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

[Mel Gorman <mel@csn.ul.ie>]

On Fri, Aug 27, 2010 at 02:38:43AM +0900, Minchan Kim wrote:
> 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. 
> 

My bad. I need to check that "next write" thing. It doesn't appear to be
happening but maybe that side of things just broke somewhere in the
distant past. I lack context of how this is meant to work so maybe
someone will educate me.

> > @@ -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.
> 

Good point, will fix.

> > +	} 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);

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab