[PATCH 3/3] XFS: Don't wake xfsbufd when idle

[Alex Elder]

Dave Chinner wrote:
> The xfsbufd wakes every xfsbufd_centisecs (once per second by
> default) for each filesystem even when the filesystem is idle.
> If the xfsbufd has nothing to do, put it into a long term sleep
> and only wake it up when there is work pending (i.e. dirty
> buffers to flush soon). This will make laptop power misers happy.

Patch generally looks good but I have a question, below.

> Signed-off-by: Dave Chinner <david at fromorbit.com>
> ---
>  fs/xfs/linux-2.6/xfs_buf.c |   20 +++++++++++++++-----
>  1 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
> index 77b8be8..b51be4e 100644
> --- a/fs/xfs/linux-2.6/xfs_buf.c
> +++ b/fs/xfs/linux-2.6/xfs_buf.c
> @@ -1595,6 +1595,11 @@ xfs_buf_delwri_queue(
>  		list_del(&bp->b_list);
>  	}
> 
> +	if (list_empty(dwq)) {
> +		/* start xfsbufd as it is about to have something to do */
> +		wake_up_process(bp->b_target->bt_task);
> +	}
> +
>  	bp->b_flags |= _XBF_DELWRI_Q;
>  	list_add_tail(&bp->b_list, dwq);
>  	bp->b_queuetime = jiffies;
> @@ -1644,6 +1649,8 @@ xfsbufd_wakeup(
>  	list_for_each_entry(btp, &xfs_buftarg_list, bt_list) {
>  		if (test_bit(XBT_FORCE_SLEEP, &btp->bt_flags))
>  			continue;
> +		if (list_empty(&btp->bt_delwrite_queue))
> +			continue;
>  		set_bit(XBT_FORCE_FLUSH, &btp->bt_flags);
>  		wake_up_process(btp->bt_task);
>  	}
> @@ -1708,6 +1715,9 @@ xfsbufd(
>  	set_freezable();
> 
>  	do {
> +		long	age = xfs_buf_age_centisecs * msecs_to_jiffies(10);
> +		long	tout = age;

Why do you switch from using xfs_buf_timer_centisecs to
using xfs_buf_age_centisecs for the timeout (in the non-empty
delwrite queue case)?


> +
>  		if (unlikely(freezing(current))) {
>  			set_bit(XBT_FORCE_SLEEP, &target->bt_flags);
>  			refrigerator();
> @@ -1715,12 +1725,12 @@ xfsbufd(
>  			clear_bit(XBT_FORCE_SLEEP, &target->bt_flags);
>  		}
> 
> -		schedule_timeout_interruptible(
> -			xfs_buf_timer_centisecs * msecs_to_jiffies(10));

Here is where the timeout was computed previously.

> -
> -		xfs_buf_delwri_split(target, &tmp,
> -				xfs_buf_age_centisecs * msecs_to_jiffies(10));
> +		/* sleep for a long time if there is nothing to do. */
> +		if (list_empty(&target->bt_delwrite_queue))
> +			tout = MAX_SCHEDULE_TIMEOUT;

Effect of the timout change is here...

> +		schedule_timeout_interruptible(tout);
> 
> +		xfs_buf_delwri_split(target, &tmp, age);
>  		count = 0;
>  		while (!list_empty(&tmp)) {
>  			bp = list_entry(tmp.next, xfs_buf_t, b_list);