perf_counter: fix perf_poll()

Impact: fix kerneltop 100% CPU usage

Only return a poll event when there's actually been one, poll_wait()
doesn't actually wait for the waitq you pass it, it only enqueues
you on it.

Only once all FDs have been iterated and none of thm returned a
poll-event will it schedule().

Also make it return POLL_HUP when there's not mmap() area to read from.

Further, fix a silly bug in the write code.

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arjan van de Ven <arjan@infradead.org>
Orig-LKML-Reference: <1237897096.24918.181.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 13 insertions, 2 deletions

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 2b5e66d5ebdf..48212c15b7d6 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -246,6 +246,7 @@ struct file;
 struct perf_mmap_data {
         struct rcu_head                 rcu_head;
         int                             nr_pages;
+        atomic_t                        wakeup;
         atomic_t                        head;
         struct perf_counter_mmap_page   *user_page;
         void                            *data_pages[0];
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 0dfe91094fd1..affe227d56a0 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1161,7 +1161,16 @@ perf_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 static unsigned int perf_poll(struct file *file, poll_table *wait)
 {
         struct perf_counter *counter = file->private_data;
-        unsigned int events = POLLIN;
+        struct perf_mmap_data *data;
+        unsigned int events;
+
+        rcu_read_lock();
+        data = rcu_dereference(counter->data);
+        if (data)
+                events = atomic_xchg(&data->wakeup, 0);
+        else
+                events = POLL_HUP;
+        rcu_read_unlock();
 
         poll_wait(file, &counter->waitq, wait);
 
@@ -1425,7 +1434,7 @@ static int perf_output_write(struct perf_counter *counter, int nmi,
 
         do {
                 offset = head = atomic_read(&data->head);
-                head += sizeof(u64);
+                head += size;
         } while (atomic_cmpxchg(&data->head, offset, head) != offset);
 
         wakeup = (offset >> PAGE_SHIFT) != (head >> PAGE_SHIFT);
@@ -1446,6 +1455,7 @@ static int perf_output_write(struct perf_counter *counter, int nmi,
          * generate a poll() wakeup for every page boundary crossed
          */
         if (wakeup) {
+                atomic_xchg(&data->wakeup, POLL_IN);
                 __perf_counter_update_userpage(counter, data);
                 if (nmi) {
                         counter->wakeup_pending = 1;