Re: [PATCH 2/6] perf tools: Store clock references for -k/--clockid option

From: Alexey Budankov
Date: Fri Jul 31 2020 - 11:52:48 EST



On 31.07.2020 0:39, Jiri Olsa wrote:
> Adding new CLOCK_DATA feature that stores reference times
> when -k/--clockid option is specified.
>
> It contains clock id and its reference time together with
> wall clock time taken at the 'same time', both values are
> in nanoseconds.
>
> The format of data is as below:
>
> struct {
> u32 version; /* version = 1 */
> u32 clockid;
> u64 clockid_time_ns;
> u64 wall_clock_ns;
> };
>
> This clock reference times will be used in following changes
> to display wall clock for perf events.
>
> It's available only for recording with clockid specified,
> because it's the only case where we can get reference time
> to wallclock time. It's can't do that with perf clock yet.
>
> Original-patch-by: David Ahern <dsahern@xxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
> .../Documentation/perf.data-file-format.txt | 13 ++
> tools/perf/builtin-record.c | 41 +++++++
> tools/perf/util/env.h | 12 ++
> tools/perf/util/header.c | 112 ++++++++++++++++++
> tools/perf/util/header.h | 1 +
> 5 files changed, 179 insertions(+)
>
> diff --git a/tools/perf/Documentation/perf.data-file-format.txt b/tools/perf/Documentation/perf.data-file-format.txt
> index b6472e463284..c484e81987c7 100644
> --- a/tools/perf/Documentation/perf.data-file-format.txt
> +++ b/tools/perf/Documentation/perf.data-file-format.txt
> @@ -389,6 +389,19 @@ struct {
> Example:
> cpu pmu capabilities: branches=32, max_precise=3, pmu_name=icelake
>
> + HEADER_CLOCK_DATA = 29,
> +
> + Contains clock id and its reference time together with wall clock
> + time taken at the 'same time', both values are in nanoseconds.
> + The format of data is as below.
> +
> +struct {
> + u32 version; /* version = 1 */
> + u32 clockid;
> + u64 clockid_time_ns;
> + u64 wall_clock_ns;
> +};
> +
> other bits are reserved and should ignored for now
> HEADER_FEAT_BITS = 256,
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 468c669519a6..f8280e721e1a 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -70,6 +70,7 @@
> #include <linux/time64.h>
> #include <linux/zalloc.h>
> #include <linux/bitmap.h>
> +#include <sys/time.h>
>
> struct switch_output {
> bool enabled;
> @@ -1203,6 +1204,9 @@ static void record__init_features(struct record *rec)
> if (!(rec->opts.use_clockid && rec->opts.clockid_res_ns))
> perf_header__clear_feat(&session->header, HEADER_CLOCKID);
>
> + if (!rec->opts.use_clockid)
> + perf_header__clear_feat(&session->header, HEADER_CLOCK_DATA);
> +
> perf_header__clear_feat(&session->header, HEADER_DIR_FORMAT);
> if (!record__comp_enabled(rec))
> perf_header__clear_feat(&session->header, HEADER_COMPRESSED);
> @@ -1551,6 +1555,40 @@ static int record__setup_sb_evlist(struct record *rec)
> return 0;
> }
>
> +static int record__init_clock(struct record *rec)
> +{
> + struct perf_session *session = rec->session;
> + struct timespec ref_clockid;
> + struct timeval ref_tod;
> + u64 ref;
> +
> + if (!rec->opts.use_clockid)
> + return 0;
> +
> + session->header.env.clock.clockid = rec->opts.clockid;
> +
> + if (gettimeofday(&ref_tod, NULL) != 0) {
> + pr_err("gettimeofday failed, cannot set reference time.\n");
> + return -1;
> + }
> +
> + if (clock_gettime(rec->opts.clockid, &ref_clockid)) {
> + pr_err("clock_gettime failed, cannot set reference time.\n");
> + return -1;
> + }

It might also want to be implemented in a loop and iteration with minimal
time delta is chosen to improve synchronization accuracy and also mitigate
possible context switches between gettimeofday() and clock_gettime() calls.

Alexei