[3.11.y.z extended stable] Patch "Don't trigger congestion wait on dirty-but-not-writeout pages" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    Don't trigger congestion wait on dirty-but-not-writeout pages

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 7703b78650e0dbaa30cc975a20509c5467f45055 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds at linux-foundation.org>
Date: Sun, 8 Jun 2014 14:17:00 -0700
Subject: Don't trigger congestion wait on dirty-but-not-writeout pages

commit b738d764652dc5aab1c8939f637112981fce9e0e upstream.

shrink_inactive_list() used to wait 0.1s to avoid congestion when all
the pages that were isolated from the inactive list were dirty but not
under active writeback.  That makes no real sense, and apparently causes
major interactivity issues under some loads since 3.11.

The ostensible reason for it was to wait for kswapd to start writing
pages, but that seems questionable as well, since the congestion wait
code seems to trigger for kswapd itself as well.  Also, the logic behind
delaying anything when we haven't actually started writeback is not
clear - it only delays actually starting that writeback.

We'll still trigger the congestion waiting if

 (a) the process is kswapd, and we hit pages flagged for immediate
     reclaim

 (b) the process is not kswapd, and the zone backing dev writeback is
     actually congested.

This probably needs to be revisited, but as it is this fixes a reported
regression.

[mhocko at suse.cz: backport to 3.12 stable tree]
Fixes: e2be15f6c3ee ('mm: vmscan: stall page reclaim and writeback pages based on dirty/writepage pages encountered')
Reported-by: Felipe Contreras <felipe.contreras at gmail.com>
Pinpointed-by: Hillf Danton <dhillf at gmail.com>
Cc: Michal Hocko <mhocko at suse.cz>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Mel Gorman <mgorman at suse.de>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Michal Hocko <mhocko at suse.cz>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 mm/vmscan.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b4241b7d7f07..370c16d21fb2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1468,19 +1468,18 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 		 * If dirty pages are scanned that are not queued for IO, it
 		 * implies that flushers are not keeping up. In this case, flag
 		 * the zone ZONE_TAIL_LRU_DIRTY and kswapd will start writing
-		 * pages from reclaim context. It will forcibly stall in the
-		 * next check.
+		 * pages from reclaim context.
 		 */
 		if (nr_unqueued_dirty == nr_taken)
 			zone_set_flag(zone, ZONE_TAIL_LRU_DIRTY);

 		/*
-		 * In addition, if kswapd scans pages marked marked for
-		 * immediate reclaim and under writeback (nr_immediate), it
-		 * implies that pages are cycling through the LRU faster than
+		 * If kswapd scans pages marked marked for immediate
+		 * reclaim and under writeback (nr_immediate), it implies
+		 * that pages are cycling through the LRU faster than
 		 * they are written so also forcibly stall.
 		 */
-		if (nr_unqueued_dirty == nr_taken || nr_immediate)
+		if (nr_immediate)
 			congestion_wait(BLK_RW_ASYNC, HZ/10);
 	}

--
1.9.1