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SubjectRe: [PATCH 3/3] writeback: Do not congestion sleep when there are no congested BDIs
On Thu, Aug 26, 2010 at 04:14:16PM +0100, Mel Gorman wrote:
> If congestion_wait() is called with no BDIs congested, the caller will
> sleep for the full timeout and this is an unnecessary sleep. This patch
> checks if there are BDIs congested. If so, it goes to sleep as normal.
> If not, it calls cond_resched() to ensure the caller is not hogging the
> CPU longer than its quota but otherwise will not sleep.
>
> This is aimed at reducing some of the major desktop stalls reported during
> IO. For example, while kswapd is operating, it calls congestion_wait()
> but it could just have been reclaiming clean page cache pages with no
> congestion. Without this patch, it would sleep for a full timeout but after
> this patch, it'll just call schedule() if it has been on the CPU too long.
> Similar logic applies to direct reclaimers that are not making enough
> progress.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
> mm/backing-dev.c | 20 ++++++++++++++------
> 1 files changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index a49167f..6abe860 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c

Function's decripton should be changed since we don't wait next write any more.

> @@ -767,13 +767,21 @@ long congestion_wait(int sync, long timeout)
> DEFINE_WAIT(wait);
> wait_queue_head_t *wqh = &congestion_wqh[sync];
>
> - /* Check if this call to congestion_wait was necessary */
> - if (atomic_read(&nr_bdi_congested[sync]) == 0)
> + /*
> + * If there is no congestion, there is no point sleeping on the queue.
> + * This call was unecessary but in case we are spinning due to a bad
> + * caller, at least call cond_reched() and sleep if our CPU quota
> + * has expired
> + */
> + if (atomic_read(&nr_bdi_congested[sync]) == 0) {
> unnecessary = true;
> -
> - prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
> - ret = io_schedule_timeout(timeout);
> - finish_wait(wqh, &wait);
> + cond_resched();
> + ret = 0;

"ret = timeout" is more proper as considering io_schedule_timeout's return value.

> + } else {
> + prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
> + ret = io_schedule_timeout(timeout);
> + finish_wait(wqh, &wait);
> + }
>
> trace_writeback_congest_waited(jiffies_to_usecs(jiffies - start),
> unnecessary);
> --
> 1.7.1
>
> --
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>

--
Kind regards,
Minchan Kim


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