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    SubjectRe: [accounting regression since rc1] scheduler updates
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    On Tue, 2007-08-21 at 10:42 +0200, Ingo Molnar wrote:
    > * Christian Borntraeger <borntraeger@de.ibm.com> wrote:
    >
    > > Am Montag, 20. August 2007 schrieb Ingo Molnar:
    > > > could you send that precise sched_clock() patch? It should be an order
    > > > of magnitude simpler than the high-precision stime/utime tracking you
    > > > already do, and it's needed for quality scheduling anyway.
    > >
    > > I have a question about that. I just played with sched_clock, and even
    > > when I intentionally slow down sched_clock by a factor of 2, my cpu
    > > bound process gets 100 % in top. If this is intentional, I dont
    > > understand how a virtualized sched_clock would fix the accounting
    > > change?
    >
    > hm, does on s390 scheduler_tick() get driven in virtual time or in real
    > time? The very latest scheduler code will enforce a minimum rate of
    > sched_clock() across two scheduler_tick() calls (in rc3 and later
    > kernels). If sched_clock() "slows down" but scheduler_tick() still has a
    > real-time frequency then that impacts the quality of scheduling. So
    > scheduler_tick() and sched_clock() must really have the same behavior
    > (either both are virtual or both are real), so that scheduling becomes
    > invariant to steal-time.

    scheduler_tick() is based on the HZ timer which uses the TOD clock =
    real time. sched_clock() currently uses the TOD clock as well so in
    regard to the new scheduler we currently do not have a problem. We have
    a problem with cpu time accounting, the change to the /proc code breaks
    the precise accounting on s390. To solve the cpu time accounting we need
    to change sched_clock() to the cpu timer = virtual time. To change the
    scheduler_tick() as well requires another patch and I fear it would
    complicate things in the s390 backend.

    And if you say that the scheduling becomes invariant to steal-time, how
    is the cpu time accounting via sum_exec supposed to work if it does not
    take steal-time into account ?

    --
    blue skies,
    Martin.

    "Reality continues to ruin my life." - Calvin.


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