linux-kernel - Re: Early test: hangs in mm/compact.c w. Linus's 12d7aacab56e9ef185c

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Date:	Sat, 08 Nov 2014 23:18:37 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"P. Christeas" <xrg@...ux.gr>
CC:	linux-mm@...ck.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
	lkml <linux-kernel@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>,
	Norbert Preining <preining@...ic.at>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Pavel Machek <pavel@....cz>
Subject: Re: Early test: hangs in mm/compact.c w. Linus's 12d7aacab56e9ef185c

On 11/08/2014 02:11 PM, P. Christeas wrote:
> On Thursday 06 November 2014, Vlastimil Babka wrote:
>>> On Wednesday 05 November 2014, Vlastimil Babka wrote:
>>>> Can you please try the following patch?
>>>> -			compaction_defer_reset(zone, order, false);
>> Oh and did I ask in this thread for /proc/zoneinfo yet? :)
> 
> Using that same kernel[1], got again into a race, gathered a few more data.
> 
> This time, I had 1x "urpmq" process [2] hung at 100% CPU , when "kwin" got 
> apparently blocked (100% CPU, too) trying to resize a GUI window. I suppose 
> the resizing operation would mean heavy memory alloc/free.
> 
> The rest of the system was responsive, I could easily get a console, login, 
> gather the files.. Then, I have *killed* -9 the "urpmq" process, which solved 
> the race and my system is still alive! "kwin" is still running, returned to 
> regular CPU load.
> 
> Attached is traces from SysRq+l (pressed a few times, wanted to "snapshot" the 
> stack) and /proc/zoneinfo + /proc/vmstat
> 
> Bisection is not yet meaningful, IMHO, because I cannot be sure that "good" 
> points are really free from this issue. I'd estimate that each test would take 
> +3days, unless I really find a deterministic way to reproduce the issue .

Hi,

I think I finally found the cause by staring into the code... CCing
people from all 4 separate threads I know about this issue.
The problem with finding the cause was that the first report I got from
Markus was about isolate_freepages_block() overhead, and later Norbert
reported that reverting a patch for isolate_freepages* helped. But the
problem seems to be that although the loop in isolate_migratepages exits
because the scanners almost meet (they are within same pageblock), they
don't truly meet, therefore compact_finished() decides to continue, but
isolate_migratepages() exits immediately... boom! But indeed e14c720efdd7
made this situation possible, as free scaner pfn can now point to a
middle of pageblock.

So I hope the attached patch will fix the soft-lockup issues in
compact_zone. Please apply on 3.18-rc3 or later without any other reverts,
and test. It probably won't help Markus and his isolate_freepages_block()
overhead though...

Thanks,
Vlastimil

------8<------
>From fbf8eb0bcd2897090312e23da6a31bad9cc6b337 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Sat, 8 Nov 2014 22:20:43 +0100
Subject: [PATCH] mm, compaction: prevent endless loop in migrate scanner

---
 mm/compaction.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index ec74cf0..1b7a1be 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1029,8 +1029,12 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
 	}
 
 	acct_isolated(zone, cc);
-	/* Record where migration scanner will be restarted */
-	cc->migrate_pfn = low_pfn;
+	/* 
+	 * Record where migration scanner will be restarted. If we end up in
+	 * the same pageblock as the free scanner, make the scanners fully
+	 * meet so that compact_finished() terminates compaction.
+	 */
+	cc->migrate_pfn = (end_pfn <= cc->free_pfn) ? low_pfn : cc->free_pfn;
 
 	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
 }
-- 
2.1.2


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