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

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Jul 31, 2020 at 06:52:36PM +0300, Alexey Budankov wrote:
> 
> 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@gmail.com>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  .../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.

right, we could make this more accurate.. I'll post some follow up
change with that

thanks,
jirka