Re: [PATCH] mm,page_alloc,cma: configurable CMA utilization

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 1/31/23 12:40, Sukadev Bhattiprolu wrote:
> 
> Commit 16867664936e ("mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations")
> added support to use CMA pages when more than 50% of total free pages in
> the zone are free CMA pages.
> 
> However, with multiplatform kernels a single binary is used across different
> targets of varying memory sizes. A low memory target using one such kernel
> would incur allocation failures even when sufficient memory is available in
> the CMA region. On these targets we would want to utilize a higher percentage
> of the CMA region and reduce the allocation failures, even if it means that a
> subsequent cma_alloc() would take longer.
> 
> Make the percentage of CMA utilization a configurable parameter to allow
> for such usecases.
> 
> Signed-off-by: Sukadev Bhattiprolu <quic_sukadev@quicinc.com>
> ---
> Note:	There was a mention about it being the last resort to making this
> 	percentage configurable (https://lkml.org/lkml/2020/3/12/751). But
> 	as explained above, multi-platform kernels for varying memory size
> 	targets would need this to be configurable.
> ---
>  include/linux/mm.h |  1 +
>  kernel/sysctl.c    |  8 ++++++++
>  mm/page_alloc.c    | 18 +++++++++++++++---
>  mm/util.c          |  2 ++
>  4 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8f857163ac89..e4e5d508e9eb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -203,6 +203,7 @@ extern unsigned long sysctl_admin_reserve_kbytes;
>  
>  extern int sysctl_overcommit_memory;
>  extern int sysctl_overcommit_ratio;
> +extern int sysctl_cma_utilization_ratio;
>  extern unsigned long sysctl_overcommit_kbytes;
>  
>  int overcommit_ratio_handler(struct ctl_table *, int, void *, size_t *,
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 137d4abe3eda..2dce6a908aa6 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -2445,6 +2445,14 @@ static struct ctl_table vm_table[] = {
>  		.extra2		= SYSCTL_ONE,
>  	},
>  #endif
> +	{
> +		.procname	= "cma_utilization_ratio",
> +		.data		= &sysctl_cma_utilization_ratio,
> +		.maxlen		= sizeof(sysctl_cma_utilization_ratio),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= SYSCTL_ONE,
> +	},
>  	{ }
>  };
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0745aedebb37..b72db3824687 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3071,6 +3071,20 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype,
>  
>  }
>  
> +static __always_inline bool zone_can_use_cma_pages(struct zone *zone)
> +{
> +	unsigned long cma_free_pages;
> +	unsigned long zone_free_pages;
> +
> +	cma_free_pages = zone_page_state(zone, NR_FREE_CMA_PAGES);
> +	zone_free_pages = zone_page_state(zone, NR_FREE_PAGES);
> +
> +	if (cma_free_pages > zone_free_pages / sysctl_cma_utilization_ratio)
> +		return true;
> +
> +	return false;
> +}
> +
>  /*
>   * Do the hard work of removing an element from the buddy allocator.
>   * Call me with the zone->lock already held.
> @@ -3087,9 +3101,7 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
>  		 * allocating from CMA when over half of the zone's free memory
>  		 * is in the CMA area.
>  		 */
> -		if (alloc_flags & ALLOC_CMA &&
> -		    zone_page_state(zone, NR_FREE_CMA_PAGES) >
> -		    zone_page_state(zone, NR_FREE_PAGES) / 2) {
> +		if (alloc_flags & ALLOC_CMA && zone_can_use_cma_pages(zone)) {
>  			page = __rmqueue_cma_fallback(zone, order);
>  			if (page)
>  				return page;
> diff --git a/mm/util.c b/mm/util.c
> index b56c92fb910f..4de81f04b249 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -781,6 +781,8 @@ void folio_copy(struct folio *dst, struct folio *src)
>  }
>  
>  int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS;
> +
> +int sysctl_cma_utilization_ratio __read_mostly = 2;

Make '2' here a macro e.g CMA_UTILIZATION_DEFAULT ? Also it might be a good
opportunity to comment why the default value is '2' i.e 50 %.

>  int sysctl_overcommit_ratio __read_mostly = 50;
>  unsigned long sysctl_overcommit_kbytes __read_mostly;
>  int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;