Re: [PATCH 6/9] mm, page_alloc: cache pageset high and batch in struct zone

[David Hildenbrand]

On 22.09.20 16:37, 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>
> ---
>  include/linux/mmzone.h |  6 ++++++
>  mm/page_alloc.c        | 16 ++++++++++++++--
>  2 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 90721f3156bc..7ad3f14dbe88 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 de3b48bda45c..901907799bdc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5824,6 +5824,8 @@ 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);
> +#define BOOT_PAGESET_HIGH	0
> +#define BOOT_PAGESET_BATCH	1

Much better. A comment would have been nice ("this disables the pcp via
the boot pageset completely.") :) (I'm pretty sure I'd forget at one
point what these values mean)

>  static DEFINE_PER_CPU(struct per_cpu_pageset, boot_pageset);
>  static DEFINE_PER_CPU(struct per_cpu_nodestat, boot_nodestats);
>  
> @@ -6213,8 +6215,8 @@ 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;
>  }
>  
>  /*
> @@ -6238,6 +6240,14 @@ 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);
> @@ -6300,6 +6310,8 @@ 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;

I do wonder if copying from any cpuvar inside boot_pageset is cleaner.

zone->pageset_high = &this_cpu_ptr(zone->pageset)->pcp.high;
...


-- 
Thanks,

David / dhildenb