ring-buffer: Add a 'dropped events' counter

The existing 'overrun' counter is incremented when the ring
buffer wraps around, with overflow on (the default). We wanted
a way to count requests lost from the buffer filling up with
overflow off, too. I decided to add a new counter instead
of retro-fitting the existing one because it seems like a
different statistic to count conceptually, and also because
of how the code was structured.

Link: http://lkml.kernel.org/r/1310765038-26399-1-git-send-email-slavapestov@google.com

Signed-off-by: Slava Pestov <slavapestov@google.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

2 files changed, 38 insertions, 6 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b979426d16c6..0ebeb1d76ddf 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -460,9 +460,10 @@ struct ring_buffer_per_cpu {
         unsigned long                   lost_events;
         unsigned long                   last_overrun;
         local_t                         entries_bytes;
-        local_t                         commit_overrun;
-        local_t                         overrun;
         local_t                         entries;
+        local_t                         overrun;
+        local_t                         commit_overrun;
+        local_t                         dropped_events;
         local_t                         committing;
         local_t                         commits;
         unsigned long                   read;
@@ -2155,8 +2156,10 @@ rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
                          * If we are not in overwrite mode,
                          * this is easy, just stop here.
                          */
-                        if (!(buffer->flags & RB_FL_OVERWRITE))
+                        if (!(buffer->flags & RB_FL_OVERWRITE)) {
+                                local_inc(&cpu_buffer->dropped_events);
                                 goto out_reset;
+                        }
 
                         ret = rb_handle_head_page(cpu_buffer,
                                                   tail_page,
@@ -2995,7 +2998,8 @@ unsigned long ring_buffer_entries_cpu(struct ring_buffer *buffer, int cpu)
 EXPORT_SYMBOL_GPL(ring_buffer_entries_cpu);
 
 /**
- * ring_buffer_overrun_cpu - get the number of overruns in a cpu_buffer
+ * ring_buffer_overrun_cpu - get the number of overruns caused by the ring
+ * buffer wrapping around (only if RB_FL_OVERWRITE is on).
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to get the number of overruns from
  */
@@ -3015,7 +3019,9 @@ unsigned long ring_buffer_overrun_cpu(struct ring_buffer *buffer, int cpu)
 EXPORT_SYMBOL_GPL(ring_buffer_overrun_cpu);
 
 /**
- * ring_buffer_commit_overrun_cpu - get the number of overruns caused by commits
+ * ring_buffer_commit_overrun_cpu - get the number of overruns caused by
+ * commits failing due to the buffer wrapping around while there are uncommitted
+ * events, such as during an interrupt storm.
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to get the number of overruns from
  */
@@ -3036,6 +3042,28 @@ ring_buffer_commit_overrun_cpu(struct ring_buffer *buffer, int cpu)
 EXPORT_SYMBOL_GPL(ring_buffer_commit_overrun_cpu);
 
 /**
+ * ring_buffer_dropped_events_cpu - get the number of dropped events caused by
+ * the ring buffer filling up (only if RB_FL_OVERWRITE is off).
+ * @buffer: The ring buffer
+ * @cpu: The per CPU buffer to get the number of overruns from
+ */
+unsigned long
+ring_buffer_dropped_events_cpu(struct ring_buffer *buffer, int cpu)
+{
+        struct ring_buffer_per_cpu *cpu_buffer;
+        unsigned long ret;
+
+        if (!cpumask_test_cpu(cpu, buffer->cpumask))
+                return 0;
+
+        cpu_buffer = buffer->buffers[cpu];
+        ret = local_read(&cpu_buffer->dropped_events);
+
+        return ret;
+}
+EXPORT_SYMBOL_GPL(ring_buffer_dropped_events_cpu);
+
+/**
  * ring_buffer_entries - get the number of entries in a buffer
  * @buffer: The ring buffer
  *
@@ -3864,9 +3892,10 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
         local_set(&cpu_buffer->reader_page->page->commit, 0);
         cpu_buffer->reader_page->read = 0;
 
-        local_set(&cpu_buffer->commit_overrun, 0);
         local_set(&cpu_buffer->entries_bytes, 0);
         local_set(&cpu_buffer->overrun, 0);
+        local_set(&cpu_buffer->commit_overrun, 0);
+        local_set(&cpu_buffer->dropped_events, 0);
         local_set(&cpu_buffer->entries, 0);
         local_set(&cpu_buffer->committing, 0);
         local_set(&cpu_buffer->commits, 0);
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index f6928edacd6d..36c213fbfce7 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4385,6 +4385,9 @@ tracing_stats_read(struct file *filp, char __user *ubuf,
         usec_rem = do_div(t, USEC_PER_SEC);
         trace_seq_printf(s, "now ts: %5llu.%06lu\n", t, usec_rem);
 
+        cnt = ring_buffer_dropped_events_cpu(tr->buffer, cpu);
+        trace_seq_printf(s, "dropped events: %ld\n", cnt);
+
         count = simple_read_from_buffer(ubuf, count, ppos, s->buffer, s->len);
 
         kfree(s);