block: add support for IO CPU affinity

This patch adds support for controlling the IO completion CPU of
either all requests on a queue, or on a per-request basis. We export
a sysfs variable (rq_affinity) which, if set, migrates completions
of requests to the CPU that originally submitted it. A bio helper
(bio_set_completion_cpu()) is also added, so that queuers can ask
for completion on that specific CPU.

In testing, this has been show to cut the system time by as much
as 20-40% on synthetic workloads where CPU affinity is desired.

This requires a little help from the architecture, so it'll only
work as designed for archs that are using the new generic smp
helper infrastructure.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 95 insertions, 31 deletions

diff --git a/block/blk-softirq.c b/block/blk-softirq.c
index 9e1c43bff662..3a1af551191e 100644
--- a/block/blk-softirq.c
+++ b/block/blk-softirq.c
@@ -13,6 +13,70 @@
 
 static DEFINE_PER_CPU(struct list_head, blk_cpu_done);
 
+/*
+ * Softirq action handler - move entries to local list and loop over them
+ * while passing them to the queue registered handler.
+ */
+static void blk_done_softirq(struct softirq_action *h)
+{
+        struct list_head *cpu_list, local_list;
+
+        local_irq_disable();
+        cpu_list = &__get_cpu_var(blk_cpu_done);
+        list_replace_init(cpu_list, &local_list);
+        local_irq_enable();
+
+        while (!list_empty(&local_list)) {
+                struct request *rq;
+
+                rq = list_entry(local_list.next, struct request, csd.list);
+                list_del_init(&rq->csd.list);
+                rq->q->softirq_done_fn(rq);
+        }
+}
+
+#if defined(CONFIG_SMP) && defined(CONFIG_USE_GENERIC_SMP_HELPERS)
+static void trigger_softirq(void *data)
+{
+        struct request *rq = data;
+        unsigned long flags;
+        struct list_head *list;
+
+        local_irq_save(flags);
+        list = &__get_cpu_var(blk_cpu_done);
+        list_add_tail(&rq->csd.list, list);
+
+        if (list->next == &rq->csd.list)
+                raise_softirq_irqoff(BLOCK_SOFTIRQ);
+
+        local_irq_restore(flags);
+}
+
+/*
+ * Setup and invoke a run of 'trigger_softirq' on the given cpu.
+ */
+static int raise_blk_irq(int cpu, struct request *rq)
+{
+        if (cpu_online(cpu)) {
+                struct call_single_data *data = &rq->csd;
+
+                data->func = trigger_softirq;
+                data->info = rq;
+                data->flags = 0;
+
+                __smp_call_function_single(cpu, data);
+                return 0;
+        }
+
+        return 1;
+}
+#else /* CONFIG_SMP && CONFIG_USE_GENERIC_SMP_HELPERS */
+static int raise_blk_irq(int cpu, struct request *rq)
+{
+        return 1;
+}
+#endif
+
 static int __cpuinit blk_cpu_notify(struct notifier_block *self,
                                     unsigned long action, void *hcpu)
 {
@@ -33,33 +97,10 @@ static int __cpuinit blk_cpu_notify(struct notifier_block *self,
         return NOTIFY_OK;
 }
 
-
-static struct notifier_block blk_cpu_notifier __cpuinitdata = {
+static struct notifier_block __cpuinitdata blk_cpu_notifier = {
         .notifier_call  = blk_cpu_notify,
 };
 
-/*
- * splice the completion data to a local structure and hand off to
- * process_completion_queue() to complete the requests
- */
-static void blk_done_softirq(struct softirq_action *h)
-{
-        struct list_head *cpu_list, local_list;
-
-        local_irq_disable();
-        cpu_list = &__get_cpu_var(blk_cpu_done);
-        list_replace_init(cpu_list, &local_list);
-        local_irq_enable();
-
-        while (!list_empty(&local_list)) {
-                struct request *rq;
-
-                rq = list_entry(local_list.next, struct request, donelist);
-                list_del_init(&rq->donelist);
-                rq->q->softirq_done_fn(rq);
-        }
-}
-
 /**
  * blk_complete_request - end I/O on a request
  * @req:      the request being processed
@@ -71,25 +112,48 @@ static void blk_done_softirq(struct softirq_action *h)
  *     through a softirq handler. The user must have registered a completion
  *     callback through blk_queue_softirq_done().
  **/
-
 void blk_complete_request(struct request *req)
 {
-        struct list_head *cpu_list;
+        struct request_queue *q = req->q;
         unsigned long flags;
+        int ccpu, cpu, group_cpu;
 
-        BUG_ON(!req->q->softirq_done_fn);
+        BUG_ON(!q->softirq_done_fn);
 
         local_irq_save(flags);
+        cpu = smp_processor_id();
+        group_cpu = blk_cpu_to_group(cpu);
 
-        cpu_list = &__get_cpu_var(blk_cpu_done);
-        list_add_tail(&req->donelist, cpu_list);
-        raise_softirq_irqoff(BLOCK_SOFTIRQ);
+        /*
+         * Select completion CPU
+         */
+        if (test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags) && req->cpu != -1)
+                ccpu = req->cpu;
+        else
+                ccpu = cpu;
+
+        if (ccpu == cpu || ccpu == group_cpu) {
+                struct list_head *list;
+do_local:
+                list = &__get_cpu_var(blk_cpu_done);
+                list_add_tail(&req->csd.list, list);
+
+                /*
+                 * if the list only contains our just added request,
+                 * signal a raise of the softirq. If there are already
+                 * entries there, someone already raised the irq but it
+                 * hasn't run yet.
+                 */
+                if (list->next == &req->csd.list)
+                        raise_softirq_irqoff(BLOCK_SOFTIRQ);
+        } else if (raise_blk_irq(ccpu, req))
+                goto do_local;
 
         local_irq_restore(flags);
 }
 EXPORT_SYMBOL(blk_complete_request);
 
-int __init blk_softirq_init(void)
+__init int blk_softirq_init(void)
 {
         int i;