linux-kernel - Re: [PATCH V2] mm/damon: simplify scheme create in lru

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Date:   Wed, 14 Sep 2022 14:22:50 +0000
From:   SeongJae Park <sj@...nel.org>
To:     Xin Hao <xhao@...ux.alibaba.com>
Cc:     sj@...nel.org, akpm@...ux-foundation.org, damon@...ts.linux.dev,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mm/damon: simplify scheme create in lru_sort.c

Hi Xin,

On Wed, 14 Sep 2022 19:38:59 +0800 Xin Hao <xhao@...ux.alibaba.com> wrote:

> In damon_lru_sort_new_hot_scheme() and damon_lru_sort_new_cold_scheme(),
> they have so much in common, so we can combine them into a single
> function, and we just need to distinguish their differences.

Thank you again for patiently waiting for my changes and reworking on this!

> 
> Signed-off-by: Xin Hao <xhao@...ux.alibaba.com>
> ---
>  mm/damon/lru_sort.c | 57 ++++++++++++++++++++++-----------------------
>  1 file changed, 28 insertions(+), 29 deletions(-)
> 
> diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
> index 07a0908963fd..2eac907e866d 100644
> --- a/mm/damon/lru_sort.c
> +++ b/mm/damon/lru_sort.c
> @@ -135,17 +135,40 @@ DEFINE_DAMON_MODULES_DAMOS_STATS_PARAMS(damon_lru_sort_cold_stat,
>  static struct damon_ctx *ctx;
>  static struct damon_target *target;
>  
> -static struct damos *damon_lru_sort_new_scheme(
> -		struct damos_access_pattern *pattern, enum damos_action action)
> +static struct damos *damon_lru_sort_new_scheme(unsigned int thres,
> +					       enum damos_action action)
>  {
> +	struct damos_access_pattern pattern = {
> +		/* Find regions having PAGE_SIZE or larger size */
> +		.min_sz_region = PAGE_SIZE,
> +		.max_sz_region = ULONG_MAX,
> +		/* and accessed for more than the threshold */

This comment would be better to be written again?

> +		.min_nr_accesses = 0,
> +		.max_nr_accesses = 0,

If we're gonna set above two fields anyway later, we could simply remove above
three lines.

> +		/* no matter its age */
> +		.min_age_region = 0,
> +		.max_age_region = UINT_MAX,
> +	};
>  	struct damos_quota quota = damon_lru_sort_quota;
>  
>  	/* Use half of total quota for hot/cold pages sorting */
>  	quota.ms = quota.ms / 2;
>  
> +	switch (action) {
> +	case DAMOS_LRU_PRIO:
> +		pattern.min_nr_accesses = thres;
> +		pattern.max_nr_accesses = UINT_MAX;
> +		break;
> +	case DAMOS_LRU_DEPRIO:
> +		pattern.min_age_region = thres;
> +		break;
> +	default:
> +		return NULL;
> +	}
> +

This switch-case makes me wondering if the 'default' case really possible case.
I think it would be clearer to set these from caller.

IMHO, it might be clearer and shorter to define a static global 'struct
damos_access_pattern' stub variable, and make the
damon_lru_sort_new_{hot,cold}_scheme() copies it to their local variable,
update the relevant fields, and pass that to 'damon_new_scheme()'.  What do you
think?

>  	return damon_new_scheme(
>  			/* find the pattern, and */
> -			pattern,
> +			&pattern,
>  			/* (de)prioritize on LRU-lists */
>  			action,
>  			/* under the quota. */
> @@ -157,37 +180,13 @@ static struct damos *damon_lru_sort_new_scheme(
>  /* Create a DAMON-based operation scheme for hot memory regions */
>  static struct damos *damon_lru_sort_new_hot_scheme(unsigned int hot_thres)
>  {
> -	struct damos_access_pattern pattern = {
> -		/* Find regions having PAGE_SIZE or larger size */
> -		.min_sz_region = PAGE_SIZE,
> -		.max_sz_region = ULONG_MAX,
> -		/* and accessed for more than the threshold */
> -		.min_nr_accesses = hot_thres,
> -		.max_nr_accesses = UINT_MAX,
> -		/* no matter its age */
> -		.min_age_region = 0,
> -		.max_age_region = UINT_MAX,
> -	};
> -
> -	return damon_lru_sort_new_scheme(&pattern, DAMOS_LRU_PRIO);
> +	return damon_lru_sort_new_scheme(hot_thres, DAMOS_LRU_PRIO);

If we follow what I suggested above, we could make this like below:

	struct damos_access_pattern pattern = damon_lru_sort_stub_access_pattern;

	pattern.min_nr_accesses = hot_thres;
	return damon_lru_sort_new_scheme(&pattern, DAMOS_LRU_PRIO);


>  }
>  
>  /* Create a DAMON-based operation scheme for cold memory regions */
>  static struct damos *damon_lru_sort_new_cold_scheme(unsigned int cold_thres)
>  {
> -	struct damos_access_pattern pattern = {
> -		/* Find regions having PAGE_SIZE or larger size */
> -		.min_sz_region = PAGE_SIZE,
> -		.max_sz_region = ULONG_MAX,
> -		/* and not accessed at all */
> -		.min_nr_accesses = 0,
> -		.max_nr_accesses = 0,
> -		/* for min_age or more micro-seconds */
> -		.min_age_region = cold_thres,
> -		.max_age_region = UINT_MAX,
> -	};
> -
> -	return damon_lru_sort_new_scheme(&pattern, DAMOS_LRU_DEPRIO);
> +	return damon_lru_sort_new_scheme(cold_thres, DAMOS_LRU_DEPRIO);

And similarly here.

>  }
>  
>  static int damon_lru_sort_apply_parameters(void)
> -- 
> 2.31.0
> 
> 


Thanks,
SJ