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    SubjectRe: [PATCH 3/6] perf: add reference time event
    On 06/17/2011 08:17 AM, Frederic Weisbecker wrote:
    > On Fri, Jun 17, 2011 at 08:04:59AM -0600, David Ahern wrote:
    >>
    >>
    >> On 06/17/2011 07:32 AM, Frederic Weisbecker wrote:
    >>> On Tue, Jun 07, 2011 at 05:55:46PM -0600, David Ahern wrote:
    >>>> For initial perf_clock to time-of-day correlation.
    >>>>
    >>>> Signed-off-by: David Ahern <dsahern@gmail.com>
    >>>> ---
    >>>> tools/perf/util/event.c | 1 +
    >>>> tools/perf/util/event.h | 8 ++++++++
    >>>> tools/perf/util/session.c | 4 ++++
    >>>> tools/perf/util/session.h | 3 ++-
    >>>> 4 files changed, 15 insertions(+), 1 deletions(-)
    >>>>
    >>>> diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
    >>>> index 3c1b8a6..1a89a04 100644
    >>>> --- a/tools/perf/util/event.c
    >>>> +++ b/tools/perf/util/event.c
    >>>> @@ -24,6 +24,7 @@ static const char *perf_event__names[] = {
    >>>> [PERF_RECORD_HEADER_TRACING_DATA] = "TRACING_DATA",
    >>>> [PERF_RECORD_HEADER_BUILD_ID] = "BUILD_ID",
    >>>> [PERF_RECORD_FINISHED_ROUND] = "FINISHED_ROUND",
    >>>> + [PERF_RECORD_REFTIME] = "REF_TIME",
    >>>> };
    >>>>
    >>>> const char *perf_event__name(unsigned int id)
    >>>> diff --git a/tools/perf/util/event.h b/tools/perf/util/event.h
    >>>> index 1d7f664..f481f90 100644
    >>>> --- a/tools/perf/util/event.h
    >>>> +++ b/tools/perf/util/event.h
    >>>> @@ -98,6 +98,7 @@ enum perf_user_event_type { /* above any possible kernel type */
    >>>> PERF_RECORD_HEADER_TRACING_DATA = 66,
    >>>> PERF_RECORD_HEADER_BUILD_ID = 67,
    >>>> PERF_RECORD_FINISHED_ROUND = 68,
    >>>> + PERF_RECORD_REFTIME = 69,
    >>>
    >>> We would like to avoid adding more custom events like these. They were very convenient
    >>> but they steal the kernel event type space. They are deemed for removal in the long term.
    >>>
    >>> Another idea to achieve what you want would be to create a new perf event header feature,
    >>> like HEADER_TRACE_INFO or HEADER_BUILD_ID are. Then use that to create a space in the perf
    >>> file to save that couple of clocks initial values.
    >>
    >> you mean like this:
    >> https://lkml.org/lkml/2010/12/7/813
    >>
    >> David
    >
    > Exactly, why did you change?

    Finally getting back to this.

    The answer to the 'why' is that putting a reference timestamp in the
    header field does not work for file appends across reboots. ie., the case:
    perf record --tod ...
    reboot
    perf record -A --tod ...

    perf_clock timestamps change across reboots so the reference time
    created by the first invocation is not valid for the append case. The
    discussion then drifted towards having a kernel side event which per
    past patch sets has its own issues.

    So to summarize the options proposed to date and issues with the proposals:
    1. reference timestamp in header
    - does not work for appends across reboots

    2. synthesized events
    - preference against them

    3. kernel side event
    - cannot generate an initial sample (with counter value and
    perf_clock timestamp) on demand - e.g., start of session; a proposal to
    use an ioctl to add one to the event stream was shot down

    At this point the only idea that comes to mind is to use a combination
    of 2 and 3: add the kernel side clock event
    (https://lkml.org/lkml/2011/2/18/11), read the realtime clock counter,
    read the monotonic clock timestamp (ie., perf_clock value), and
    synthesize a perf sample that is written to the file. The append case
    (with mismatch in --tod options between record invocations) would be
    handled by having the kernel side clock event in the event list
    (perf_evlist__equal would fail if --tod was not used for all invocations).

    David


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