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    SubjectRe: [ANNOUNCE] sched: schedtop utility
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    On Mon, 2008-06-02 at 18:18 +0530, Ankita Garg wrote:
    > Hi Gregory,
    >
    > On Thu, May 22, 2008 at 08:06:44AM -0600, Gregory Haskins wrote:
    > > Hi all scheduler developers,
    > > I had an itch to scratch w.r.t. watching the stats in /proc/schedstats, and it appears that the perl scripts referenced in Documentation/scheduler/sched-stats.txt do not support v14 from HEAD so I whipped up a little utility I call "schedtop".
    > >
    >
    > Nice tool! Helps in better visualization of the data in schedstats.
    >
    > Using the tool, realized that most of the timing related stats therein
    > might not be completely usable in many scenarios, as might already be
    > known.
    >
    > Without any additional load on the system, all the stats are nice and
    > sane. But, as soon as I ran my particular testcase, the data
    > pertaining to the delta of run_delay/cpu_time went haywire! I understand
    > that all the values are based on top of rq->clock, which relies on tsc that
    > is not synced across cpus and would result in skews/incorrect values.
    > But, turns out to be not so reliable data for debugging. This is
    > ofcourse nothing related to the tool, but for schedstat in
    > general...rather just adding on to the already existing woes with non-syned
    > tscs :-)

    Thing is, things runtime should be calculated by using per cpu deltas.
    You take a stamp when you get scheduled on the cpu and another one when
    you stop running, then the delta is added to runtime.

    This is always on the same cpu - when you get migrated you're stopped
    and re-scheduled so that should work out nicely.

    So in that sense it shouldn't matter that the rq->clock values can get
    skewed between cpus.

    So I'm still a little puzzled by your observations; though it could be
    that the schedstat stuff got broken - I've never really looked too
    closely at it.





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