perf_counter: uncouple data_head updates from wakeups

Keep data_head up-to-date irrespective of notifications. This fixes
the case where you disable a counter and don't get a notification for
the last few pending events, and it also allows polling usage.

[ Impact: increase precision of perfcounter mmap-ed fields ]

Suggested-by: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20090505155436.925084300@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index a356fa6..17b6310 100644 (file)
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -362,9 +362,11 @@ struct perf_mmap_data {
        atomic_t                        head;           /* write position    */
        atomic_t                        events;         /* event limit       */
 
-       atomic_t                        wakeup_head;    /* completed head    */
+       atomic_t                        done_head;      /* completed head    */
        atomic_t                        lock;           /* concurrent writes */
 
+       atomic_t                        wakeup;         /* needs a wakeup    */
+
        struct perf_counter_mmap_page   *user_page;
        void                            *data_pages[0];
 };

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 5f86a11..ba5e921 100644 (file)
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1696,7 +1696,6 @@ struct perf_output_handle {
        struct perf_mmap_data   *data;
        unsigned int            offset;
        unsigned int            head;
-       int                     wakeup;
        int                     nmi;
        int                     overflow;
        int                     locked;
@@ -1752,8 +1751,7 @@ static void perf_output_unlock(struct perf_output_handle *handle)
        struct perf_mmap_data *data = handle->data;
        int head, cpu;
 
-       if (handle->wakeup)
-               data->wakeup_head = data->head;
+       data->done_head = data->head;
 
        if (!handle->locked)
                goto out;
@@ -1764,13 +1762,11 @@ again:
         * before we publish the new head, matched by a rmb() in userspace when
         * reading this position.
         */
-       while ((head = atomic_xchg(&data->wakeup_head, 0))) {
+       while ((head = atomic_xchg(&data->done_head, 0)))
                data->user_page->data_head = head;
-               handle->wakeup = 1;
-       }
 
        /*
-        * NMI can happen here, which means we can miss a wakeup_head update.
+        * NMI can happen here, which means we can miss a done_head update.
         */
 
        cpu = atomic_xchg(&data->lock, 0);
@@ -1779,7 +1775,7 @@ again:
        /*
         * Therefore we have to validate we did not indeed do so.
         */
-       if (unlikely(atomic_read(&data->wakeup_head))) {
+       if (unlikely(atomic_read(&data->done_head))) {
                /*
                 * Since we had it locked, we can lock it again.
                 */
@@ -1789,7 +1785,7 @@ again:
                goto again;
        }
 
-       if (handle->wakeup)
+       if (atomic_xchg(&data->wakeup, 0))
                perf_output_wakeup(handle);
 out:
        local_irq_restore(handle->flags);
@@ -1824,7 +1820,9 @@ static int perf_output_begin(struct perf_output_handle *handle,
 
        handle->offset  = offset;
        handle->head    = head;
-       handle->wakeup  = (offset >> PAGE_SHIFT) != (head >> PAGE_SHIFT);
+
+       if ((offset >> PAGE_SHIFT) != (head >> PAGE_SHIFT))
+               atomic_set(&data->wakeup, 1);
 
        return 0;
 
@@ -1882,7 +1880,7 @@ static 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);
-                       handle->wakeup = 1;
+                       atomic_set(&data->wakeup, 1);
                }
        }