Date:   Mon, 18 Sep 2023 13:10:35 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Stefan Roesch <shr@...kernel.io>, kernel-team@...com
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, riel@...riel.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 1/4] mm/ksm: add "smart" page scanning mode

On 12.09.23 19:52, Stefan Roesch wrote:
> This change adds a "smart" page scanning mode for KSM. So far all the
> candidate pages are continuously scanned to find candidates for
> de-duplication. There are a considerably number of pages that cannot be
> de-duplicated. This is costly in terms of CPU. By using smart scanning
> considerable CPU savings can be achieved.
> 
> This change takes the history of scanning pages into account and skips
> the page scanning of certain pages for a while if de-deduplication for
> this page has not been successful in the past.
> 
> To do this it introduces two new fields in the ksm_rmap_item structure:
> age and skip_age. age, is the KSM age and skip_page is the age for how
> long page scanning of this page is skipped. The age field is incremented
> each time the page is scanned and the page cannot be de-duplicated.
> 
> How often a page is skipped is dependent how often de-duplication has
> been tried so far and the number of skips is currently limited to 8.
> This value has shown to be effective with different workloads.
> 
> The feature is currently disable by default and can be enabled with the
> new smart_scan knob.
> 
> The feature has shown to be very effective: upt to 25% of the page scans
> can be eliminated; the pages_to_scan rate can be reduced by 40 - 50% and
> a similar de-duplication rate can be maintained.
> 
> Signed-off-by: Stefan Roesch <shr@...kernel.io>
> ---
>   mm/ksm.c | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 75 insertions(+)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 981af9c72e7a..bfd5087c7d5a 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -56,6 +56,8 @@
>   #define DO_NUMA(x)	do { } while (0)
>   #endif
>   
> +typedef u8 rmap_age_t;
> +
>   /**
>    * DOC: Overview
>    *
> @@ -193,6 +195,8 @@ struct ksm_stable_node {
>    * @node: rb node of this rmap_item in the unstable tree
>    * @head: pointer to stable_node heading this list in the stable tree
>    * @hlist: link into hlist of rmap_items hanging off that stable_node
> + * @age: number of scan iterations since creation
> + * @skip_age: skip rmap item until age reaches skip_age
>    */
>   struct ksm_rmap_item {
>   	struct ksm_rmap_item *rmap_list;
> @@ -212,6 +216,8 @@ struct ksm_rmap_item {
>   			struct hlist_node hlist;
>   		};
>   	};
> +	rmap_age_t age;
> +	rmap_age_t skip_age;
>   };
>   
>   #define SEQNR_MASK	0x0ff	/* low bits of unstable tree seqnr */
> @@ -281,6 +287,9 @@ static unsigned int zero_checksum __read_mostly;
>   /* Whether to merge empty (zeroed) pages with actual zero pages */
>   static bool ksm_use_zero_pages __read_mostly;
>   
> +/* Skip pages that couldn't be de-duplicated previously  */
> +static bool ksm_smart_scan;
> +
>   /* The number of zero pages which is placed by KSM */
>   unsigned long ksm_zero_pages;
>   
> @@ -2305,6 +2314,45 @@ static struct ksm_rmap_item *get_next_rmap_item(struct ksm_mm_slot *mm_slot,
>   	return rmap_item;
>   }
>   
> +static unsigned int inc_skip_age(rmap_age_t age)
> +{
> +	if (age <= 3)
> +		return 1;
> +	if (age <= 5)
> +		return 2;
> +	if (age <= 8)
> +		return 4;
> +
> +	return 8;
> +}
> +
> +static bool skip_rmap_item(struct page *page, struct ksm_rmap_item *rmap_item)
> +{
> +	rmap_age_t age;
> +
> +	if (!ksm_smart_scan)
> +		return false;
> +
> +	if (PageKsm(page))
> +		return false;


I'm a bit confused about this check here. scan_get_next_rmap_item() 
would return a PageKsm() page and call cmp_and_merge_page().

cmp_and_merge_page() says: "first see if page can be merged into the 
stable tree"

... but shouldn't a PageKsm page *already* be in the stable tree?

Maybe that's what cmp_and_merge_page() does via:

	kpage = stable_tree_search(page);
	if (kpage == page && rmap_item->head == stable_node) {
		put_page(kpage);
		return;
	}


Hoping you can enlighten me :)

> +
> +	age = rmap_item->age++;

Can't we overflow here? Is that desired, or would you want to stop at 
the maximum you can store?

> +	if (age < 3)
> +		return false;
> +
> +	if (rmap_item->skip_age == age) {
> +		rmap_item->skip_age = 0;
> +		return false;
> +	}
> +
> +	if (rmap_item->skip_age == 0) {
> +		rmap_item->skip_age = age + inc_skip_age(age);

Can't you overflow here as well?

> +		remove_rmap_item_from_tree(rmap_item);


Can you enlighten me why that is required?

> +	}
> +
> +	return true;
> +}
> +


-- 
Cheers,

David / dhildenb

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