linux-kernel - Re: [PATCH] percpu: simplify the logic of pcpu_alloc_first

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Date: Tue, 23 Apr 2024 22:12:00 -0700
From: Dennis Zhou <dennis@...nel.org>
To: Yuntao Wang <ytcoode@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] percpu: simplify the logic of pcpu_alloc_first_chunk()

Hi Yuntao,

On Tue, Apr 23, 2024 at 09:55:25PM +0800, Yuntao Wang wrote:
> In the logic for hiding the end of the bitmap, there are several places
> where the same value 'region_bits - offset_bits' is calculated over and
> over again using different methods. Eliminate these redundant calculations
> to improve code readability.
> 
> Signed-off-by: Yuntao Wang <ytcoode@...il.com>
> ---
>  mm/percpu.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 4e11fc1e6def..2a051f00f68d 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -1421,15 +1421,13 @@ static struct pcpu_chunk * __init pcpu_alloc_first_chunk(unsigned long tmp_addr,
>  	if (chunk->end_offset) {
>  		/* hide the end of the bitmap */
>  		offset_bits = chunk->end_offset / PCPU_MIN_ALLOC_SIZE;
> -		bitmap_set(chunk->alloc_map,
> -			   pcpu_chunk_map_bits(chunk) - offset_bits,
> -			   offset_bits);
> -		set_bit((start_offset + map_size) / PCPU_MIN_ALLOC_SIZE,
> -			chunk->bound_map);
> +		start_offset = region_bits - offset_bits;

Generally I think this makes sense, but I'm less inclined to mix the
start_offset variable name in here.

A helper function might make this cleaner, consolidating the
start_offset and end_offset logic.

static void pcpu_chunk_hide_region(chunk, bit_off, bits);

Thanks,
Dennis

> +
> +		bitmap_set(chunk->alloc_map, start_offset, offset_bits);
> +		set_bit(start_offset, chunk->bound_map);
>  		set_bit(region_bits, chunk->bound_map);
>  
> -		pcpu_block_update_hint_alloc(chunk, pcpu_chunk_map_bits(chunk)
> -					     - offset_bits, offset_bits);
> +		pcpu_block_update_hint_alloc(chunk, start_offset, offset_bits);
>  	}
>  
>  	return chunk;
> -- 
> 2.44.0
>