clock offset: print negative value in metadata for lttng-ust metadata

[lttng-tools.git] / src / bin / lttng-sessiond / ust-metadata.c

diff --git a/src/bin/lttng-sessiond/ust-metadata.c b/src/bin/lttng-sessiond/ust-metadata.c
index 984d5f813ccf9277e6e221b7299f8f80a7d50954..e5d4944ed931ca9c2177cf124734a4dc0d3f6cf2 100644 (file)
--- a/src/bin/lttng-sessiond/ust-metadata.c
+++ b/src/bin/lttng-sessiond/ust-metadata.c
@@ -41,7 +41,7 @@
 #define NR_CLOCK_OFFSET_SAMPLES                10
 
 struct offset_sample {
-       uint64_t offset;                /* correlation offset */
+       int64_t offset;                 /* correlation offset */
        uint64_t measure_delta;         /* lower is better */
 };
 
@@ -604,10 +604,14 @@ int _lttng_event_header_declare(struct ust_registry_session *session)
        );
 }
 
+/*
+ * The offset between monotonic and realtime clock can be negative if
+ * the system sets the REALTIME clock to 0 after boot.
+ */
 static
 int measure_single_clock_offset(struct offset_sample *sample)
 {
-       uint64_t offset, monotonic[2], measure_delta, realtime;
+       uint64_t monotonic_avg, monotonic[2], measure_delta, realtime;
        uint64_t tcf = trace_clock_freq();
        struct timespec rts = { 0, 0 };
        int ret;
@@ -626,15 +630,14 @@ int measure_single_clock_offset(struct offset_sample *sample)
                 */
                return 0;
        }
-       offset = (monotonic[0] + monotonic[1]) >> 1;
+       monotonic_avg = (monotonic[0] + monotonic[1]) >> 1;
        realtime = (uint64_t) rts.tv_sec * tcf;
        if (tcf == NSEC_PER_SEC) {
                realtime += rts.tv_nsec;
        } else {
                realtime += (uint64_t) rts.tv_nsec * tcf / NSEC_PER_SEC;
        }
-       offset = realtime - offset;
-       sample->offset = offset;
+       sample->offset = (int64_t) realtime - monotonic_avg;
        sample->measure_delta = measure_delta;
        return 0;
 }
@@ -643,9 +646,10 @@ int measure_single_clock_offset(struct offset_sample *sample)
  * Approximation of NTP time of day to clock monotonic correlation,
  * taken at start of trace. Keep the measurement that took the less time
  * to complete, thus removing imprecision caused by preemption.
+ * May return a negative offset.
  */
 static
-uint64_t measure_clock_offset(void)
+int64_t measure_clock_offset(void)
 {
        int i;
        struct offset_sample offset_best_sample = {
@@ -793,7 +797,7 @@ int ust_metadata_session_statedump(struct ust_registry_session *session,
                "       description = \"%s\";\n"
                "       freq = %" PRIu64 "; /* Frequency, in Hz */\n"
                "       /* clock value offset from Epoch is: offset * (1/freq) */\n"
-               "       offset = %" PRIu64 ";\n"
+               "       offset = %" PRId64 ";\n"
                "};\n\n",
                trace_clock_description(),
                trace_clock_freq(),