linux-kernel - Re: [PATCH 4/4] mm/memcg: refine mem_cgroup_threshold_ary->current

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Date:   Tue, 11 Jan 2022 09:44:45 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     hannes@...xchg.org, vdavydov.dev@...il.com,
        akpm@...ux-foundation.org, shakeelb@...gle.com, guro@...com,
        vbabka@...e.cz, willy@...radead.org, songmuchun@...edance.com,
        shy828301@...il.com, surenb@...gle.com,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 4/4] mm/memcg: refine
 mem_cgroup_threshold_ary->current_threshold calculation

On Tue 11-01-22 01:03:02, Wei Yang wrote:
> mem_cgroup_threshold_ary->current_threshold points to the last entry
> who's threshold is less or equal to usage.
> 
> Instead of iterating entries to get the correct index, we can leverage
> primary->current_threshold to get it. If the threshold added is less or
> equal to usage, current_threshold should increase by one. Otherwise, it
> doesn't change.

Why do we want/need this change?

> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
> ---
>  mm/memcontrol.c | 31 +++++++++++++++++--------------
>  1 file changed, 17 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a504616f904a..ce7060907df2 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4161,7 +4161,7 @@ static int __mem_cgroup_usage_register_event(struct mem_cgroup *memcg,
>  	struct mem_cgroup_threshold_ary *new;
>  	unsigned long threshold;
>  	unsigned long usage;
> -	int i, size, ret;
> +	int size, ret;
>  
>  	ret = page_counter_memparse(args, "-1", &threshold);
>  	if (ret)
> @@ -4193,9 +4193,13 @@ static int __mem_cgroup_usage_register_event(struct mem_cgroup *memcg,
>  	new->size = size;
>  
>  	/* Copy thresholds (if any) to new array */
> -	if (thresholds->primary)
> +	if (thresholds->primary) {
>  		memcpy(new->entries, thresholds->primary->entries,
>  		       flex_array_size(new, entries, size - 1));
> +		new->current_threshold = thresholds->primary->current_threshold;
> +	} else {
> +		new->current_threshold = -1;
> +	}
>  
>  	/* Add new threshold */
>  	new->entries[size - 1].eventfd = eventfd;
> @@ -4205,18 +4209,17 @@ static int __mem_cgroup_usage_register_event(struct mem_cgroup *memcg,
>  	sort(new->entries, size, sizeof(*new->entries),
>  			compare_thresholds, NULL);
>  
> -	/* Find current threshold */
> -	new->current_threshold = -1;
> -	for (i = 0; i < size; i++) {
> -		if (new->entries[i].threshold <= usage) {
> -			/*
> -			 * new->current_threshold will not be used until
> -			 * rcu_assign_pointer(), so it's safe to increment
> -			 * it here.
> -			 */
> -			++new->current_threshold;
> -		} else
> -			break;
> +	/*
> +	 * If the threshold added here is less or equal to usage, this means
> +	 * current_threshold need to increase by one.
> +	 */
> +	if (threshold <= usage) {
> +		/*
> +		 * new->current_threshold will not be used until
> +		 * rcu_assign_pointer(), so it's safe to increment
> +		 * it here.
> +		 */
> +		new->current_threshold++;
>  	}
>  
>  	/* Free old spare buffer and save old primary buffer as spare */
> -- 
> 2.33.1

-- 
Michal Hocko
SUSE Labs