[Qemu-devel] [PATCH] trace: only create simple backend trace files when an event is emitted.

[Hollis Blanchard <hollis_blanchard@mentor.com>]

Previously, trace record files (created by st_set_trace_file_enabled(true))
were created by the simple backend at initialization time, even if no events
were actually enabled.

As a consequence, the working directory would become littered with trace-PID
files just by enabling the simple backend at configure time, even when no
-trace options were used.

---
I've been using this to avoid creation of empty trace record files. I haven't
done any scientific performance studies, but an extra function
call/conditional/return when recording every trace event doesn't seem to have
any noticeable effect.

Signed-off-by: Hollis Blanchard <hollis_blanchard@mentor.com>
---
 trace/simple.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/trace/simple.c b/trace/simple.c
index 3fdcc82..67ccc3c 100644
--- a/trace/simple.c
+++ b/trace/simple.c
@@ -209,6 +209,8 @@ int trace_record_start(TraceBufferRecord *rec, TraceEventID event, size_t datasi
     uint64_t event_u64 = event;
     uint64_t timestamp_ns = get_clock();
 
+    st_set_trace_file_enabled(true);
+
     do {
         old_idx = g_atomic_int_get(&trace_idx);
         smp_rmb();
@@ -320,6 +322,7 @@ void st_set_trace_file_enabled(bool enable)
  */
 void st_set_trace_file(const char *file)
 {
+    /* Tracing will be re-enabled when the next event is written. */
     st_set_trace_file_enabled(false);
 
     g_free(trace_file_name);
@@ -330,8 +333,6 @@ void st_set_trace_file(const char *file)
     } else {
         trace_file_name = g_strdup_printf("%s", file);
     }
-
-    st_set_trace_file_enabled(true);
 }
 
 void st_print_trace_file_status(FILE *stream, int (*stream_printf)(FILE *stream, const char *fmt, ...))
-- 
1.9.1