linux-kernel - Re: [PATCH 1/2] mm: add per-order mTHP split counters

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Date: Mon, 1 Jul 2024 09:16:48 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Lance Yang <ioworker0@...il.com>, Barry Song <baohua@...nel.org>
Cc: akpm@...ux-foundation.org, david@...hat.com,
 baolin.wang@...ux.alibaba.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm: add per-order mTHP split counters

On 30/06/2024 12:34, Lance Yang wrote:
> Hi Barry,
> 
> Thanks for following up!
> 
> On Sun, Jun 30, 2024 at 5:48 PM Barry Song <baohua@...nel.org> wrote:
>>
>> On Thu, Apr 25, 2024 at 3:41 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>>
>>> + Barry
>>>
>>> On 24/04/2024 14:51, Lance Yang wrote:
>>>> At present, the split counters in THP statistics no longer include
>>>> PTE-mapped mTHP. Therefore, this commit introduces per-order mTHP split
>>>> counters to monitor the frequency of mTHP splits. This will assist
>>>> developers in better analyzing and optimizing system performance.
>>>>
>>>> /sys/kernel/mm/transparent_hugepage/hugepages-<size>/stats
>>>>         split_page
>>>>         split_page_failed
>>>>         deferred_split_page
>>>>
>>>> Signed-off-by: Lance Yang <ioworker0@...il.com>
>>>> ---
>>>>  include/linux/huge_mm.h |  3 +++
>>>>  mm/huge_memory.c        | 14 ++++++++++++--
>>>>  2 files changed, 15 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>> index 56c7ea73090b..7b9c6590e1f7 100644
>>>> --- a/include/linux/huge_mm.h
>>>> +++ b/include/linux/huge_mm.h
>>>> @@ -272,6 +272,9 @@ enum mthp_stat_item {
>>>>       MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
>>>>       MTHP_STAT_ANON_SWPOUT,
>>>>       MTHP_STAT_ANON_SWPOUT_FALLBACK,
>>>> +     MTHP_STAT_SPLIT_PAGE,
>>>> +     MTHP_STAT_SPLIT_PAGE_FAILED,
>>>> +     MTHP_STAT_DEFERRED_SPLIT_PAGE,
>>>>       __MTHP_STAT_COUNT
>>>>  };
>>>>
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 055df5aac7c3..52db888e47a6 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -557,6 +557,9 @@ DEFINE_MTHP_STAT_ATTR(anon_fault_fallback, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>  DEFINE_MTHP_STAT_ATTR(anon_fault_fallback_charge, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>>>>  DEFINE_MTHP_STAT_ATTR(anon_swpout, MTHP_STAT_ANON_SWPOUT);
>>>>  DEFINE_MTHP_STAT_ATTR(anon_swpout_fallback, MTHP_STAT_ANON_SWPOUT_FALLBACK);
>>>> +DEFINE_MTHP_STAT_ATTR(split_page, MTHP_STAT_SPLIT_PAGE);
>>>> +DEFINE_MTHP_STAT_ATTR(split_page_failed, MTHP_STAT_SPLIT_PAGE_FAILED);
>>>> +DEFINE_MTHP_STAT_ATTR(deferred_split_page, MTHP_STAT_DEFERRED_SPLIT_PAGE);
>>>>
>>>>  static struct attribute *stats_attrs[] = {
>>>>       &anon_fault_alloc_attr.attr,
>>>> @@ -564,6 +567,9 @@ static struct attribute *stats_attrs[] = {
>>>>       &anon_fault_fallback_charge_attr.attr,
>>>>       &anon_swpout_attr.attr,
>>>>       &anon_swpout_fallback_attr.attr,
>>>> +     &split_page_attr.attr,
>>>> +     &split_page_failed_attr.attr,
>>>> +     &deferred_split_page_attr.attr,
>>>>       NULL,
>>>>  };
>>>>
>>>> @@ -3083,7 +3089,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>>       XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
>>>>       struct anon_vma *anon_vma = NULL;
>>>>       struct address_space *mapping = NULL;
>>>> -     bool is_thp = folio_test_pmd_mappable(folio);
>>>> +     int order = folio_order(folio);
>>>>       int extra_pins, ret;
>>>>       pgoff_t end;
>>>>       bool is_hzp;
>>>> @@ -3262,8 +3268,10 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>>               i_mmap_unlock_read(mapping);
>>>>  out:
>>>>       xas_destroy(&xas);
>>>> -     if (is_thp)
>>>> +     if (order >= HPAGE_PMD_ORDER)
>>>>               count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
>>>> +     count_mthp_stat(order, !ret ? MTHP_STAT_SPLIT_PAGE :
>>>> +                                   MTHP_STAT_SPLIT_PAGE_FAILED);
>>>>       return ret;
>>>>  }
>>>>
>>>> @@ -3327,6 +3335,8 @@ void deferred_split_folio(struct folio *folio)
>>>>       if (list_empty(&folio->_deferred_list)) {
>>>>               if (folio_test_pmd_mappable(folio))
>>>>                       count_vm_event(THP_DEFERRED_SPLIT_PAGE);
>>>> +             count_mthp_stat(folio_order(folio),
>>>> +                             MTHP_STAT_DEFERRED_SPLIT_PAGE);
>>>
>>> There is a very long conversation with Barry about adding a 'global "mTHP became
>>> partially mapped 1 or more processes" counter (inc only)', which terminates at
>>> [1]. There is a lot of discussion about the required semantics around the need
>>> for partial map to cover alignment and contiguity as well as whether all pages
>>> are mapped, and to trigger once it becomes partial in at least 1 process.
>>>
>>> MTHP_STAT_DEFERRED_SPLIT_PAGE is giving much simpler semantics, but less
>>> information as a result. Barry, what's your view here? I'm guessing this doesn't
>>> quite solve what you are looking for?
>>
>> This doesn't quite solve what I am looking for but I still think the
>> patch has its value.
>>
>> I'm looking for a solution that can:
>>
>>   *  Count the amount of memory in the system for each mTHP size.
>>   *  Determine how much memory for each mTHP size is partially unmapped.
>>
>> For example, in a system with 16GB of memory, we might find that we have 3GB
>> of 64KB mTHP, and within that, 512MB is partially unmapped, potentially wasting
>> memory at this moment.  I'm uncertain whether Lance is interested in
>> this job :-)
> 
> Nice, that's an interesting/valuable job for me ;)
> 
> Let's do it separately, as 'split' and friends probably can’t be the
> solution you
> mentioned above, IMHO.
> 
> Hmm... I don't have a good idea about the solution for now, but will
> think it over
> and come back to discuss it here.

I have a grad starting in a couple of weeks and I had been planning to initially
ask him to look at this to help him get up to speed on mTHP/mm stuff. But I have
plenty of other things for him to do if Lance wants to take this :)

> 
>>
>> Counting deferred_split remains valuable as it can signal whether the system is
>> experiencing significant partial unmapping.
> 
> Have a nice weekend!
> Lance
> 
>>
>>>
>>> [1] https://lore.kernel.org/linux-mm/6cc7d781-884f-4d8f-a175-8609732b87eb@arm.com/
>>>
>>> Thanks,
>>> Ryan
>>>
>>>>               list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
>>>>               ds_queue->split_queue_len++;
>>>>  #ifdef CONFIG_MEMCG
>>>
>>
>> Thanks
>> Barry