Re: [PATCH RFC] mm: Implement balance_dirty_pages() through waiting for flusher thread

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2010-06-17 at 20:04 +0200, Jan Kara wrote:
> +/* Wait until write_chunk is written or we get below dirty limits */
> +void bdi_wait_written(struct backing_dev_info *bdi, long write_chunk)
> +{
> +       struct bdi_written_count wc = {
> +                                       .list = LIST_HEAD_INIT(wc.list),
> +                                       .written = write_chunk,
> +                               };
> +       DECLARE_WAITQUEUE(wait, current);
> +       int pause = 1;
> +
> +       bdi_add_writer(bdi, &wc, &wait);
> +       for (;;) {
> +               if (signal_pending_state(TASK_KILLABLE, current))
> +                       break;
> +
> +               /*
> +                * Make the task just killable so that tasks cannot circumvent
> +                * throttling by sending themselves non-fatal signals...
> +                */
> +               __set_current_state(TASK_KILLABLE);
> +               io_schedule_timeout(pause);
> +
> +               /*
> +                * The following check is save without wb_written_wait.lock
> +                * because once bdi_remove_writer removes us from the list
> +                * noone will touch us and it's impossible for list_empty check
> +                * to trigger as false positive. The barrier is there to avoid
> +                * missing the wakeup when we are removed from the list.
> +                */
> +               smp_rmb();
> +               if (list_empty(&wc.list))
> +                       break;
> +
> +               if (!dirty_limits_exceeded(bdi))
> +                       break;
> +
> +               /*
> +                * Increase the delay for each loop, up to our previous
> +                * default of taking a 100ms nap.
> +                */
> +               pause <<= 1;
> +               if (pause > HZ / 10)
> +                       pause = HZ / 10;
> +       }
> +
> +       spin_lock_irq(&bdi->wb_written_wait.lock);
> +       __remove_wait_queue(&bdi->wb_written_wait, &wait);
> +       if (!list_empty(&wc.list))
> +               bdi_remove_writer(bdi, &wc);
> +       spin_unlock_irq(&bdi->wb_written_wait.lock);
> +} 

OK, so the whole pause thing is simply because we don't get a wakeup
when we drop below the limit, right?


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