linux-kernel - Re: [PATCH v4 2/4] mm/ksm: add sysfs knobs for advisor

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Date: Mon, 18 Dec 2023 12:25:23 +0100
From: David Hildenbrand <david@...hat.com>
To: Stefan Roesch <shr@...kernel.io>, kernel-team@...com
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, riel@...riel.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 2/4] mm/ksm: add sysfs knobs for advisor

On 13.12.23 19:27, Stefan Roesch wrote:
> This adds four new knobs for the KSM advisor to influence its behaviour.
> 
> The knobs are:
> - advisor_mode:
>      none:      no advisor (default)
>      scan-time: scan time advisor
> - advisor_max_cpu: 70 (default, cpu usage percent)
> - advisor_min_pages_to_scan: 500 (default)
> - advisor_max_pages_to_scan: 30000 (default)
> - advisor_target_scan_time: 200 (default in seconds)
> 
> The new values will take effect on the next scan round.
> 
> Signed-off-by: Stefan Roesch <shr@...kernel.io>
> ---
>   mm/ksm.c | 142 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 139 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 4f7b71a1f3112..f7387a6d02050 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -302,10 +302,10 @@ unsigned long ksm_zero_pages;
>   static unsigned long ksm_pages_skipped;
>   
>   /* Don't scan more than max pages per batch. */
> -static unsigned long ksm_advisor_max_pages = 30000;
> +static unsigned long ksm_advisor_max_pages_to_scan = 30000;
>   
>   /* At least scan this many pages per batch. */
> -static unsigned long ksm_advisor_min_pages = 500;
> +static unsigned long ksm_advisor_min_pages_to_scan = 500;
>   

That renaming should go into the previous patch.

>   /* Min CPU for scanning pages per scan */
>   static unsigned int ksm_advisor_min_cpu =  10;
> @@ -341,6 +341,16 @@ enum ksm_advisor_type {
>   };
>   static enum ksm_advisor_type ksm_advisor;
>   
> +static void set_advisor_defaults(void)
> +{
> +	if (ksm_advisor == KSM_ADVISOR_NONE) {
> +		ksm_thread_pages_to_scan = DEFAULT_PAGES_TO_SCAN;
> +	} else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME) {
> +		advisor_ctx = (const struct advisor_ctx){ 0 };
> +		ksm_thread_pages_to_scan = ksm_advisor_min_pages_to_scan;
> +	}
> +}
> +
>   static inline void advisor_start_scan(void)
>   {
>   	if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
> @@ -434,7 +444,7 @@ static void scan_time_advisor(void)
>   
>   	pages = min(pages, per_page_cost * ksm_advisor_max_cpu);
>   	pages = max(pages, per_page_cost * ksm_advisor_min_cpu);
> -	pages = min(pages, ksm_advisor_max_pages);
> +	pages = min(pages, ksm_advisor_max_pages_to_scan);
>   
>   	/* Update advisor context */
>   	advisor_ctx.change = change;
> @@ -3722,6 +3732,127 @@ static ssize_t smart_scan_store(struct kobject *kobj,
>   }
>   KSM_ATTR(smart_scan);
>   
> +static ssize_t advisor_mode_show(struct kobject *kobj,
> +				 struct kobj_attribute *attr, char *buf)
> +{
> +	const char *output;
> +
> +	if (ksm_advisor == KSM_ADVISOR_NONE)
> +		output = "[none] scan-time";
> +	else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
> +		output = "none [scan-time]";
> +
> +	return sysfs_emit(buf, "%s\n", output);
> +}
> +
> +static ssize_t advisor_mode_store(struct kobject *kobj,
> +				  struct kobj_attribute *attr, const char *buf,
> +				  size_t count)
> +{
> +	if (sysfs_streq("scan-time", buf))
> +		ksm_advisor = KSM_ADVISOR_SCAN_TIME;
> +	else if (sysfs_streq("none", buf))
> +		ksm_advisor = KSM_ADVISOR_NONE;
> +	else
> +		return -EINVAL;
> +
> +	/* Set advisor default values */
> +	set_advisor_defaults();

It probably makes sense to not reset the defaults if the ksm_advisor 
didn't change.

Apart form that, LGTM.

-- 
Cheers,

David / dhildenb