linux-kernel - Re: [PATCH] memblock: avoid some repeat when add new range

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Date:   Mon, 13 Jun 2022 07:45:18 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     tjytimi <tjytimi@....com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memblock: avoid some repeat when add new range

On Thu, Jun 09, 2022 at 10:41:22AM +0800, tjytimi wrote:
> The worst case is that the new memory range overlaps all existing
> regions,which need type->cnt + 1 free area of struct memblock_region.
> So if type->cnt + 1 + type->cnt is less than type->max,we can insert
> regions directly.And becase of merge operation in the end of function,
> tpye->cnt increase slowly for many cases.So this patch can avoid
> unnecessary repeat for many cases when add new memory range.
> 
> Signed-off-by: tjytimi <tjytimi@....com>
> ---
>  mm/memblock.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index e4f03a6e8..243cd7de5 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -602,6 +602,9 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>  	base = obase;
>  	nr_new = 0;
>  
> +	if (type->cnt<<1 < type->max - 1)
> +		insert = true;
> +

Several things here:
- cnt * 2 looks clearer than cnt << 1 and the performance difference (if
  any) is negligible
- I think checking that cnt + 1 < max is easier to read
- there should be space around operators, i.e. cnt << 1

I'd also appreciate a comment in the code with the explanation similar
to the changelog

>  	for_each_memblock_type(idx, type, rgn) {
>  		phys_addr_t rbase = rgn->base;
>  		phys_addr_t rend = rbase + rgn->size;
> -- 
> 2.32.0
> 

-- 
Sincerely yours,
Mike.