mmiotrace: count events lost due to not recording

Impact: enhances lost events counting in mmiotrace

The tracing framework, or the ring buffer facility it uses, has a switch
to stop recording data. When recording is off, the trace events will be
lost. The framework does not count these, so mmiotrace has to count them
itself.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_mmiotrace.c

@@ -9,6 +9,7 @@
 #include <linux/kernel.h>
 #include <linux/mmiotrace.h>
 #include <linux/pci.h>
+#include <asm/atomic.h>
 
 #include "trace.h"
 
@@ -19,6 +20,7 @@ struct header_iter {
 static struct trace_array *mmio_trace_array;
 static bool overrun_detected;
 static unsigned long prev_overruns;
+static atomic_t dropped_count;
 
 static void mmio_reset_data(struct trace_array *tr)
 {
@@ -121,11 +123,11 @@ static void mmio_close(struct trace_iterator *iter)
 
 static unsigned long count_overruns(struct trace_iterator *iter)
 {
-	unsigned long cnt = 0;
+	unsigned long cnt = atomic_xchg(&dropped_count, 0);
 	unsigned long over = ring_buffer_overruns(iter->tr->buffer);
 
 	if (over > prev_overruns)
-		cnt = over - prev_overruns;
+		cnt += over - prev_overruns;
 	prev_overruns = over;
 	return cnt;
 }
@@ -310,8 +312,10 @@ static void __trace_mmiotrace_rw(struct trace_array *tr,
 
 	event	= ring_buffer_lock_reserve(tr->buffer, sizeof(*entry),
 					   &irq_flags);
-	if (!event)
+	if (!event) {
+		atomic_inc(&dropped_count);
 		return;
+	}
 	entry	= ring_buffer_event_data(event);
 	tracing_generic_entry_update(&entry->ent, 0, preempt_count());
 	entry->ent.type			= TRACE_MMIO_RW;
@@ -338,8 +342,10 @@ static void __trace_mmiotrace_map(struct trace_array *tr,
 
 	event	= ring_buffer_lock_reserve(tr->buffer, sizeof(*entry),
 					   &irq_flags);
-	if (!event)
+	if (!event) {
+		atomic_inc(&dropped_count);
 		return;
+	}
 	entry	= ring_buffer_event_data(event);
 	tracing_generic_entry_update(&entry->ent, 0, preempt_count());
 	entry->ent.type			= TRACE_MMIO_MAP;