Re: DAMON: problems when running DAMON on ARM64 with 'transparent_hugepage' enabled

[Xiongfeng Wang <wangxiongfeng2@huawei.com>]

Hi SeongJae,

Thanks for your reply !

On 2021/11/5 21:57, SeongJae Park wrote:
> Hi Xiongfeng,
> 
> On Wed, 27 Oct 2021 08:06:36 +0000 SeongJae Park <sj@kernel.org> wrote:
> 
>> Hello Xiongfeng,
>>
>> On Wed, 27 Oct 2021 14:14:57 +0800 Xiongfeng Wang <wangxiongfeng2@huawei.com> wrote:
>>
>>> Sorry, I forgot to Cc the maillist. Cc it in this mail.
>>>
>>> On 2021/10/27 10:19, Xiongfeng Wang wrote:
>>>> Hi SeongJae,
>>>>
>>>> Sorry to disturb you. It's just that I came across some problems when running
>>>> DAMON, but still didn't find the solution after several days.
>>
>> You're not disturbing but helping me!  Please don't say so! :)
>>
>>>>
>>>> A short description is that the result of DAMON is not as expected when running
>>>> on ARM64 with 'transparent_hugepage' enabled. But the result is correct when
>>>> 'transparent_hugepage' is disabled.
>>>>
>>>> The following are the steps I came across the problems.
>>>> 1. Firstly, I use 'damo record' to sample the 'stairs' demo.
>>>>   damo record "./masim ./configs/stairs.cfg"
>>>> 2. Then I use 'damo report' to show the results.
>>>>   damo report heats --address_range xxx  xxx  --time_range xxx xxx    --heatmap
>>>> stdout    --stdout_heatmap_color emotion
>>>> The result doesn't show like a stair. I wrote a userspace demo to access a
>>>> certain address range in loop and use DAMON to sample the demo. I added
>>>> trace_print in 'damon_va_check_access()' and found out the pages in the address
>>>> range are not always detected as accessed, which is not expected. When I disable
>>>> transparent_hugepage by chance, the pages are marked as accessed. Then I test
>>>> the 'stairs' demo again, the result is correct. It seems that, only when
>>>> transparent_hugepage' is disabled, the access check works. I don't know where
>>>> the bug is, the software or the hardware ? Appreciate it if you have time to
>>>> reply. Thanks !
>>
>> Thank you for this report!  I have a theory, but would like to test first.
>> Will check and get back to you soon.
> 
> Sorry for late response.  I also confirmed the issue is reproducible on my
> ARM64 test machine.  My theory is, enabling THP reduced page table walks, and
> therefore the PTE Accessed bits are not frequently updated.  To verify this, I
> made below experimental change.  After applying the change on my test machine,
> I was able to show the expected access pattern regardless of THP enablement.
> 
>     --- a/mm/damon/vaddr.c
>     +++ b/mm/damon/vaddr.c
>     @@ -429,6 +429,7 @@ void damon_va_prepare_access_checks(struct damon_ctx *ctx)
>                             continue;
>                     damon_for_each_region(r, t)
>                             damon_va_prepare_access_check(ctx, mm, r);
>     +               flush_tlb_mm(mm);
>                     mmput(mm);
>             }
>      }
> 
> Could you please test this on your machine and let me know the result?

Yes, it works. After I add 'flush_tlb_mm()' as above, the result is as expected.

> 
> Again, please note that this change is only for proof of the theory, rather
> than the complete fix.

Thanks for let me know. I will wait for the official version.

Thanks,
Xiongfeng

> 
> 
> Thanks,
> SJ
> 
>>
>>
>> Thanks,
>> SJ
>>
>>>>
>>>> Thanks,
>>>> Xiongfeng
> .
>