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

1 files changed, 49 insertions, 90 deletions

diff --git a/kernel/smp.c b/kernel/smp.c
index 75c8dde58c55..9f0eafed1399 100644
--- 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);