Fix: measure UST clock offset with best sample (v2)

Fixes #729

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>

diff --git a/src/bin/lttng-sessiond/ust-metadata.c b/src/bin/lttng-sessiond/ust-metadata.c
index b0f83d2721654b0351c872f082997bef2b0dc881..f430ae1f1e8ed390df410b71370f1771286afc66 100644 (file)
--- a/src/bin/lttng-sessiond/ust-metadata.c
+++ b/src/bin/lttng-sessiond/ust-metadata.c
@@ -37,6 +37,13 @@
 #define max_t(type, a, b)      ((type) ((a) > (b) ? (a) : (b)))
 #endif
 
+#define NR_CLOCK_OFFSET_SAMPLES                10
+
+struct offset_sample {
+       uint64_t offset;                /* correlation offset */
+       uint64_t measure_delta;         /* lower is better */
+};
+
 static inline
 int fls(unsigned int x)
 {
@@ -490,31 +497,57 @@ int _lttng_event_header_declare(struct ust_registry_session *session)
        );
 }
 
-/*
- * Approximation of NTP time of day to clock monotonic correlation,
- * taken at start of trace.
- * Yes, this is only an approximation. Yes, we can (and will) do better
- * in future versions.
- */
 static
-uint64_t measure_clock_offset(void)
+int measure_single_clock_offset(struct offset_sample *sample)
 {
-       uint64_t offset, monotonic[2], realtime;
+       uint64_t offset, monotonic[2], measure_delta, realtime;
        struct timespec rts = { 0, 0 };
        int ret;
 
        monotonic[0] = trace_clock_read64();
        ret = clock_gettime(CLOCK_REALTIME, &rts);
-       if (ret < 0)
-               return 0;
+       if (ret < 0) {
+               return ret;
+       }
        monotonic[1] = trace_clock_read64();
+       measure_delta = monotonic[1] - monotonic[0];
+       if (measure_delta > sample->measure_delta) {
+               /*
+                * Discard value if it took longer to read than the best
+                * sample so far.
+                */
+               return 0;
+       }
        offset = (monotonic[0] + monotonic[1]) >> 1;
        realtime = (uint64_t) rts.tv_sec * 1000000000ULL;
        realtime += rts.tv_nsec;
        offset = realtime - offset;
-       return offset;
+       sample->offset = offset;
+       sample->measure_delta = measure_delta;
+       return 0;
 }
 
+/*
+ * 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.
+ */
+static
+uint64_t measure_clock_offset(void)
+{
+       int i;
+       struct offset_sample offset_best_sample = {
+               .offset = 0,
+               .measure_delta = UINT64_MAX,
+       };
+
+       for (i = 0; i < NR_CLOCK_OFFSET_SAMPLES; i++) {
+               if (measure_single_clock_offset(&offset_best_sample)) {
+                       return 0;
+               }
+       }
+       return offset_best_sample.offset;
+}
 
 /*
  * Should be called with session registry mutex held.