_function_mask() to smp_call_function_many().
We avoid cpumasks on the stack in this version.

(S390 has its own version, but that's going away apparently).

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 (which needs a cpumask on
stack).

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

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>
Cc: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Cc: npiggin@suse.de
Cc: axboe@kernel.dk

diff --git a/include/linux/smp.h b/include/linux/smp.h
index 2f85f3b..b824669 100644 (file)
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -67,15 +67,16 @@ 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 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 --git a/kernel/smp.c b/kernel/smp.c
index 75c8dde..9f0eafe 100644 (file)
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -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 {
@@ -110,13 +110,13 @@ void generic_smp_call_function_interrupt(void)
        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;
@@ -266,51 +266,13 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
        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
  * we fall back to on-stack allocation.
@@ -319,53 +281,57 @@ static void smp_call_function_mask_quiesce_stack(cpumask_t mask)
  * 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);
-
-       /*
-        * 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);
+       /* 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;
+
+       /* 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);
+
+       /* Fastpath: do that cpu by itself. */
+       if (next_cpu >= nr_cpu_ids) {
+               smp_call_function_single(cpu, func, info, wait);
+               return;
        }
 
-       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;
+       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);
@@ -377,18 +343,13 @@ int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,
        smp_mb();
 
        /* 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.
@@ -396,7 +357,7 @@ 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
@@ -407,12 +368,10 @@ 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);