Re: [PATCH v2 5/7] mm, page_alloc: cache pageset high and batch in struct zone

[Michal Hocko]

On Thu 08-10-20 13:41:59, Vlastimil Babka wrote:
> All per-cpu pagesets for a zone use the same high and batch values, that are
> duplicated there just for performance (locality) reasons. This patch adds the
> same variables also to struct zone as a shared copy.
> 
> This will be useful later for making possible to disable pcplists temporarily
> by setting high value to 0, while remembering the values for restoring them
> later. But we can also immediately benefit from not updating pagesets of all
> possible cpus in case the newly recalculated values (after sysctl change or
> memory online/offline) are actually unchanged from the previous ones.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

I would consider the check flipped with early return more pleasing to my
eyes but nothing to lose sleep over.

> ---
>  include/linux/mmzone.h |  6 ++++++
>  mm/page_alloc.c        | 17 +++++++++++++++--
>  2 files changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index fb3bf696c05e..c63863794afc 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -470,6 +470,12 @@ struct zone {
>  #endif
>  	struct pglist_data	*zone_pgdat;
>  	struct per_cpu_pageset __percpu *pageset;
> +	/*
> +	 * the high and batch values are copied to individual pagesets for
> +	 * faster access
> +	 */
> +	int pageset_high;
> +	int pageset_batch;
>  
>  #ifndef CONFIG_SPARSEMEM
>  	/*
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f33c36312eb5..5b98dd5ab006 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5900,6 +5900,9 @@ static void build_zonelists(pg_data_t *pgdat)
>   * Other parts of the kernel may not check if the zone is available.
>   */
>  static void pageset_init(struct per_cpu_pageset *p);
> +/* These effectively disable the pcplists in the boot pageset completely */
> +#define BOOT_PAGESET_HIGH	0
> +#define BOOT_PAGESET_BATCH	1
>  static DEFINE_PER_CPU(struct per_cpu_pageset, boot_pageset);
>  static DEFINE_PER_CPU(struct per_cpu_nodestat, boot_nodestats);
>  
> @@ -6289,8 +6292,8 @@ static void pageset_init(struct per_cpu_pageset *p)
>  	 * need to be as careful as pageset_update() as nobody can access the
>  	 * pageset yet.
>  	 */
> -	pcp->high = 0;
> -	pcp->batch = 1;
> +	pcp->high = BOOT_PAGESET_HIGH;
> +	pcp->batch = BOOT_PAGESET_BATCH;
>  }
>  
>  /*
> @@ -6314,6 +6317,14 @@ static void zone_set_pageset_high_and_batch(struct zone *zone)
>  		new_batch = max(1UL, 1 * new_batch);
>  	}
>  
> +	if (zone->pageset_high != new_high ||
> +	    zone->pageset_batch != new_batch) {
> +		zone->pageset_high = new_high;
> +		zone->pageset_batch = new_batch;
> +	} else {
> +		return;
> +	}
> +
>  	for_each_possible_cpu(cpu) {
>  		p = per_cpu_ptr(zone->pageset, cpu);
>  		pageset_update(&p->pcp, new_high, new_batch);
> @@ -6374,6 +6385,8 @@ static __meminit void zone_pcp_init(struct zone *zone)
>  	 * offset of a (static) per cpu variable into the per cpu area.
>  	 */
>  	zone->pageset = &boot_pageset;
> +	zone->pageset_high = BOOT_PAGESET_HIGH;
> +	zone->pageset_batch = BOOT_PAGESET_BATCH;
>  
>  	if (populated_zone(zone))
>  		printk(KERN_DEBUG "  %s zone: %lu pages, LIFO batch:%u\n",
> -- 
> 2.28.0

-- 
Michal Hocko
SUSE Labs