linux-kernel - [PATCH] mm: use global_dirty_limit in throttle_vm

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Message-ID: <20120302061451.GA6468@localhost>
Date:	Fri, 2 Mar 2012 14:14:51 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
	Greg Thelen <gthelen@...gle.com>,
	Ying Han <yinghan@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] mm: use global_dirty_limit in throttle_vm_writeout()

When starting a memory hog task, a desktop box w/o swap is found to go
unresponsive for a long time. It's solely caused by lots of congestion
waits in throttle_vm_writeout():

 gnome-system-mo-4201 553.073384: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
 gnome-system-mo-4201 553.073386: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000
           gtali-4237 553.080377: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
           gtali-4237 553.080378: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000
            Xorg-3483 553.103375: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
            Xorg-3483 553.103377: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000

The root cause is, the dirty threshold is knocked down a lot by the
memory hog task. Fixed by using global_dirty_limit which decreases
gradually on such events and can guarantee we stay above (the also
decreasing) nr_dirty in the progress of following down to the new
dirty threshold.

Signed-off-by: Fengguang Wu <fengguang.wu@...el.com>
---
 mm/page-writeback.c |    1 +
 1 file changed, 1 insertion(+)

--- linux.orig/mm/page-writeback.c	2012-03-02 14:05:01.633763187 +0800
+++ linux/mm/page-writeback.c	2012-03-02 14:11:52.929772962 +0800
@@ -1472,6 +1472,7 @@ void throttle_vm_writeout(gfp_t gfp_mask
 
         for ( ; ; ) {
 		global_dirty_limits(&background_thresh, &dirty_thresh);
+		dirty_thresh = hard_dirty_limit(dirty_thresh);
 
                 /*
                  * Boost the allowable dirty threshold a bit for page
--
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