Re: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 6 Mar 2020 15:01:02 -0500 Rik van Riel <riel@surriel.com> wrote:

> Posting this one for Roman so I can deal with any upstream feedback and
> create a v2 if needed, while scratching my head over the next piece of
> this puzzle :)
> 
> ---8<---
> 
> From: Roman Gushchin <guro@fb.com>
> 
> Currently a cma area is barely used by the page allocator because
> it's used only as a fallback from movable, however kswapd tries
> hard to make sure that the fallback path isn't used.
> 
> This results in a system evicting memory and pushing data into swap,
> while lots of CMA memory is still available. This happens despite the
> fact that alloc_contig_range is perfectly capable of moving any movable
> allocations out of the way of an allocation.
> 
> To effectively use the cma area let's alter the rules: if the zone
> has more free cma pages than the half of total free pages in the zone,
> use cma pageblocks first and fallback to movable blocks in the case of
> failure.
> 
> Signed-off-by: Rik van Riel <riel@surriel.com>
> Co-developed-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Roman Gushchin <guro@fb.com>

fyi, the signoffs are in an unconventional order - usually the primary
author comes first.

> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2711,6 +2711,18 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
>  {
>  	struct page *page;
>  
> +	/*
> +	 * Balance movable allocations between regular and CMA areas by
> +	 * allocating from CMA when over half of the zone's free memory
> +	 * is in the CMA area.
> +	 */
> +	if (migratetype == MIGRATE_MOVABLE &&
> +	    zone_page_state(zone, NR_FREE_CMA_PAGES) >
> +	    zone_page_state(zone, NR_FREE_PAGES) / 2) {
> +		page = __rmqueue_cma_fallback(zone, order);
> +		if (page)
> +			return page;
> +	}
>  retry:
>  	page = __rmqueue_smallest(zone, order, migratetype);
>  	if (unlikely(!page)) {

__rmqueue() is a hot path (as much as any per-page operation can be a
hot path).  What is the impact here?