linux-kernel - Re: [PATCH v3 5/6] percpu: factor out pcpu_check_chunk

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Date:   Fri, 16 Apr 2021 21:15:25 +0000
From:   Dennis Zhou <dennis@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 5/6] percpu: factor out pcpu_check_chunk_hint()

Hello,

On Wed, Apr 07, 2021 at 08:57:35PM -0700, Roman Gushchin wrote:
> Factor out the pcpu_check_chunk_hint() helper, which will be useful
> in the future. The new function checks if the allocation can likely
> fit the given chunk.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> ---
>  mm/percpu.c | 30 +++++++++++++++++++++---------
>  1 file changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index e20119668c42..357fd6994278 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -306,6 +306,26 @@ static unsigned long pcpu_block_off_to_off(int index, int off)
>  	return index * PCPU_BITMAP_BLOCK_BITS + off;
>  }
>  
> +/**
> + * pcpu_check_chunk_hint - check that allocation can fit a chunk
> + * @chunk_md: chunk's block

nit for consistency: @block: block of interest

> + * @bits: size of request in allocation units
> + * @align: alignment of area (max PAGE_SIZE)
> + *
> + * Check to see if the allocation can fit in the chunk's contig hint.
> + * This is an optimization to prevent scanning by assuming if it
> + * cannot fit in the global hint, there is memory pressure and creating
> + * a new chunk would happen soon.
> + */

It occurred to me, That I converged block_md and chunk_md to be the same
object as 1 is just a degenerative case of the other. Can we rename this
to be pcpu_check_block_hint() and have it take in pcpu_block_md?

> +static bool pcpu_check_chunk_hint(struct pcpu_block_md *chunk_md, int bits,
> +				  size_t align)
> +{
> +	int bit_off = ALIGN(chunk_md->contig_hint_start, align) -
> +		chunk_md->contig_hint_start;
> +
> +	return bit_off + bits <= chunk_md->contig_hint;
> +}
> +
>  /*
>   * pcpu_next_hint - determine which hint to use
>   * @block: block of interest
> @@ -1065,15 +1085,7 @@ static int pcpu_find_block_fit(struct pcpu_chunk *chunk, int alloc_bits,
>  	struct pcpu_block_md *chunk_md = &chunk->chunk_md;
>  	int bit_off, bits, next_off;
>  
> -	/*
> -	 * Check to see if the allocation can fit in the chunk's contig hint.
> -	 * This is an optimization to prevent scanning by assuming if it
> -	 * cannot fit in the global hint, there is memory pressure and creating
> -	 * a new chunk would happen soon.
> -	 */
> -	bit_off = ALIGN(chunk_md->contig_hint_start, align) -
> -		  chunk_md->contig_hint_start;
> -	if (bit_off + alloc_bits > chunk_md->contig_hint)
> +	if (!pcpu_check_chunk_hint(chunk_md, alloc_bits, align))
>  		return -1;
>  
>  	bit_off = pcpu_next_hint(chunk_md, alloc_bits);
> -- 
> 2.30.2
> 

Thanks,
Dennis