Re: [PATCHv2] mm: optimization on page allocation when CMA enabled

[Roman Gushchin <roman.gushchin@linux.dev>]

On Thu, May 04, 2023 at 06:09:54PM +0800, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> Let us look at the series of scenarios below with WMARK_LOW=25MB,WMARK_MIN=5MB
> (managed pages 1.9GB). We can know that current 'fixed 1/2 ratio' start to use
> CMA since C which actually has caused U&R lower than WMARK_LOW (this should be
> deemed as against current memory policy, that is, U&R should either stay around
> WATERMARK_LOW when no allocation or do reclaim via enter slowpath)
> 
> free_cma/free_pages(MB)      A(12/30)     B(12/25)     C(12/20)
> fixed 1/2 ratio                 N             N           Y
> this commit                     Y             Y           Y
> 
> Suggested-by: Roman Gushchin <roman.gushchin@linux.dev>

I didn't suggest it in this form, please, drop this tag.

> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> ---
> v2: do proportion check when zone_watermark_ok, update commit message
> ---
> ---
>  mm/page_alloc.c | 36 ++++++++++++++++++++++++++++++++----
>  1 file changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0745aed..d0baeab 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3071,6 +3071,34 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
>  
>  }
>  
> +#ifdef CONFIG_CMA
> +static bool __if_use_cma_first(struct zone *zone, unsigned int order, unsigned int alloc_flags)
> +{
> +	unsigned long cma_proportion = 0;
> +	unsigned long cma_free_proportion = 0;
> +	unsigned long watermark = 0;
> +	long count = 0;
> +	bool cma_first = false;
> +
> +	watermark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
> +	/*check if GFP_MOVABLE pass previous watermark check via the help of CMA*/
> +	if (!zone_watermark_ok(zone, order, watermark, 0, alloc_flags & (~ALLOC_CMA)))
> +		/* WMARK_LOW failed lead to using cma first, this helps U&R stay
> +		 * around low when being drained by GFP_MOVABLE
> +		 */
> +		cma_first = true;

This part looks reasonable to me.

> +	else {
> +		/*check proportion when zone_watermark_ok*/
> +		count = atomic_long_read(&zone->managed_pages);
> +		cma_proportion = zone->cma_pages * 100 / count;
> +		cma_free_proportion = zone_page_state(zone, NR_FREE_CMA_PAGES) * 100
> +			/  zone_page_state(zone, NR_FREE_PAGES);
> +		cma_first = (cma_free_proportion >= cma_proportion * 2

Why *2? Please, explain.

> +				|| cma_free_proportion >= 50);

It will heavily boost the use of cma at early stages of uptime, when there is a lot of !cma
memory, making continuous (e.g. hugetlb) allocations fail more often. Not a good idea.

Thanks!