Re: [RFC PATCH 03/26] mm: make pageblock_order 2M per default

[Vlastimil Babka <vbabka@suse.cz>]

On 4/18/23 21:12, Johannes Weiner wrote:
> pageblock_order can be of various sizes, depending on configuration,
> but the default is MAX_ORDER-1. Given 4k pages, that comes out to
> 4M. This is a large chunk for the allocator/reclaim/compaction to try
> to keep grouped per migratetype. It's also unnecessary as the majority
> of higher order allocations - THP and slab - are smaller than that.

Well in my experience the kernel usually has hugetlbfs config-enabled so it
uses 2MB pageblocks (on x86) even if hugetlbfs is unused at runtime and THP
is used instead. But sure, we can set a better default that's not tied to
hugetlbfs.

> Before subsequent patches increase the effort that goes into
> maintaining migratetype isolation, it's important to first set the
> defrag block size to what's likely to have common consumers.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/pageblock-flags.h | 4 ++--
>  mm/page_alloc.c                 | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
> index 5f1ae07d724b..05b6811f8cee 100644
> --- a/include/linux/pageblock-flags.h
> +++ b/include/linux/pageblock-flags.h
> @@ -47,8 +47,8 @@ extern unsigned int pageblock_order;
>  
>  #else /* CONFIG_HUGETLB_PAGE */
>  
> -/* If huge pages are not used, group by MAX_ORDER_NR_PAGES */
> -#define pageblock_order		(MAX_ORDER-1)
> +/* Manage fragmentation at the 2M level */
> +#define pageblock_order		ilog2(2U << (20 - PAGE_SHIFT))
>  
>  #endif /* CONFIG_HUGETLB_PAGE */
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ac03571e0532..5e04a69f6a26 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7634,7 +7634,7 @@ static inline void setup_usemap(struct zone *zone) {}
>  /* Initialise the number of pages represented by NR_PAGEBLOCK_BITS */
>  void __init set_pageblock_order(void)
>  {
> -	unsigned int order = MAX_ORDER - 1;
> +	unsigned int order = ilog2(2U << (20 - PAGE_SHIFT));
>  
>  	/* Check that pageblock_nr_pages has not already been setup */
>  	if (pageblock_order)