lkml.org 
[lkml]   [2015]   [Jan]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 3/3] perf: Sample additional clock value
    On 1/23/15 10:06 AM, Pawel Moll wrote:
    > As far as I understand (John?) POSIX timers can be used on any clockid?
    > So it would be possible to obtain a dynamic clock id, for example for my
    > exotic trace hardware (by any means necessary, like opening a char
    > device) and create a timer firing every 1 ms (in the trace time domain).
    > Than this event would be somehow associated with a perf session (for
    > example, by passing the timerid via perf's ioctl) and then, every when
    > timer fires, a perf record (something like PERF_RECORD_TIMER?)
    > containing the timer/clock's value*and* the normal perf timestamp,
    > would be injected into the circular buffer.

    Like this: https://lkml.org/lkml/2011/2/27/158 ? note the date -- 4
    years ago. This is has been dragging on for a long time.

    A few problems with that approach:
    1. I would like to see a sample generated immediately to get the
    perf_clock -> timeofday correlation immediately rather than have to wait
    N (milli)seconds and have perf scan forward through an M-(giga)byte file
    looking for the one sample that gives the correlation.

    I tried to address that problem with an ioctl to force a sample into the
    stream:
    https://lkml.org/lkml/2011/2/27/159
    it did not go over very well.

    2. there is a risk that the realtime samples dominate a stream.

    Another issue that has been raised is updates to xtime by ntp / user. I
    have suggested tracepoints to catch those:
    https://lkml.org/lkml/2011/6/7/636
    I don't believe there were ever any comments on the tracepoints.

    David


    \
     
     \ /
      Last update: 2015-01-23 19:21    [W:5.796 / U:0.116 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site