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From:   Stefan Roesch <shr@devkernel.io>
To:     David Hildenbrand <david@redhat.com>
Cc:     kernel-team@fb.com, akpm@linux-foundation.org, hannes@cmpxchg.org,
        riel@surriel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v1 1/4] mm/ksm: add "smart" page scanning mode
Date:   Mon, 18 Sep 2023 09:18:27 -0700
In-reply-to: <6447ab02-1a22-efe4-68c9-4f595e2499fc@redhat.com>
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David Hildenbrand <david@redhat.com> writes:

> 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@devkernel.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 :)
>

The above description sounds correct. During each scan we go through all
the candidate pages and this includes rmap_items that maps to KSM pages.
The above check simply skips these pages.

>> +
>> +	age = rmap_item->age++;
>
> Can't we overflow here? Is that desired, or would you want to stop at the
> maximum you can store?
>

Yes, we can overflow here and it was a deliberate choice. If we overflow
after we tried unsuccessfully for 255 times, we re-start with shorter
skip values, but that should be fine. In return we avoid an if statement.
The age is defined as unsigned.

>> +	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?
>

Yes, you can. See the above discussion. This skip_age is also an
unsigned value.

>> +		remove_rmap_item_from_tree(rmap_item);
>
>
> Can you enlighten me why that is required?
>

This is required for age calculation and BUG_ON check in
remove_rmap_item_from_tree. If we don't call remove_rmap_item_from_tree,
we will hit the BUG_ON for the skipped pages later on.

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