Re: ODEBUG: Out of memory. ODEBUG disabled

[Qian Cai <cai@gmx.us>]

> On Nov 12, 2018, at 11:33 PM, Qian Cai <cai@gmx.us> wrote:
> 
> 
> 
>> On Nov 10, 2018, at 9:11 AM, Qian Cai <cai@gmx.us> wrote:
>> 
>> On 11/10/18 at 8:59 AM, Waiman Long wrote:
>> 
>>> On 11/09/2018 08:45 PM, Qian Cai wrote:
>>>>> Sent: Friday, November 09, 2018 at 5:08 PM
>>>>> From: "Waiman Long" <longman@redhat.com>
>>>>> To: "Qian Cai" <cai@gmx.us>, "Yang Shi" <yang.shi@linux.alibaba.com>
>>>>> Cc: "open list" <linux-kernel@vger.kernel.org>, "Thomas Gleixner" <tglx@linutronix.de>, "Arnd Bergmann" <arnd@arndb.de>, "Joel Fernandes (Google)" <joel@joelfernandes.org>, "Zhong Jiang" <zhongjiang@huawei.com>
>>>>> Subject: Re: ODEBUG: Out of memory. ODEBUG disabled
>>>>> 
>>>>> On 11/09/2018 04:51 PM, Qian Cai wrote:
>>>>>>> On Nov 9, 2018, at 4:42 PM, Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>> On 11/9/18 1:36 PM, Qian Cai wrote:
>>>>>>>> It is a bit annoying on this aarch64 server with 64 CPUs that is
>>>>>>>> booting the latest mainline (3541833fd1f2) causes object debugging
>>>>>>>> always running out of memory.
>>>>>>> May you please paste the detail failure log?
>>>>>> I assume you mean dmesg.
>>>>>> 
>>>>>> Here is the dmesg for 64 CPUs,
>>>>>> https://paste.ubuntu.com/p/BnhvXXhn7k/
>>>>>>>> I have to boot the kernel with only 16 CPUs instead (nr_cpus=16)
>>>>>>>> to make it work. Is it expected that object debugging is not going
>>>>>>>> to work with large machines?
>>>>>>> I don't think so. I'm supposed it works well with large CPU number on x86.
>>>>>> Here is the one with nr_cpus workaround,
>>>>>> https://paste.ubuntu.com/p/qMpd2CCPSV/
>>>>> The debugobjects code have a set of 1024 statically allocated debug
>>>>> objects that can be used in early boot before the slab memory allocator
>>>>> is initialized. Apparently, the system may have used up all the
>>>>> statically allocated objects. Try double ODEBUG_POOL_SIZE to see if it
>>>>> helps.
>>>> Great, you are right. Doubling the size makes it work. Does it make sense
>>>> to have a kconfig option instead?
>>> 
>>> First, I think you need to figure out what your system needed to use up
>>> so many debug objects in early boot. If there is a legitimate reason for
>>> this behavior, we can talk about having a kconfig option to increase that.
>> Anybody else not getting ODEBUG OOM with more than 64-CPU? As
>> mentioned, restricting to 16-CPU works fine. How can I figure out why the
>> system uses so much debug objects?
> On another aarch64 server with 256-CPU, even double the size of
> ODEBUG_POOL_SIZE, i.e., 2048 will get "ODEBUG: Out of memory. ODEBUG
> disabled”.

OK, here is the problem.

In order to get aarch64 work, the initial ODEBUG_POOL_SIZE on

64-CPU:  need 2048
256-CPU: need 8192 (4096 too small)

This commit 97dd552eb23c

+        * Increase the thresholds for allocating and freeing objects
+        * according to the number of possible CPUs available in the system.
+        */
+       debug_objects_pool_size += num_possible_cpus() * 32;

Why magic number 32?

It needs to be bigger than that for aarch64.

(2048 + 64 x 32 - 1024) / 64 = 48 (work on 64-cpu)
(4096 + 256 x 32 - 1024) / 256 = 48 (not work on 256-cpu)
(8196 + 256 x 32 - 1024) / 256 = 60 (work on 256-cpu)