Fixes #729
Signed-off-by: Mathieu Desnoyers <[email protected]>
---
src/bin/lttng-sessiond/ust-metadata.c | 52 +++++++++++++++++++++++++++------
1 file changed, 43 insertions(+), 9 deletions(-)
diff --git a/src/bin/lttng-sessiond/ust-metadata.c
b/src/bin/lttng-sessiond/ust-metadata.c
index b0f83d2..f511253 100644
--- a/src/bin/lttng-sessiond/ust-metadata.c
+++ b/src/bin/lttng-sessiond/ust-metadata.c
@@ -24,6 +24,7 @@
#include <string.h>
#include <stdarg.h>
#include <stdio.h>
+#include <stdlib.h>
#include <limits.h>
#include <unistd.h>
#include <inttypes.h>
@@ -37,6 +38,8 @@
#define max_t(type, a, b) ((type) ((a) > (b) ? (a) : (b)))
#endif
+#define NR_CLOCK_OFFSET_SAMPLES 20
+
static inline
int fls(unsigned int x)
{
@@ -490,14 +493,21 @@ 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 sort_uint64(const void *_a, const void *_b)
+{
+ const uint64_t *a = _a, *b = _b;
+
+ if (a < b)
+ return -1;
+ else if (a > b)
+ return 1;
+ else
+ return 0;
+}
+
+static
+int measure_single_clock_offset(uint64_t *offset_sample)
{
uint64_t offset, monotonic[2], realtime;
struct timespec rts = { 0, 0 };
@@ -506,15 +516,39 @@ uint64_t measure_clock_offset(void)
monotonic[0] = trace_clock_read64();
ret = clock_gettime(CLOCK_REALTIME, &rts);
if (ret < 0)
- return 0;
+ return ret;
monotonic[1] = trace_clock_read64();
offset = (monotonic[0] + monotonic[1]) >> 1;
realtime = (uint64_t) rts.tv_sec * 1000000000ULL;
realtime += rts.tv_nsec;
offset = realtime - offset;
- return offset;
+ *offset_sample = offset;
+ return 0;
}
+/*
+ * Approximation of NTP time of day to clock monotonic correlation,
+ * taken at start of trace.
+ * Take the median of many measurements to remove uncertainty caused by
+ * preemption.
+ */
+static
+uint64_t measure_clock_offset(void)
+{
+ uint64_t offset_samples[NR_CLOCK_OFFSET_SAMPLES];
+ int i;
+
+ for (i = 0; i < NR_CLOCK_OFFSET_SAMPLES; i++) {
+ if (measure_single_clock_offset(&offset_samples[i])) {
+ return 0;
+ }
+ }
+
+ /* Find median (using qsort() which is already implemented) */
+ qsort(offset_samples, NR_CLOCK_OFFSET_SAMPLES,
+ sizeof(*offset_samples), sort_uint64);
+ return offset_samples[NR_CLOCK_OFFSET_SAMPLES >> 1];
+}
/*
* Should be called with session registry mutex held.
--
1.7.10.4
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