linux-kernel - Re: [PATCHv2 1/6] zsmalloc: remove insert

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Date:   Thu, 23 Feb 2023 15:09:32 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv2 1/6] zsmalloc: remove insert_zspage() ->inuse
 optimization

On Thu, Feb 23, 2023 at 12:04:46PM +0900, Sergey Senozhatsky wrote:
> This optimization has no effect. It only ensures that
> when a page was added to its corresponding fullness
> list, its "inuse" counter was higher or lower than the
> "inuse" counter of the page at the head of the list.
> The intention was to keep busy pages at the head, so
> they could be filled up and moved to the ZS_FULL
> fullness group more quickly. However, this doesn't work
> as the "inuse" counter of a page can be modified by

                              zspage

Let's use term zspage instead of page to prevent confusing.

> obj_free() but the page may still belong to the same
> fullness list. So, fix_fullness_group() won't change

Yes. I didn't expect it should be perfect from the beginning
but would help just little optimization.

> the page's position in relation to the head's "inuse"
> counter, leading to a largely random order of pages
> within the fullness list.

Good point.

> 
> For instance, consider a printout of the "inuse"
> counters of the first 10 pages in a class that holds
> 93 objects per zspage:
> 
>  ZS_ALMOST_EMPTY:  36  67  68  64  35  54  63  52
> 
> As we can see the page with the lowest "inuse" counter
> is actually the head of the fullness list.

Let's write what the patch is doing cleary

"So, let's remove the pointless optimization" or something better word.

> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> ---
>  mm/zsmalloc.c | 29 ++++++++---------------------
>  1 file changed, 8 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 3aed46ab7e6c..b57a89ed6f30 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -753,37 +753,24 @@ static enum fullness_group get_fullness_group(struct size_class *class,
>  }
>  
>  /*
> - * Each size class maintains various freelists and zspages are assigned
> - * to one of these freelists based on the number of live objects they
> - * have. This functions inserts the given zspage into the freelist
> - * identified by <class, fullness_group>.
> + * This function adds the given zspage to the fullness list identified
> + * by <class, fullness_group>.
>   */
>  static void insert_zspage(struct size_class *class,
> -				struct zspage *zspage,
> -				enum fullness_group fullness)
> +			  struct zspage *zspage,
> +			  enum fullness_group fullness)

Unnecessary changes

>  {
> -	struct zspage *head;
> -
>  	class_stat_inc(class, fullness, 1);
> -	head = list_first_entry_or_null(&class->fullness_list[fullness],
> -					struct zspage, list);
> -	/*
> -	 * We want to see more ZS_FULL pages and less almost empty/full.
> -	 * Put pages with higher ->inuse first.
> -	 */
> -	if (head && get_zspage_inuse(zspage) < get_zspage_inuse(head))
> -		list_add(&zspage->list, &head->list);
> -	else
> -		list_add(&zspage->list, &class->fullness_list[fullness]);
> +	list_add(&zspage->list, &class->fullness_list[fullness]);
>  }
>  
>  /*
> - * This function removes the given zspage from the freelist identified
> + * This function removes the given zspage from the fullness list identified
>   * by <class, fullness_group>.
>   */
>  static void remove_zspage(struct size_class *class,
> -				struct zspage *zspage,
> -				enum fullness_group fullness)
> +			  struct zspage *zspage,
> +			  enum fullness_group fullness)

Ditto.

Other than that, looks good to me.