ring-buffer: convert cpu buffer entries to local_t

The entries counter in cpu buffer is not atomic. It can be updated by
other interrupts or from another CPU (readers).

But making entries into "atomic_t" causes an atomic operation that can
hurt performance. Instead we convert it to a local_t that will increment
a counter with a local CPU atomic operation (if the arch supports it).

Instead of fighting with readers and overwrites that decrement the counter,
I added a "read" counter. Every time a reader reads an entry it is
incremented.

We already have a overrun counter and with that, the entries counter and
the read counter, we can calculate the total number of entries in the
buffer with:

  (entries - overrun) - read

As long as the total number of entries in the ring buffer is less than
the word size, this will work. But since the entries counter was previously
a long, this is no different than what we had before.

Thanks to Andrew Morton for pointing out in the first version that
atomic_t does not replace unsigned long. I switched to atomic_long_t
even though it is signed. A negative count is most likely a bug.

[ Impact: keep accurate count of cpu buffer entries ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 12 insertions, 9 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 26e1359fe193..c792ea893b01 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -405,7 +405,8 @@ struct ring_buffer_per_cpu {
         unsigned long                   nmi_dropped;
         unsigned long                   commit_overrun;
         unsigned long                   overrun;
-        unsigned long                   entries;
+        unsigned long                   read;
+        local_t                         entries;
         u64                             write_stamp;
         u64                             read_stamp;
         atomic_t                        record_disabled;
@@ -997,7 +998,6 @@ static void rb_update_overflow(struct ring_buffer_per_cpu *cpu_buffer)
                 if (event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
                         continue;
                 cpu_buffer->overrun++;
-                cpu_buffer->entries--;
         }
 }
 
@@ -1588,7 +1588,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_lock_reserve);
 static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer,
                       struct ring_buffer_event *event)
 {
-        cpu_buffer->entries++;
+        local_inc(&cpu_buffer->entries);
 
         /* Only process further if we own the commit */
         if (!rb_is_commit(cpu_buffer, event))
@@ -1722,7 +1722,7 @@ void ring_buffer_discard_commit(struct ring_buffer *buffer,
          * The commit is still visible by the reader, so we
          * must increment entries.
          */
-        cpu_buffer->entries++;
+        local_inc(&cpu_buffer->entries);
  out:
         /*
          * If a write came in and pushed the tail page
@@ -1902,7 +1902,8 @@ unsigned long ring_buffer_entries_cpu(struct ring_buffer *buffer, int cpu)
                 return 0;
 
         cpu_buffer = buffer->buffers[cpu];
-        ret = cpu_buffer->entries;
+        ret = (local_read(&cpu_buffer->entries) - cpu_buffer->overrun)
+                - cpu_buffer->read;
 
         return ret;
 }
@@ -1985,7 +1986,8 @@ unsigned long ring_buffer_entries(struct ring_buffer *buffer)
         /* if you care about this being correct, lock the buffer */
         for_each_buffer_cpu(buffer, cpu) {
                 cpu_buffer = buffer->buffers[cpu];
-                entries += cpu_buffer->entries;
+                entries += (local_read(&cpu_buffer->entries) -
+                            cpu_buffer->overrun) - cpu_buffer->read;
         }
 
         return entries;
@@ -2225,7 +2227,7 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
 
         if (event->type_len <= RINGBUF_TYPE_DATA_TYPE_LEN_MAX
                         || rb_discarded_event(event))
-                cpu_buffer->entries--;
+                cpu_buffer->read++;
 
         rb_update_read_stamp(cpu_buffer, event);
 
@@ -2642,7 +2644,8 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
         cpu_buffer->nmi_dropped = 0;
         cpu_buffer->commit_overrun = 0;
         cpu_buffer->overrun = 0;
-        cpu_buffer->entries = 0;
+        cpu_buffer->read = 0;
+        local_set(&cpu_buffer->entries, 0);
 
         cpu_buffer->write_stamp = 0;
         cpu_buffer->read_stamp = 0;
@@ -2813,7 +2816,7 @@ static void rb_remove_entries(struct ring_buffer_per_cpu *cpu_buffer,
                 /* Only count data entries */
                 if (event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
                         continue;
-                cpu_buffer->entries--;
+                cpu_buffer->read++;
         }
         __raw_spin_unlock(&cpu_buffer->lock);
 }