Re: [PATCH 3/3] writeback: Do not congestion sleep when there are no congested BDIs

[Minchan Kim <minchan.kim@gmail.com>]

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