perf: Optimize the perf_output() path by removing IRQ-disables

Since we can now assume there is only a single writer
to each buffer, we can remove per-cpu lock thingy and
use a simply nest-count to the same effect.

This removes the need to disable IRQs.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 28 insertions, 66 deletions

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index ff5d430d45a7..8cf737da3ec4 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -2519,8 +2519,6 @@ perf_mmap_data_init(struct perf_event *event, struct perf_mmap_data *data)
 {
         long max_size = perf_data_size(data);
 
-        atomic_set(&data->lock, -1);
-
         if (event->attr.watermark) {
                 data->watermark = min_t(long, max_size,
                                         event->attr.wakeup_watermark);
@@ -2906,82 +2904,56 @@ static void perf_output_wakeup(struct perf_output_handle *handle)
 }
 
 /*
- * Curious locking construct.
- *
  * We need to ensure a later event_id doesn't publish a head when a former
- * event_id isn't done writing. However since we need to deal with NMIs we
+ * event isn't done writing. However since we need to deal with NMIs we
  * cannot fully serialize things.
  *
- * What we do is serialize between CPUs so we only have to deal with NMI
- * nesting on a single CPU.
- *
  * We only publish the head (and generate a wakeup) when the outer-most
- * event_id completes.
+ * event completes.
  */
-static void perf_output_lock(struct perf_output_handle *handle)
+static void perf_output_get_handle(struct perf_output_handle *handle)
 {
         struct perf_mmap_data *data = handle->data;
-        int cur, cpu = get_cpu();
 
-        handle->locked = 0;
-
-        for (;;) {
-                cur = atomic_cmpxchg(&data->lock, -1, cpu);
-                if (cur == -1) {
-                        handle->locked = 1;
-                        break;
-                }
-                if (cur == cpu)
-                        break;
-
-                cpu_relax();
-        }
+        preempt_disable();
+        atomic_inc(&data->nest);
 }
 
-static void perf_output_unlock(struct perf_output_handle *handle)
+static void perf_output_put_handle(struct perf_output_handle *handle)
 {
         struct perf_mmap_data *data = handle->data;
         unsigned long head;
-        int cpu;
-
-        data->done_head = data->head;
-
-        if (!handle->locked)
-                goto out;
 
 again:
-        /*
-         * The xchg implies a full barrier that ensures all writes are done
-         * before we publish the new head, matched by a rmb() in userspace when
-         * reading this position.
-         */
-        while ((head = atomic_long_xchg(&data->done_head, 0)))
-                data->user_page->data_head = head;
+        head = atomic_long_read(&data->head);
 
         /*
-         * NMI can happen here, which means we can miss a done_head update.
+         * IRQ/NMI can happen here, which means we can miss a head update.
          */
 
-        cpu = atomic_xchg(&data->lock, -1);
-        WARN_ON_ONCE(cpu != smp_processor_id());
+        if (!atomic_dec_and_test(&data->nest))
+                return;
 
         /*
-         * Therefore we have to validate we did not indeed do so.
+         * Publish the known good head. Rely on the full barrier implied
+         * by atomic_dec_and_test() order the data->head read and this
+         * write.
          */
-        if (unlikely(atomic_long_read(&data->done_head))) {
-                /*
-                 * Since we had it locked, we can lock it again.
-                 */
-                while (atomic_cmpxchg(&data->lock, -1, cpu) != -1)
-                        cpu_relax();
+        data->user_page->data_head = head;
 
+        /*
+         * Now check if we missed an update, rely on the (compiler)
+         * barrier in atomic_dec_and_test() to re-read data->head.
+         */
+        if (unlikely(head != atomic_long_read(&data->head))) {
+                atomic_inc(&data->nest);
                 goto again;
         }
 
         if (atomic_xchg(&data->wakeup, 0))
                 perf_output_wakeup(handle);
-out:
-        put_cpu();
+
+        preempt_enable();
 }
 
 void perf_output_copy(struct perf_output_handle *handle,
@@ -3063,7 +3035,7 @@ int perf_output_begin(struct perf_output_handle *handle,
         if (have_lost)
                 size += sizeof(lost_event);
 
-        perf_output_lock(handle);
+        perf_output_get_handle(handle);
 
         do {
                 /*
@@ -3083,7 +3055,7 @@ int perf_output_begin(struct perf_output_handle *handle,
         handle->head    = head;
 
         if (head - tail > data->watermark)
-                atomic_set(&data->wakeup, 1);
+                atomic_inc(&data->wakeup);
 
         if (have_lost) {
                 lost_event.header.type = PERF_RECORD_LOST;
@@ -3099,7 +3071,7 @@ int perf_output_begin(struct perf_output_handle *handle,
 
 fail:
         atomic_inc(&data->lost);
-        perf_output_unlock(handle);
+        perf_output_put_handle(handle);
 out:
         rcu_read_unlock();
 
@@ -3117,11 +3089,11 @@ void perf_output_end(struct perf_output_handle *handle)
                 int events = atomic_inc_return(&data->events);
                 if (events >= wakeup_events) {
                         atomic_sub(wakeup_events, &data->events);
-                        atomic_set(&data->wakeup, 1);
+                        atomic_inc(&data->wakeup);
                 }
         }
 
-        perf_output_unlock(handle);
+        perf_output_put_handle(handle);
         rcu_read_unlock();
 }
 
@@ -3457,22 +3429,13 @@ static void perf_event_task_output(struct perf_event *event,
 {
         struct perf_output_handle handle;
         struct task_struct *task = task_event->task;
-        unsigned long flags;
         int size, ret;
 
-        /*
-         * If this CPU attempts to acquire an rq lock held by a CPU spinning
-         * in perf_output_lock() from interrupt context, it's game over.
-         */
-        local_irq_save(flags);
-
         size  = task_event->event_id.header.size;
         ret = perf_output_begin(&handle, event, size, 0, 0);
 
-        if (ret) {
-                local_irq_restore(flags);
+        if (ret)
                 return;
-        }
 
         task_event->event_id.pid = perf_event_pid(event, task);
         task_event->event_id.ppid = perf_event_pid(event, current);
@@ -3483,7 +3446,6 @@ static void perf_event_task_output(struct perf_event *event,
         perf_output_put(&handle, task_event->event_id);
 
         perf_output_end(&handle);
-        local_irq_restore(flags);
 }
 
 static int perf_event_task_match(struct perf_event *event)