Re: [patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[Shaohua Li <shaohua.li@intel.com>]

On Fri, Jul 29, 2011 at 04:50:43PM +0800, Mel Gorman wrote:
> On Fri, Jul 29, 2011 at 08:35:25AM +0800, Shaohua Li wrote:
> > On Thu, 2011-07-28 at 18:56 +0800, Mel Gorman wrote:
> > > On Thu, Jul 28, 2011 at 04:13:01PM +0800, Shaohua Li wrote:
> > > > correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> > > > should clear ZONE_CONGESTED regardless if this is a high order
> > > > allocation, because pages can be reclaimed in other tasks but
> > > > ZONE_CONGESTED is only cleared in kswapd.
> > > > 
> > > 
> > > What problem does this solve?
> > > 
> > > As it is, for high order allocations it takes the following steps
> > > 
> > > If reclaiming at high order {
> > > 	for each zone {
> > > 		if all_unreclaimable
> > > 			skip
> > > 		if watermark is not met
> > > 			order = 0
> > > 			loop again
> > > 		
> > > 		/* watermark is met */
> > > 		clear congested
> > > 	}
> > > }
> > > 
> > > If high orders are failing, kswapd balances for order-0 where there
> > > is already a cleaning of ZONE_CONGESTED if the zone was shrunk and
> > > became balanced. I see the case for hunk 1 of the patch because now
> > > it'll clear ZONE_CONGESTED for zones that are already balanced which
> > > might have a noticable effect on wait_iff_congested. Is this what
> > > you see? Even if it is, it does not explain hunk 2 of the patch.
> > I first looked at the hunk 2 place and thought we don't clear
> > ZONE_CONGESTED there. I then figured out we need do the same thing for
> > the hunk 1. But you are correct, with hunk 1, hunk 2 isn't required.
> > updated patch.
> > 
> > 
> > 
> > correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> > should clear ZONE_CONGESTED because pages can be reclaimed in
> > other tasks but ZONE_CONGESTED is only cleared in kswapd.
> > 
> > Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> 
> It would be nice if the changelog was expanded a bit to explain
> why the patch is necessary. You say this is to "correctly clear
> ZONE_CONGESTED" but do not explain why the current code is wrong
> or what the user-visible impact is. For example, even cutting and
> pasting bits of the discussion like the following would have been
> an improvement.
> 
> ==== CUT HERE ===
> kswapd is responsible for clearing ZONE_CONGESTED after it balances
> a zone. Unfortunately, if ZONE_CONGESTED was set during a high-order
> allocation, it is possible that kswapd misses clearing it.
> 
> At the end of balance_pgdat(), kswapd uses the following logic;
> 
>  If reclaiming at high order {
>      for each zone {
>              if all_unreclaimable
>                      skip
>              if watermark is not met
>                      order = 0
>                      loop again
>              
>              /* watermark is met */
>              clear congested
>      }
>  }
> 
> i.e. it clears ZONE_CONGESTED if it the zone is balanced. if not,
> it restarts balancing at order-0. However, if the higher zones are
> balanced for order-0, kswapd will miss clearing ZONE_CONGESTED
> as that only happens after a zone is shrunk. This can mean that
> wait_iff_congested() stalls unnecessarily. This patch makes kswapd
> clear ZONE_CONGESTED during its initial highmem->dma scan for zones
> that are already balanced.
> 
> ==== CUT HERE ====
> 
> This makes review a lot easier and will be helpful in the future if
> someone uses git blame.
> 
> Whether you update the changelog or not;
> 
> Acked-by: Mel Gorman <mgorman@suse.de>
Ok, updated the changelog.


ZONE_CONGESTED is only cleared in kswapd, but pages can be freed in any task.
It's possible ZONE_CONGESTED isn't cleared in some cases:
1. the zone is already balanced just entering balance_pgdat() for order-0 because
concurrent tasks free memory. In this case, later check will skip the zone as
it's balanced so the flag isn't cleared.
2. high order balance fallbacks to order-0. quote from Mel:
At the end of balance_pgdat(), kswapd uses the following logic;

 If reclaiming at high order {
     for each zone {
             if all_unreclaimable
                     skip
             if watermark is not met
                     order = 0
                     loop again

             /* watermark is met */
             clear congested
     }
 }

i.e. it clears ZONE_CONGESTED if it the zone is balanced. if not,
it restarts balancing at order-0. However, if the higher zones are
balanced for order-0, kswapd will miss clearing ZONE_CONGESTED
as that only happens after a zone is shrunk.
This can mean that wait_iff_congested() stalls unnecessarily. This patch
makes kswapd clear ZONE_CONGESTED during its initial highmem->dma scan
for zones that are already balanced.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/vmscan.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux/mm/vmscan.c
===================================================================
--- linux.orig/mm/vmscan.c	2011-07-29 08:24:10.000000000 +0800
+++ linux/mm/vmscan.c	2011-07-29 08:26:29.000000000 +0800
@@ -2494,6 +2494,9 @@ loop_again:
 					high_wmark_pages(zone), 0, 0)) {
 				end_zone = i;
 				break;
+			} else {
+				/* If balanced, clear the congested flag */
+				zone_clear_flag(zone, ZONE_CONGESTED);
 			}
 		}
 		if (i < 0)

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