Date:   Mon, 18 Sep 2023 18:54:09 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Stefan Roesch <shr@...kernel.io>
Cc:     kernel-team@...com, 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 18.09.23 18:18, Stefan Roesch wrote:
> 
> David Hildenbrand <david@...hat.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@...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 :)
>>
> 
> 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.

Can we add a comment why we don't skip them? Like

/*
  * Never skip pages that are already KSM; pages cmp_and_merge_page()
  * will essentially ignore them, but we still have to process them
  * properly.
  */

> 
>>> +
>>> +	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.

Can we make that explicit instead? Dealing with implicit overflows 
really makes the code harder to grasp.

> 
>>> +	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.

Dito.

> 
>>> +		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.

I see, thanks!


-- 
Cheers,

David / dhildenb

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