Re: [uml-devel] BUG: soft lockup for a user mode linux image

[Richard Weinberger <richard@nod.at>]

Am 09.10.2013 23:47, schrieb Jan Kara:
> On Wed 09-10-13 20:43:50, Richard Weinberger wrote:
>> CC'ing mm folks.
>> Please see below.
>   Added Fenguang to CC since he is the author of this code.

Thx, get_maintainer.pl didn't list him.

>> Am 09.10.2013 19:26, schrieb Toralf Forster:
>>> On 10/08/2013 10:07 PM, Geert Uytterhoeven wrote:
>>>> On Sun, Oct 6, 2013 at 11:01 PM, Toralf Forster <toralf.foerster@gmx.de> wrote:
>>>>>> Hmm, now pages_dirtied is zero, according to the backtrace, but the BUG_ON()
>>>>>> asserts its strict positive?!?
>>>>>>
>>>>>> Can you please try the following instead of the BUG_ON():
>>>>>>
>>>>>> if (pause < 0) {
>>>>>>         printk("pages_dirtied = %lu\n", pages_dirtied);
>>>>>>         printk("task_ratelimit = %lu\n", task_ratelimit);
>>>>>>         printk("pause = %ld\n", pause);
>>>>>> }
>>>>>>
>>>>>> Gr{oetje,eeting}s,
>>>>>>
>>>>>>                         Geert
>>>>> I tried it in different ways already - I'm completely unsuccessful in getting any printk output.
>>>>> As soon as the issue happens I do have a
>>>>>
>>>>> BUG: soft lockup - CPU#0 stuck for 22s! [trinity-child0:1521]
>>>>>
>>>>> at stderr of the UML and then no further input is accepted. With uml_mconsole I'm however able
>>>>> to run very basic commands like a crash dump, sysrq ond so on.
>>>>
>>>> You may get an idea of the magnitude of pages_dirtied by using a chain of
>>>> BUG_ON()s, like:
>>>>
>>>> BUG_ON(pages_dirtied > 2000000000);
>>>> BUG_ON(pages_dirtied > 1000000000);
>>>> BUG_ON(pages_dirtied > 100000000);
>>>> BUG_ON(pages_dirtied > 10000000);
>>>> BUG_ON(pages_dirtied > 1000000);
>>>>
>>>> Probably 1 million is already too much for normal operation?
>>>>
>>> period = HZ * pages_dirtied / task_ratelimit;
>>> 		BUG_ON(pages_dirtied > 2000000000);
>>> 		BUG_ON(pages_dirtied > 1000000000);      <-------------- this is line 1467
>>
>> Summary for mm people:
>>
>> Toralf runs trinty on UML/i386.
>> After some time pages_dirtied becomes very large.
>> More than 1000000000 pages in this case.
>   Huh, this is really strange. pages_dirtied is passed into
> balance_dirty_pages() from current->nr_dirtied. So I wonder how a value
> over 10^9 can get there. After all that is over 4TB so I somewhat doubt the
> task was ever able to dirty that much during its lifetime (but correct me
> if I'm wrong here, with UML and memory backed disks it is not totally
> impossible)... I went through the logic of handling ->nr_dirtied but
> I didn't find any obvious problem there. Hum, maybe one thing - what
> 'task_ratelimit' values do you see in balance_dirty_pages? If that one was
> huge, we could possibly accumulate huge current->nr_dirtied.

Toralf, you can try a snipplet like this one to get the values printed out:
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index f5236f8..a80e520 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1463,6 +1463,12 @@ static void balance_dirty_pages(struct address_space *mapping,
                        goto pause;
                }
                period = HZ * pages_dirtied / task_ratelimit;
+
+               {
+                       extern int printf(char *, ...);
+                       printf("---> task_ratelimit: %lu\n", task_ratelimit);
+               }
+
                pause = period;
                if (current->dirty_paused_when)
                        pause -= now - current->dirty_paused_when;


Yes, printf(), not printk().
Using this hack we print directly to host's stdout. :)

Thanks,
//richard

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