[Qemu-devel] [PATCH v3 09/18] trace: emit name <-> ID mapping in simpletrace header

["Daniel P. Berrange" <berrange@redhat.com>]

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>
---
 scripts/simpletrace.py                       | 49 ++++++++++++++++++++--------
 scripts/tracetool/format/simpletrace_stap.py | 23 +++++++++++--
 trace/simple.c                               | 32 ++++++++++++++++--
 3 files changed, 86 insertions(+), 18 deletions(-)

diff --git a/scripts/simpletrace.py b/scripts/simpletrace.py
index 3916c6d..98abb7a 100755
--- a/scripts/simpletrace.py
+++ b/scripts/simpletrace.py
@@ -19,6 +19,9 @@ header_event_id = 0xffffffffffffffff
 header_magic    = 0xf2b177cb0aa429b4
 dropped_event_id = 0xfffffffffffffffe
 
+record_type_mapping = 0
+record_type_event = 1
+
 log_header_fmt = '=QQQ'
 rec_header_fmt = '=QQII'
 
@@ -30,14 +33,15 @@ def read_header(fobj, hfmt):
         return None
     return struct.unpack(hfmt, hdr)
 
-def get_record(edict, rechdr, fobj):
-    """Deserialize a trace record from a file into a tuple (event_num, timestamp, pid, arg1, ..., arg6)."""
+def get_record(edict, idtoname, rechdr, fobj):
+    """Deserialize a trace record from a file into a tuple (name, timestamp, pid, arg1, ..., arg6)."""
     if rechdr is None:
         return None
-    rec = (rechdr[0], rechdr[1], rechdr[3])
     if rechdr[0] != dropped_event_id:
         event_id = rechdr[0]
-        event = edict[event_id]
+        name = idtoname[event_id]
+        rec = (name, rechdr[1], rechdr[3])
+        event = edict[name]
         for type, name in event.args:
             if is_string(type):
                 l = fobj.read(4)
@@ -48,15 +52,22 @@ def get_record(edict, rechdr, fobj):
                 (value,) = struct.unpack('=Q', fobj.read(8))
                 rec = rec + (value,)
     else:
+        rec = ("dropped", rechdr[1], rechdr[3])
         (value,) = struct.unpack('=Q', fobj.read(8))
         rec = rec + (value,)
     return rec
 
+def get_mapping(fobj):
+    (event_id, ) = struct.unpack('=Q', fobj.read(8))
+    (len, ) = struct.unpack('=L', fobj.read(4))
+    name = fobj.read(len)
+
+    return (event_id, name)
 
-def read_record(edict, fobj):
+def read_record(edict, idtoname, fobj):
     """Deserialize a trace record from a file into a tuple (event_num, timestamp, pid, arg1, ..., arg6)."""
     rechdr = read_header(fobj, rec_header_fmt)
-    return get_record(edict, rechdr, fobj) # return tuple of record elements
+    return get_record(edict, idtoname, rechdr, fobj) # return tuple of record elements
 
 def read_trace_header(fobj):
     """Read and verify trace file header"""
@@ -67,20 +78,30 @@ def read_trace_header(fobj):
         raise ValueError('Not a valid trace file!')
 
     log_version = header[2]
-    if log_version not in [0, 2, 3]:
+    if log_version not in [0, 2, 3, 4]:
         raise ValueError('Unknown version of tracelog format!')
-    if log_version != 3:
+    if log_version != 4:
         raise ValueError('Log format %d not supported with this QEMU release!'
                          % log_version)
 
 def read_trace_records(edict, fobj):
     """Deserialize trace records from a file, yielding record tuples (event_num, timestamp, pid, arg1, ..., arg6)."""
+    idtoname = {
+        dropped_event_id: "dropped"
+    }
     while True:
-        rec = read_record(edict, fobj)
-        if rec is None:
+        t = fobj.read(8)
+        if len(t) == 0:
             break
 
-        yield rec
+        (rectype, ) = struct.unpack('=Q', t)
+        if rectype == record_type_mapping:
+            mapping = get_mapping(fobj)
+            idtoname[mapping[0]] = mapping[1]
+        else:
+            rec = read_record(edict, idtoname, fobj)
+
+            yield rec
 
 class Analyzer(object):
     """A trace file analyzer which processes trace records.
@@ -115,10 +136,10 @@ def process(events, log, analyzer, read_header=True):
         read_trace_header(log)
 
     dropped_event = Event.build("Dropped_Event(uint64_t num_events_dropped)")
-    edict = {dropped_event_id: dropped_event}
+    edict = {"dropped": dropped_event}
 
-    for num, event in enumerate(events):
-        edict[num] = event
+    for event in events:
+        edict[event.name] = event
 
     def build_fn(analyzer, event):
         if isinstance(event, str):
diff --git a/scripts/tracetool/format/simpletrace_stap.py b/scripts/tracetool/format/simpletrace_stap.py
index 7e44bc1..0573f43 100644
--- a/scripts/tracetool/format/simpletrace_stap.py
+++ b/scripts/tracetool/format/simpletrace_stap.py
@@ -21,6 +21,24 @@ from tracetool.format.stap import stap_escape
 
 def generate(events, backend):
     out('/* This file is autogenerated by tracetool, do not edit. */',
+        '',
+        'global event_name_to_id_map',
+        'global event_next_id',
+        'function simple_trace_map_event(name)',
+        '',
+        '{',
+        '    if (!([name] in event_name_to_id_map)) {',
+        '        event_name_to_id_map[name] = event_next_id',
+        '        name_len = strlen(name)',
+        '        printf("%%8b%%8b%%4b%%.*s", 0, event_next_id, name_len, name_len, name)',
+        '        event_next_id = event_next_id + 1',
+        '    }',
+        '    return event_name_to_id_map[name]',
+        '}',
+        'probe begin',
+        '{',
+        '    printf("%%8b%%8b%%8b", 0xffffffffffffffff, 0xf2b177cb0aa429b4, 4)',
+        '}',
         '')
 
     for event_id, e in enumerate(events):
@@ -29,6 +47,7 @@ def generate(events, backend):
 
         out('probe %(probeprefix)s.simpletrace.%(name)s = %(probeprefix)s.%(name)s ?',
             '{',
+            '    id = simple_trace_map_event("%(name)s")',
             probeprefix=probeprefix(),
             name=e.name)
 
@@ -48,7 +67,7 @@ def generate(events, backend):
         sizestr = ' + '.join(sizes)
 
         # Generate format string and value pairs for record header and arguments
-        fields = [('8b', str(event_id)),
+        fields = [('8b', 'id'),
                   ('8b', 'gettimeofday_ns()'),
                   ('4b', sizestr),
                   ('4b', 'pid()')]
@@ -63,7 +82,7 @@ def generate(events, backend):
         # Emit the entire record in a single SystemTap printf()
         fmt_str = '%'.join(fmt for fmt, _ in fields)
         arg_str = ', '.join(arg for _, arg in fields)
-        out('    printf("%%%(fmt_str)s", %(arg_str)s)',
+        out('    printf("%%8b%%%(fmt_str)s", 1, %(arg_str)s)',
             fmt_str=fmt_str, arg_str=arg_str)
 
         out('}')
diff --git a/trace/simple.c b/trace/simple.c
index 716b66d..81caa5a 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 +279,27 @@ 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 +324,8 @@ 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;
-- 
2.7.4