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

[Ryan Roberts <ryan.roberts@arm.com>]

+ 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@gmail.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?

[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