[lttng-dev] [PATCH lttng-tools] Fix: measure UST clock offset with best sample (v2)

[David Goulet]

Merged.

Backported to 2.4 and 2.3

On 30 Jan (17:06:30), Mathieu Desnoyers wrote:
> Fixes #729
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> ---
>  src/bin/lttng-sessiond/ust-metadata.c |   55 ++++++++++++++++++++++++++-------
>  1 file changed, 44 insertions(+), 11 deletions(-)
> 
> diff --git a/src/bin/lttng-sessiond/ust-metadata.c b/src/bin/lttng-sessiond/ust-metadata.c
> index b0f83d2..f430ae1 100644
> --- 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.
> -- 
> 1.7.10.4
> 
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