trace: emit name <-> ID mapping in simpletrace header

Currently simpletrace assumes that events are given IDs
starting from 0, based on the order in which they appear
in the trace-events file, with no gaps. When the
trace-events file is split up, this assumption becomes
problematic.

To deal with this, extend the simpletrace format so that
it outputs a table of event name <-> ID mappings. That
will allow QEMU to assign arbitrary IDs to events without
breaking simpletrace parsing.

The v3 simple trace format was

  FILE HEADER
  EVENT TRACE RECORD 0
  EVENT TRACE RECORD 1
  ...
  EVENT TRACE RECORD N

The v4 simple trace format is now

  FILE HEADER
  EVENT MAPPING RECORD 0
  EVENT MAPPING RECORD 1
  ...
  EVENT MAPPING RECORD M
  EVENT TRACE RECORD RECORD 0
  EVENT TRACE RECORD RECORD 1
  ...
  EVENT TRACE RECORD N

Although this shows all the mapping records being emitted
upfront, this is not required by the format. While the main
simpletrace backend will emit all mappings at startup,
the systemtap simpletrace.stp script will emit the mappings
at first use. eg

  FILE HEADER
  ...
  EVENT MAPPING RECORD 0
  EVENT TRACE RECORD RECORD 0
  EVENT TRACE RECORD RECORD 1
  EVENT MAPPING RECORD 1
  EVENT TRACE RECORD RECORD 2
  ...
  EVENT TRACE RECORD N

This is more space efficient given that most trace records
only include a subset of events.

In modifying the systemtap simpletrace code, a 'begin' probe
was added to emit the trace event header, so you no longer
need to add '--no-header' when running simpletrace.py for
systemtap generated trace files.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1475588159-30598-12-git-send-email-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 31 insertions, 2 deletions

diff --git a/trace/simple.c b/trace/simple.c
index 2ec32e159c..b263622fa9 100644
--- a/trace/simple.c
+++ b/trace/simple.c
@@ -24,7 +24,7 @@
 #define HEADER_MAGIC 0xf2b177cb0aa429b4ULL
 
 /** Trace file version number, bump if format changes */
-#define HEADER_VERSION 3
+#define HEADER_VERSION 4
 
 /** Records were dropped event ID */
 #define DROPPED_EVENT_ID (~(uint64_t)0 - 1)
@@ -56,6 +56,9 @@ static uint32_t trace_pid;
 static FILE *trace_fp;
 static char *trace_file_name;
 
+#define TRACE_RECORD_TYPE_MAPPING 0
+#define TRACE_RECORD_TYPE_EVENT   1
+
 /* * Trace buffer entry */
 typedef struct {
     uint64_t event; /* event ID value */
@@ -160,6 +163,7 @@ static gpointer writeout_thread(gpointer opaque)
     unsigned int idx = 0;
     int dropped_count;
     size_t unused __attribute__ ((unused));
+    uint64_t type = TRACE_RECORD_TYPE_EVENT;
 
     for (;;) {
         wait_for_trace_records_available();
@@ -174,10 +178,12 @@ static gpointer writeout_thread(gpointer opaque)
             } while (!g_atomic_int_compare_and_exchange(&dropped_events,
                                                         dropped_count, 0));
             dropped.rec.arguments[0] = dropped_count;
+            unused = fwrite(&type, sizeof(type), 1, trace_fp);
             unused = fwrite(&dropped.rec, dropped.rec.length, 1, trace_fp);
         }
 
         while (get_trace_record(idx, &recordptr)) {
+            unused = fwrite(&type, sizeof(type), 1, trace_fp);
             unused = fwrite(recordptr, recordptr->length, 1, trace_fp);
             writeout_idx += recordptr->length;
             free(recordptr); /* don't use g_free, can deadlock when traced */
@@ -273,6 +279,28 @@ void trace_record_finish(TraceBufferRecord *rec)
     }
 }
 
+static int st_write_event_mapping(void)
+{
+    uint64_t type = TRACE_RECORD_TYPE_MAPPING;
+    TraceEventIter iter;
+    TraceEvent *ev;
+
+    trace_event_iter_init(&iter, NULL);
+    while ((ev = trace_event_iter_next(&iter)) != NULL) {
+        uint64_t id = trace_event_get_id(ev);
+        const char *name = trace_event_get_name(ev);
+        uint32_t len = strlen(name);
+        if (fwrite(&type, sizeof(type), 1, trace_fp) != 1 ||
+            fwrite(&id, sizeof(id), 1, trace_fp) != 1 ||
+            fwrite(&len, sizeof(len), 1, trace_fp) != 1 ||
+            fwrite(name, len, 1, trace_fp) != 1) {
+            return -1;
+        }
+    }
+
+    return 0;
+}
+
 void st_set_trace_file_enabled(bool enable)
 {
     if (enable == !!trace_fp) {
@@ -297,7 +325,8 @@ void st_set_trace_file_enabled(bool enable)
             return;
         }
 
-        if (fwrite(&header, sizeof header, 1, trace_fp) != 1) {
+        if (fwrite(&header, sizeof header, 1, trace_fp) != 1 ||
+            st_write_event_mapping() < 0) {
             fclose(trace_fp);
             trace_fp = NULL;
             return;