Re: [PATCH] mm/vmscan: simplify the calculation of fractions for SCAN_FRACT

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Feb 28, 2024 at 10:55:00AM +0900, Byungchul Park wrote:
> The current way to calculate fractions for SACN_FRACT is little readable
> and more complicated than it should be.  It also performs unnecessary
> division and adjustment to avoid zero operands.  Prune away by
> multiplying the fractions by 'anon_cost * file_cost / (3 * total_cost)':
> 
> where:
>    total_cost = sc->anon_cost + sc->file_cost
>    anon_cost = total_cost + sc->anon_cost
>    file_cost = total_cost + sc->file_cost
> 
> before:
>    fraction[0] = swappiness * 3 * total_cost / anon_cost
>    fraction[1] = (200 - swappiness) * 3 * total_cost / file_cost
> 
> after:
>    fraction[0] = swappiness * file_cost
>    fraction[1] = (200 - swappiness) * anon_cost
> 
> Worth noting that this patch doesn't change the formula.
> 
> Signed-off-by: Byungchul Park <byungchul@sk.com>
> ---
>  mm/vmscan.c | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 4657440854db..7b33fcc1cbdc 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2339,7 +2339,6 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  	u64 fraction[ANON_AND_FILE];
>  	u64 denominator = 0;	/* gcc */
>  	enum scan_balance scan_balance;
> -	unsigned long ap, fp;
>  	enum lru_list lru;
>  
>  	/*
> @@ -2416,17 +2415,10 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  	total_cost = sc->anon_cost + sc->file_cost;
>  	anon_cost = total_cost + sc->anon_cost;
>  	file_cost = total_cost + sc->file_cost;
> -	total_cost = anon_cost + file_cost;
>  
> -	ap = swappiness * (total_cost + 1);
> -	ap /= anon_cost + 1;
> -
> -	fp = (200 - swappiness) * (total_cost + 1);
> -	fp /= file_cost + 1;
> -
> -	fraction[0] = ap;
> -	fraction[1] = fp;
> -	denominator = ap + fp;
> +	fraction[0] = swappiness * file_cost;
> +	fraction[1] = (200 - swappiness) * anon_cost;

Unfortunately, I don't think that

anon = swappiness * file_cost
file = (200 - swappiness) * anon_cost

is more readable. Sure it's the same, but I think it's clearer to
actually see that `anon = total_cost / anon_cost` ratio in the code.