perf: Fix loss of notification with multi-event
When you do: $ perf record -e cycles,cycles,cycles noploop 10 You expect about 10,000 samples for each event, i.e., 10s at 1000samples/sec. However, this is not what's happening. You get much fewer samples, maybe 3700 samples/event: $ perf report -D | tail -15 Aggregated stats: TOTAL events: 10998 MMAP events: 66 COMM events: 2 SAMPLE events: 10930 cycles stats: TOTAL events: 3644 SAMPLE events: 3644 cycles stats: TOTAL events: 3642 SAMPLE events: 3642 cycles stats: TOTAL events: 3644 SAMPLE events: 3644 On a Intel Nehalem or even AMD64, there are 4 counters capable of measuring cycles, so there is plenty of space to measure those events without multiplexing (even with the NMI watchdog active). And even with multiplexing, we'd expect roughly the same number of samples per event. The root of the problem was that when the event that caused the buffer to become full was not the first event passed on the cmdline, the user notification would get lost. The notification was sent to the file descriptor of the overflowed event but the perf tool was not polling on it. The perf tool aggregates all samples into a single buffer, i.e., the buffer of the first event. Consequently, it assumes notifications for any event will come via that descriptor. The seemingly straight forward solution of moving the waitq into the ringbuffer object doesn't work because of life-time issues. One could perf_event_set_output() on a fd that you're also blocking on and cause the old rb object to be freed while its waitq would still be referenced by the blocked thread -> FAIL. Therefore link all events to the ringbuffer and broadcast the wakeup from the ringbuffer object to all possible events that could be waited upon. This is rather ugly, and we're open to better solutions but it works for now. Reported-by: Stephane Eranian <eranian@google.com> Finished-by: Stephane Eranian <eranian@google.com> Reviewed-by: Stephane Eranian <eranian@google.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Link: http://lkml.kernel.org/r/20111126014731.GA7030@quad Signed-off-by: Ingo Molnar <mingo@elte.hu>
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@ -822,6 +822,7 @@ struct perf_event {
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int mmap_locked;
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struct user_struct *mmap_user;
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struct ring_buffer *rb;
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struct list_head rb_entry;
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/* poll related */
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wait_queue_head_t waitq;
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@ -185,6 +185,9 @@ static void cpu_ctx_sched_in(struct perf_cpu_context *cpuctx,
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static void update_context_time(struct perf_event_context *ctx);
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static u64 perf_event_time(struct perf_event *event);
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static void ring_buffer_attach(struct perf_event *event,
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struct ring_buffer *rb);
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void __weak perf_event_print_debug(void) { }
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extern __weak const char *perf_pmu_name(void)
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@ -3191,12 +3194,33 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
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struct ring_buffer *rb;
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unsigned int events = POLL_HUP;
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/*
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* Race between perf_event_set_output() and perf_poll(): perf_poll()
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* grabs the rb reference but perf_event_set_output() overrides it.
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* Here is the timeline for two threads T1, T2:
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* t0: T1, rb = rcu_dereference(event->rb)
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* t1: T2, old_rb = event->rb
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* t2: T2, event->rb = new rb
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* t3: T2, ring_buffer_detach(old_rb)
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* t4: T1, ring_buffer_attach(rb1)
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* t5: T1, poll_wait(event->waitq)
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*
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* To avoid this problem, we grab mmap_mutex in perf_poll()
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* thereby ensuring that the assignment of the new ring buffer
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* and the detachment of the old buffer appear atomic to perf_poll()
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*/
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mutex_lock(&event->mmap_mutex);
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rcu_read_lock();
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rb = rcu_dereference(event->rb);
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if (rb)
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if (rb) {
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ring_buffer_attach(event, rb);
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events = atomic_xchg(&rb->poll, 0);
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}
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rcu_read_unlock();
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mutex_unlock(&event->mmap_mutex);
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poll_wait(file, &event->waitq, wait);
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return events;
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@ -3497,6 +3521,49 @@ unlock:
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return ret;
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}
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static void ring_buffer_attach(struct perf_event *event,
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struct ring_buffer *rb)
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{
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unsigned long flags;
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if (!list_empty(&event->rb_entry))
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return;
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spin_lock_irqsave(&rb->event_lock, flags);
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if (!list_empty(&event->rb_entry))
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goto unlock;
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list_add(&event->rb_entry, &rb->event_list);
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unlock:
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spin_unlock_irqrestore(&rb->event_lock, flags);
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}
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static void ring_buffer_detach(struct perf_event *event,
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struct ring_buffer *rb)
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{
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unsigned long flags;
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if (list_empty(&event->rb_entry))
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return;
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spin_lock_irqsave(&rb->event_lock, flags);
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list_del_init(&event->rb_entry);
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wake_up_all(&event->waitq);
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spin_unlock_irqrestore(&rb->event_lock, flags);
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}
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static void ring_buffer_wakeup(struct perf_event *event)
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{
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struct ring_buffer *rb;
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rcu_read_lock();
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rb = rcu_dereference(event->rb);
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list_for_each_entry_rcu(event, &rb->event_list, rb_entry) {
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wake_up_all(&event->waitq);
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}
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rcu_read_unlock();
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}
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static void rb_free_rcu(struct rcu_head *rcu_head)
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{
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struct ring_buffer *rb;
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@ -3522,9 +3589,19 @@ static struct ring_buffer *ring_buffer_get(struct perf_event *event)
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static void ring_buffer_put(struct ring_buffer *rb)
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{
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struct perf_event *event, *n;
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unsigned long flags;
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if (!atomic_dec_and_test(&rb->refcount))
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return;
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spin_lock_irqsave(&rb->event_lock, flags);
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list_for_each_entry_safe(event, n, &rb->event_list, rb_entry) {
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list_del_init(&event->rb_entry);
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wake_up_all(&event->waitq);
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}
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spin_unlock_irqrestore(&rb->event_lock, flags);
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call_rcu(&rb->rcu_head, rb_free_rcu);
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}
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@ -3547,6 +3624,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
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atomic_long_sub((size >> PAGE_SHIFT) + 1, &user->locked_vm);
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vma->vm_mm->pinned_vm -= event->mmap_locked;
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rcu_assign_pointer(event->rb, NULL);
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ring_buffer_detach(event, rb);
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mutex_unlock(&event->mmap_mutex);
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ring_buffer_put(rb);
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@ -3701,7 +3779,7 @@ static const struct file_operations perf_fops = {
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void perf_event_wakeup(struct perf_event *event)
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{
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wake_up_all(&event->waitq);
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ring_buffer_wakeup(event);
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if (event->pending_kill) {
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kill_fasync(&event->fasync, SIGIO, event->pending_kill);
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@ -5823,6 +5901,8 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
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INIT_LIST_HEAD(&event->group_entry);
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INIT_LIST_HEAD(&event->event_entry);
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INIT_LIST_HEAD(&event->sibling_list);
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INIT_LIST_HEAD(&event->rb_entry);
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init_waitqueue_head(&event->waitq);
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init_irq_work(&event->pending, perf_pending_event);
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@ -6029,6 +6109,8 @@ set:
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old_rb = event->rb;
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rcu_assign_pointer(event->rb, rb);
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if (old_rb)
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ring_buffer_detach(event, old_rb);
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ret = 0;
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unlock:
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mutex_unlock(&event->mmap_mutex);
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@ -22,6 +22,9 @@ struct ring_buffer {
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local_t lost; /* nr records lost */
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long watermark; /* wakeup watermark */
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/* poll crap */
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spinlock_t event_lock;
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struct list_head event_list;
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struct perf_event_mmap_page *user_page;
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void *data_pages[0];
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@ -209,6 +209,9 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
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rb->writable = 1;
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atomic_set(&rb->refcount, 1);
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INIT_LIST_HEAD(&rb->event_list);
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spin_lock_init(&rb->event_lock);
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}
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#ifndef CONFIG_PERF_USE_VMALLOC
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