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    SubjectRe: [RFC][PATCH] iowait statistics
    Rik van Riel wrote:
    >
    > Hi,
    >
    > the following patch implements iowait statistics in a simple way:
    >
    > 1) if we go to sleep while waiting on a page or buffer, we
    > increment nr_iowait_tasks, note that this is done only in
    > the slow path so overhead shouldn't even be measurable
    >
    > 2) if no process is running, the timer interrupt adds a jiffy
    > to the iowait time
    >
    > 3) iowait time is counted separately from user/system/idle and
    > can overlap with either system or idle (when no process is
    > running the system can still be busy processing interrupts)
    >
    > 4) on SMP systems the iowait time can be overestimated, no big
    > deal IMHO but cheap suggestions for improvement are welcome

    I suspect that a number of these statistical accounting mechanisms
    are going to break. The new irq-affinity code works awfully well.

    The kernel profiler in 2.5 doesn't work very well at present.
    When investigating this, I ran a busy-wait process. It attached
    itself to CPU #3 and that CPU received precisely zero interrupts
    across a five minute period. So the profiler cunningly avoids profiling
    busy CPUs, which is rather counter-productive. Fortunate that oprofile
    uses NMI.

    > ...
    > ===== fs/buffer.c 1.64 vs edited =====
    > --- 1.64/fs/buffer.c Mon May 13 19:04:59 2002
    > +++ edited/fs/buffer.c Mon May 13 19:16:57 2002
    > @@ -156,8 +156,10 @@
    > get_bh(bh);
    > add_wait_queue(&bh->b_wait, &wait);
    > do {
    > + atomic_inc(&nr_iowait_tasks);
    > run_task_queue(&tq_disk);
    > set_task_state(tsk, TASK_UNINTERRUPTIBLE);
    > + atomic_dec(&nr_iowait_tasks);
    > if (!buffer_locked(bh))
    > break;
    > schedule();

    Shouldn't the atomic_inc cover the schedule()?


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