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Date:   Wed, 13 Sep 2023 14:07:50 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Stefan Roesch <shr@devkernel.io>
Cc:     kernel-team@fb.com, david@redhat.com, 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
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        <20230912175228.952039-2-shr@devkernel.io>
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On Tue, 12 Sep 2023 10:52:25 -0700 Stefan Roesch <shr@devkernel.io> 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

s/skip_page/skip_age/

> 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.
> 

All seems nice.  I'll sit out v1, see what people have to say.

Some nits:

> --- a/mm/ksm.c
> +++ b/mm/ksm.c
>
> ...
>
> @@ -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;
> +}

"inc_skip_age" sounds like it increments something.  Can we give it a
better name?

And a nice comment explaining its role in life.

> +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;
> +
> +	age = rmap_item->age++;
> +	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);
> +		remove_rmap_item_from_tree(rmap_item);
> +	}
> +
> +	return true;
> +}

Would a better name be should_skip_rmap_item()?

But even that name implies that the function is idempotent (has no
side-effects).  Again, an explanatory comment would be good.  And
simple comments over each non-obvious `if' statement.

>
> ...
>