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    SubjectRe: [RFC][PATCH] iowait statistics
    On Mon, May 13, 2002 at 10:19:26PM -0300, Rik van Riel wrote:
    > 2) if no process is running, the timer interrupt adds a jiffy
    > to the iowait time
    > 4) on SMP systems the iowait time can be overestimated, no big
    > deal IMHO but cheap suggestions for improvement are welcome

    This appears to be global across all cpu's. Maybe nr_iowait_tasks
    should be accounted on a per-cpu basis, where

    (1) If a task sleeps for an io while bound to a cpu it
    counts toward the cpu's number of iowait tasks.

    (2) iowait time is accounted and reports are generated already
    on a per-cpu basis, so there's nothing to do there.

    (3) The global statistic does not need to be entirely accurate;
    a lockfree approximation by summing across all cpus'
    local counters should suffice for global iowait. I also
    suspect it will not fluctuate rapidly enough for truly
    horribly inaccurate results to occur.

    (4) A per-cpu nr_iowait_tasks counter may still well need
    to be atomic as other cpu's may be stealing sleeping
    tasks purportedly bound to a given cpu at migration
    time (in order to prevent going negative) and in that
    process altering other cpus' counters.

    (5) A flag marking a task as in iowait may well need to be kept
    in the task_struct so that at migration time the
    appropriate counter adjustments can be made.

    (6) Given sufficient cpu affinity in the scheduler the case
    where one cpu's counter needs alteration from another
    should be relatively uncommon.

    The scheduler already participates in keeping per_cpu_user[],
    per_cpu_system[], and per_cpu_nice[] up-to-date, so it's not
    unreasonable to expect its support for per_cpu_iowait[].

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