Re: Allocation failure of ring buffer for trace

[YASUAKI ISHIMATSU]

On 11/14/2017 10:53 AM, Mel Gorman wrote:
> On Tue, Nov 14, 2017 at 10:39:19AM -0500, YASUAKI ISHIMATSU wrote:
>>
>>
>> On 11/14/2017 06:46 AM, Mel Gorman wrote:
>>> On Mon, Nov 13, 2017 at 12:48:36PM -0500, YASUAKI ISHIMATSU wrote:
>>>> When using trace_buf_size= boot option, memory allocation of ring buffer
>>>> for trace fails as follows:
>>>>
>>>> [ ] x86: Booting SMP configuration:
>>>> <SNIP>
>>>>
>>>> In my server, there are 384 CPUs, 512 GB memory and 8 nodes. And
>>>> "trace_buf_size=100M" is set.
>>>>
>>>> When using trace_buf_size=100M, kernel allocates 100 MB memory
>>>> per CPU before calling free_are_init_core(). Kernel tries to
>>>> allocates 38.4GB (100 MB * 384 CPU) memory. But available memory
>>>> at this time is about 16GB (2 GB * 8 nodes) due to the following commit:
>>>>
>>>>   3a80a7fa7989 ("mm: meminit: initialise a subset of struct pages
>>>>                  if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set")
>>>>
>>>
>>> 1. What is the use case for such a large trace buffer being allocated at
>>>    boot time?
>>
>> I'm not sure the use case. I found the following commit log:
>>
>>   commit 864b9a393dcb5aed09b8fd31b9bbda0fdda99374
>>   Author: Michal Hocko <mhocko@xxxxxxxx>
>>   Date:   Fri Jun 2 14:46:49 2017 -0700
>>
>>       mm: consider memblock reservations for deferred memory initialization sizing
>>
>> So I thought similar memory exhaustion may occurs on other boot option.
>> And I reproduced the issue.
>>
> 
> That was different, it was a premature OOM caused by reservations that
> were of a known size. It's not related to trace_buf_size in any fashion.

Yes. I know there are different bugs. I thought memory exhaustion at boot time
may occur by other boot option. So I tried trace_buf_size boot option.

> 
>>
>>> 2. Is disabling CONFIG_DEFERRED_STRUCT_PAGE_INIT at compile time an
>>>    option for you given that it's a custom-built kernel and not a
>>>    distribution kernel?
>>
>> The issue also occurred on distribution kernels. So we have to fix the issue.
>>
> 
> I'm aware of now bugs against a distribution kernel. However, does the
> patch work for you?
> 

I'll apply it.

Thanks,
Yasuaki Ishimatsu