From: David Hildenbrand <david@redhat.com>
To: Yang Shi <shy828301@gmail.com>
Cc: David Hildenbrand <david@redhat.com>, Zi Yan <ziy@nvidia.com>,
Linux MM <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-kselftest@vger.kernel.org,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Shuah Khan <shuah@kernel.org>, John Hubbard <jhubbard@nvidia.com>,
Sandipan Das <sandipan@linux.ibm.com>,
David Rientjes <rientjes@google.com>,
Alex Shi <alex.shi@linux.alibaba.com>
Subject: Re: [PATCH] mm: huge_memory: a new debugfs interface for splitting THP tests.
Date: Mon, 8 Mar 2021 22:58:54 +0100 [thread overview]
Message-ID: <B7E9E82B-70B1-491E-99B9-B6B30AE0DE68@redhat.com> (raw)
In-Reply-To: <CAHbLzkpfR=xbnepbriu1b8n5ii8Kp0EgEa3D7+mU2xcS9iGpGg@mail.gmail.com>
> Am 08.03.2021 um 22:25 schrieb Yang Shi <shy828301@gmail.com>:
>
> On Mon, Mar 8, 2021 at 12:36 PM David Hildenbrand <david@redhat.com> wrote:
>>
>>
>>>> Am 08.03.2021 um 21:18 schrieb Yang Shi <shy828301@gmail.com>:
>>>
>>> On Mon, Mar 8, 2021 at 11:30 AM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>>> On 08.03.21 20:11, Yang Shi wrote:
>>>>> On Mon, Mar 8, 2021 at 11:01 AM Zi Yan <ziy@nvidia.com> wrote:
>>>>>>
>>>>>> On 8 Mar 2021, at 13:11, David Hildenbrand wrote:
>>>>>>
>>>>>>> On 08.03.21 18:49, Zi Yan wrote:
>>>>>>>> On 8 Mar 2021, at 11:17, David Hildenbrand wrote:
>>>>>>>>
>>>>>>>>> On 08.03.21 16:22, Zi Yan wrote:
>>>>>>>>>> From: Zi Yan <ziy@nvidia.com>
>>>>>>>>>>
>>>>>>>>>> By writing "<pid>,<vaddr_start>,<vaddr_end>" to
>>>>>>>>>> <debugfs>/split_huge_pages_in_range_pid, THPs in the process with the
>>>>>>>>>> given pid and virtual address range are split. It is used to test
>>>>>>>>>> split_huge_page function. In addition, a selftest program is added to
>>>>>>>>>> tools/testing/selftests/vm to utilize the interface by splitting
>>>>>>>>>> PMD THPs and PTE-mapped THPs.
>>>>>>>>>
>>>>>>>>> Won't something like
>>>>>>>>>
>>>>>>>>> 1. MADV_HUGEPAGE
>>>>>>>>>
>>>>>>>>> 2. Access memory
>>>>>>>>>
>>>>>>>>> 3. MADV_NOHUGEPAGE
>>>>>>>>>
>>>>>>>>> Have a similar effect? What's the benefit of this?
>>>>>>>>
>>>>>>>> Thanks for checking the patch.
>>>>>>>>
>>>>>>>> No, MADV_NOHUGEPAGE just replaces VM_HUGEPAGE with VM_NOHUGEPAGE,
>>>>>>>> nothing else will be done.
>>>>>>>
>>>>>>> Ah, okay - maybe my memory was tricking me. There is some s390x KVM code that forces MADV_NOHUGEPAGE and force-splits everything.
>>>>>>>
>>>>>>> I do wonder, though, if this functionality would be worth a proper user interface (e.g., madvise), though. There might be actual benefit in having this as a !debug interface.
>>>>>>>
>>>>>>> I think you aware of the discussion in https://lkml.kernel.org/r/d098c392-273a-36a4-1a29-59731cdf5d3d@google.com
>>>>>>
>>>>>> Yes. Thanks for bringing this up.
>>>>>>
>>>>>>>
>>>>>>> If there will be an interface to collapse a THP -- "this memory area is worth extra performance now by collapsing a THP if possible" -- it might also be helpful to have the opposite functionality -- "this memory area is not worth a THP, rather use that somehwere else".
>>>>>>>
>>>>>>> MADV_HUGE_COLLAPSE vs. MADV_HUGE_SPLIT
>>>>>>
>>>>>> I agree that MADV_HUGE_SPLIT would be useful as the opposite of COLLAPSE when user might just want PAGESIZE mappings.
>>>>>> Right now, HUGE_SPLIT is implicit from mapping changes like mprotect or MADV_DONTNEED.
>>>>>
>>>>> IMHO, it sounds not very useful. MADV_DONTNEED would split PMD for any
>>>>> partial THP. If the range covers the whole THP, the whole THP is going
>>>>> to be freed anyway. All other places in kernel which need split THP
>>>>> have been covered. So I didn't realize any usecase from userspace for
>>>>> just splitting PMD to PTEs.
>>>>
>>>> THP are a limited resource. So indicating which virtual memory regions
>>>> are not performance sensitive right now (e.g., cold pages in a databse)
>>>> and not worth a THP might be quite valuable, no?
>>>
>>> Such functionality could be achieved by MADV_COLD or MADV_PAGEOUT,
>>> right? Then a subsequent call to MADV_NOHUGEPAGE would prevent from
>>> collapsing or allocating THP for that area.
>>>
>>
>> I remember these deal with optimizing swapping. Not sure how they interact with THP, especially on systems without swap - I would guess they don‘t as of now.
>
> Yes, MADV_PAGEOUT would just swap the THP or sub pages out. I think I
> just forgot to mention MADV_FREE which would be more suitable for this
> usecase.
>
>>
Can you elaborate? MADV_FREE is destructive, just like a delayed MADV_DONTNEED. How would that help here?
>>>>
>>>> --
>>>> Thanks,
>>>>
>>>> David / dhildenb
>>>>
>>>
>>
>
next prev parent reply other threads:[~2021-03-08 21:59 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-08 15:22 [PATCH] mm: huge_memory: a new debugfs interface for splitting THP tests Zi Yan
2021-03-08 16:17 ` David Hildenbrand
2021-03-08 17:49 ` Zi Yan
2021-03-08 18:11 ` David Hildenbrand
2021-03-08 19:01 ` Zi Yan
2021-03-08 19:11 ` Yang Shi
2021-03-08 19:30 ` David Hildenbrand
2021-03-08 20:18 ` Yang Shi
2021-03-08 20:35 ` David Hildenbrand
2021-03-08 21:25 ` Yang Shi
2021-03-08 21:58 ` David Hildenbrand [this message]
2021-03-08 22:20 ` Yang Shi
[not found] ` <590175ca-ccf8-45d2-c108-e6225451e68a@nextfour.com>
[not found] ` <efac8763-8706-7b0b-17b9-4b0a4538fbf1@nextfour.com>
2021-03-08 18:46 ` Zi Yan
2021-03-08 19:23 ` kernel test robot
2021-03-08 19:36 ` Zi Yan
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