Re: [patch] vmscan: fix zone shrinking exit when scan work is done

[Andrea Arcangeli]

On Wed, Feb 09, 2011 at 04:46:56PM +0000, Mel Gorman wrote:
> On Wed, Feb 09, 2011 at 04:46:06PM +0100, Johannes Weiner wrote:
> > Hi,
> > 
> > I think this should fix the problem of processes getting stuck in
> > reclaim that has been reported several times.
> 
> I don't think it's the only source but I'm basing this on seeing
> constant looping in balance_pgdat() and calling congestion_wait() a few
> weeks ago that I haven't rechecked since. However, this looks like a
> real fix for a real problem.

Agreed. Just yesterday I spent some time on the lumpy compaction
changes after wondering about Michal's khugepaged 100% report, and I
expected some fix was needed in this area (as I couldn't find any bug
in khugepaged yet, so the lumpy compaction looked the next candidate
for bugs).

I've also been wondering about the !nr_scanned check in
should_continue_reclaim too but I didn't look too much into the caller
(I was tempted to remove it all together). I don't see how checking
nr_scanned can be safe even after we fix the caller to avoid passing
non-zero values if "goto restart".

nr_scanned is incremented even for !page_evictable... so it's not
really useful to insist, just because we scanned something, in my
view. It looks bogus... So my proposal would be below.

====
Subject: mm: stop checking nr_scanned in should_continue_reclaim

From: Andrea Arcangeli <aarcange@xxxxxxxxxx>

nr_scanned is incremented even for !page_evictable... so it's not
really useful to insist, just because we scanned something.

Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 148c6e6..9741884 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1831,7 +1831,6 @@ out:
  */
 static inline bool should_continue_reclaim(struct zone *zone,
 					unsigned long nr_reclaimed,
-					unsigned long nr_scanned,
 					struct scan_control *sc)
 {
 	unsigned long pages_for_compaction;
@@ -1841,15 +1840,8 @@ static inline bool should_continue_reclaim(struct zone *zone,
 	if (!(sc->reclaim_mode & RECLAIM_MODE_COMPACTION))
 		return false;
 
-	/*
-	 * If we failed to reclaim and have scanned the full list, stop.
-	 * NOTE: Checking just nr_reclaimed would exit reclaim/compaction far
-	 *       faster but obviously would be less likely to succeed
-	 *       allocation. If this is desirable, use GFP_REPEAT to decide
-	 *       if both reclaimed and scanned should be checked or just
-	 *       reclaimed
-	 */
-	if (!nr_reclaimed && !nr_scanned)
+	/* If we failed to reclaim stop. */
+	if (!nr_reclaimed)
 		return false;
 
 	/*
@@ -1884,7 +1876,6 @@ static void shrink_zone(int priority, struct zone *zone,
 	enum lru_list l;
 	unsigned long nr_reclaimed;
 	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
-	unsigned long nr_scanned = sc->nr_scanned;
 
 restart:
 	nr_reclaimed = 0;
@@ -1923,8 +1914,7 @@ restart:
 		shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
 
 	/* reclaim/compaction might need reclaim to continue */
-	if (should_continue_reclaim(zone, nr_reclaimed,
-					sc->nr_scanned - nr_scanned, sc))
+	if (should_continue_reclaim(zone, nr_reclaimed, sc))
 		goto restart;
 
 	throttle_vm_writeout(sc->gfp_mask);


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