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

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Date:	Sun, 09 Nov 2014 10:43:00 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Pavel Machek <pavel@....cz>
CC:	"P. Christeas" <xrg@...ux.gr>, 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>
Subject: Re: Early test: hangs in mm/compact.c w. Linus's 12d7aacab56e9ef185c

On 11/09/2014 09:27 AM, Pavel Machek wrote:
> Hi!
> 
>>>> 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.
> 
> Ok, it seems it happened second time now, again shortly after
> resume. I guess I should apply your patch after all.

Thanks.

> (Or... instead it should go to Linus ASAP -- it fixes known problem
> that is affected people, and we want it in soon in case it is not
> complete fix.)

I don't want to send untested fix, and wasn't able to reproduce the bug
myself. I think Norbert could do it rather quickly so I hope he can tell
us soon.

> Dmesg is in the attachment, perhaps it helps.
> 									Pavel

It looks the same as before, so no surprises there, which is good.

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