Date: Fri, 21 Jun 2024 08:58:49 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Barry Song <21cnbao@...il.com>
Cc: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
 shuah@...nel.org, linux-mm@...ck.org, chrisl@...nel.org, hughd@...gle.com,
 kaleshsingh@...gle.com, kasong@...cent.com, linux-kernel@...r.kernel.org,
 ying.huang@...el.com, linux-kselftest@...r.kernel.org,
 Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH] selftests/mm: Introduce a test program to assess swap
 entry allocation for thp_swapout

On 21/06/2024 08:47, Barry Song wrote:
> On Fri, Jun 21, 2024 at 7:25 PM Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> On 20/06/2024 12:34, David Hildenbrand wrote:
>>> On 20.06.24 11:04, Ryan Roberts wrote:
>>>> On 20/06/2024 01:26, Barry Song wrote:
>>>>> From: Barry Song <v-songbaohua@...o.com>
>>>>>
>>>>> Both Ryan and Chris have been utilizing the small test program to aid
>>>>> in debugging and identifying issues with swap entry allocation. While
>>>>> a real or intricate workload might be more suitable for assessing the
>>>>> correctness and effectiveness of the swap allocation policy, a small
>>>>> test program presents a simpler means of understanding the problem and
>>>>> initially verifying the improvements being made.
>>>>>
>>>>> Let's endeavor to integrate it into the self-test suite. Although it
>>>>> presently only accommodates 64KB and 4KB, I'm optimistic that we can
>>>>> expand its capabilities to support multiple sizes and simulate more
>>>>> complex systems in the future as required.
>>>>
>>>> I'll try to summarize the thread with Huang Ying by suggesting this test program
>>>> is "neccessary but not sufficient" to exhaustively test the mTHP swap-out path.
>>>> I've certainly found it useful and think it would be a valuable addition to the
>>>> tree.
>>>>
>>>> That said, I'm not convinced it is a selftest; IMO a selftest should provide a
>>>> clear pass/fail result against some criteria and must be able to be run
>>>> automatically by (e.g.) a CI system.
>>>
>>> Likely we should then consider moving other such performance-related thingies
>>> out of the selftests?
>>
>> Yes, that would get my vote. But of the 4 tests you mentioned that use
>> clock_gettime(), it looks like transhuge-stress is the only one that doesn't
>> have a pass/fail result, so is probably the only candidate for moving.
>>
>> The others either use the times as a timeout and determines failure if the
>> action didn't occur within the timeout (e.g. ksm_tests.c) or use it to add some
>> supplemental performance information to an otherwise functionality-oriented test.
> 
> Thank you very much, Ryan. I think you've found a better home for this
> tool . I will
> send v2, relocating it to tools/mm and adding a function to swap in
> either the whole
> mTHPs or a portion of mTHPs by "-a"(aligned swapin).
> 
> So basically, we will have
> 
> 1. Use MADV_PAGEPUT for rapid swap-out, putting the swap allocation code under
> high exercise in a short time.
> 
> 2. Use MADV_DONTNEED to simulate the behavior of libc and Java heap in freeing
> memory, as well as for munmap, app exits, or OOM killer scenarios. This ensures
> new mTHP is always generated, released or swapped out, similar to the behavior
> on a PC or Android phone where many applications are frequently started and
> terminated.
> 
> 3. Swap in with or without the "-a" option to observe how fragments
> due to swap-in
> and the incoming swap-in of large folios will impact swap-out fallback.
> 
> And many thanks to Chris for the suggestion on improving it within
> selftest, though I
> prefer to place it in tools/mm.

All sounds good to me!

If, (for future) you also wanted to test the vmscan swap-out path, the way I've
been doing that is to run the workload in a memory-constrained cgroup. That
means you don't need to exhaust all your phsical ram so speeds things up a lot.


> 
> Thanks
> Barry

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