linux-kernel - Re: [PATCH v2 1/2] mm/page_alloc: Do not calculate node's total pages and memmap pages when empty

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Date:   Tue, 21 Jun 2022 09:44:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/page_alloc: Do not calculate node's total pages
 and memmap pages when empty

On 21.06.22 06:17, Oscar Salvador wrote:
> free_area_init_node() calls calculate_node_totalpages() and
> free_area_init_core(). The former to get node's {spanned,present}_pages,
> and the latter to calculate, among other things, how many pages per zone
> we spent on memmap_pages, which is used to substract zone's free pages.
> 
> On memoryless-nodes, it is pointless to perform such a bunch of work, so
> make sure we skip the calculations when having a node or empty zone.
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>  mm/page_alloc.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e008a3df0485..2b9b2422ba32 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7361,6 +7361,10 @@ static void __init calculate_node_totalpages(struct pglist_data *pgdat,
>  	unsigned long realtotalpages = 0, totalpages = 0;
>  	enum zone_type i;
>  
> +	/* Skip calculation for memoryless nodes */
> +	if (pgdat_is_empty(pgdat))
> +		goto no_pages;
> +
>  	for (i = 0; i < MAX_NR_ZONES; i++) {
>  		struct zone *zone = pgdat->node_zones + i;
>  		unsigned long zone_start_pfn, zone_end_pfn;
> @@ -7393,6 +7397,7 @@ static void __init calculate_node_totalpages(struct pglist_data *pgdat,
>  		realtotalpages += real_size;
>  	}
>  
> +no_pages:
>  	pgdat->node_spanned_pages = totalpages;
>  	pgdat->node_present_pages = realtotalpages;
>  	pr_debug("On node %d totalpages: %lu\n", pgdat->node_id, realtotalpages);
> @@ -7610,6 +7615,12 @@ static void __init free_area_init_core(struct pglist_data *pgdat)
>  		size = zone->spanned_pages;
>  		freesize = zone->present_pages;
>  
> +		/* No pages? Nothing to calculate then. */
> +		if (!size) {
> +			zone_init_internals(zone, j, nid, 0);
> +			continue;
> +		}
> +
>  		/*
>  		 * Adjust freesize so that it accounts for how much memory
>  		 * is used by this zone for memmap. This affects the watermark
> @@ -7647,9 +7658,6 @@ static void __init free_area_init_core(struct pglist_data *pgdat)
>  		 */
>  		zone_init_internals(zone, j, nid, freesize);
>  
> -		if (!size)
> -			continue;
> -
>  		set_pageblock_order();
>  		setup_usemap(zone);
>  		init_currently_empty_zone(zone, zone->zone_start_pfn, size);
> @@ -7730,7 +7738,7 @@ static void __init free_area_init_node(int nid)
>  	pgdat->node_start_pfn = start_pfn;
>  	pgdat->per_cpu_nodestats = NULL;
>  
> -	if (start_pfn != end_pfn) {
> +	if (!pgdat_is_empty(pgdat)) {
>  		pr_info("Initmem setup node %d [mem %#018Lx-%#018Lx]\n", nid,
>  			(u64)start_pfn << PAGE_SHIFT,
>  			end_pfn ? ((u64)end_pfn << PAGE_SHIFT) - 1 : 0);


It's worth noting that the check in pgdat_is_empty() is slightly
different. I *think* it doesn't matter in practice, yet I wonder if we
should simply fixup (currently unused) pgdat_is_empty().

Anyhow

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb