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Subject: Re: [PATCH v3 5/7] mm, page_alloc: cache pageset high and batch in
 struct zone
To:     Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Michal Hocko <mhocko@kernel.org>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Oscar Salvador <osalvador@suse.de>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Michal Hocko <mhocko@suse.com>
References: <20201111092812.11329-1-vbabka@suse.cz>
 <20201111092812.11329-6-vbabka@suse.cz>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <34b9c883-8b77-435f-73b9-ccaa1677fc52@redhat.com>
Date:   Thu, 12 Nov 2020 17:26:14 +0100
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On 11.11.20 10:28, 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@suse.cz>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
>   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 7385871768d4..bb9188de2718 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 8b43d6d1a288..25bc9bb77696 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5912,6 +5912,9 @@ 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);
>   
> @@ -6301,8 +6304,8 @@ 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;
>   }
>   
>   /*
> @@ -6326,6 +6329,13 @@ 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)
> +		return;
> +
> +	zone->pageset_high = new_high;
> +	zone->pageset_batch = new_batch;
> +
>   	for_each_possible_cpu(cpu) {
>   		p = per_cpu_ptr(zone->pageset, cpu);
>   		pageset_update(&p->pcp, new_high, new_batch);
> @@ -6386,6 +6396,8 @@ 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",
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb