linux-kernel - Re: [PATCH 3/3] vmscan: Put kswapd to sleep on its own waitqueue, not congestion

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Message-ID: <20100309102145.GB4883@csn.ul.ie>
Date:	Tue, 9 Mar 2010 10:21:46 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-mm@...ck.org,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] vmscan: Put kswapd to sleep on its own waitqueue,
	not congestion

On Tue, Mar 09, 2010 at 09:00:44PM +1100, Nick Piggin wrote:
> On Mon, Mar 08, 2010 at 11:48:23AM +0000, Mel Gorman wrote:
> > If kswapd is raising its priority to get the zone over the high
> > watermark, it may call congestion_wait() ostensibly to allow congestion
> > to clear. However, there is no guarantee that the queue is congested at
> > this point because it depends on kswapds previous actions as well as the
> > rest of the system. Kswapd could simply be working hard because there is
> > a lot of SYNC traffic in which case it shouldn't be sleeping.
> > 
> > Rather than waiting on congestion and potentially sleeping for longer
> > than it should, this patch puts kswapd back to sleep on the kswapd_wait
> > queue for the timeout. If direct reclaimers are in trouble, kswapd will
> > be rewoken as it should instead of sleeping when there is work to be
> > done.
> 
> Well but it is quite possible that many allocators are coming in to
> wake it up. So with your patch, I think we'd need to consider the case
> where the timeout approaches 0 here (if it's always being woken).
> 

True, similar to how zonepressure_wait() rechecks the watermarks if
there is still a timeout left and deciding whether to sleep again or
not.

> Direct reclaimers need not be involved because the pages might be
> hovering around the asynchronous reclaim watermarks (which would be
> the ideal case of system operation).
> 
> In which case, can you explain how this change makes sense? Why is
> it a good thing not to wait when we previously did wait?
> 

Well, it makes sense from the perspective it's better for kswapd to be doing
work than direct reclaim. If processes are hitting the watermarks then
why should kswapd be asleep?

That said, if the timeout was non-zero it should be able to make some decision
on whether it should be really awake. Putting the page allocator and kswapd
patches into the same series was a mistake because it's conflating two
different problems as one. I'm going to drop this one for the moment and
treat the page allocator patch in isolation.

> > 
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > ---
> >  mm/vmscan.c |   11 +++++++----
> >  1 files changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 4f92a48..894d366 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1955,7 +1955,7 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
> >   * interoperates with the page allocator fallback scheme to ensure that aging
> >   * of pages is balanced across the zones.
> >   */
> > -static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
> > +static unsigned long balance_pgdat(pg_data_t *pgdat, wait_queue_t *wait, int order)
> >  {
> >  	int all_zones_ok;
> >  	int priority;
> > @@ -2122,8 +2122,11 @@ loop_again:
> >  		if (total_scanned && (priority < DEF_PRIORITY - 2)) {
> >  			if (has_under_min_watermark_zone)
> >  				count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);
> > -			else
> > -				congestion_wait(BLK_RW_ASYNC, HZ/10);
> > +			else {
> > +				prepare_to_wait(&pgdat->kswapd_wait, wait, TASK_INTERRUPTIBLE);
> > +				schedule_timeout(HZ/10);
> > +				finish_wait(&pgdat->kswapd_wait, wait);
> > +			}
> >  		}
> >  
> >  		/*
> > @@ -2272,7 +2275,7 @@ static int kswapd(void *p)
> >  		 * after returning from the refrigerator
> >  		 */
> >  		if (!ret)
> > -			balance_pgdat(pgdat, order);
> > +			balance_pgdat(pgdat, &wait, order);
> >  	}
> >  	return 0;
> >  }
> > -- 
> > 1.6.5
> 

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