RE: [PATCH 12/12] percpu: use chunk scan_hint to skip some scanning

[Peng Fan]

> -----Original Message-----
> From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On
> Behalf Of Dennis Zhou
> Sent: 2019年2月28日 10:19
> To: Dennis Zhou <dennis@xxxxxxxxxx>; Tejun Heo <tj@xxxxxxxxxx>; Christoph
> Lameter <cl@xxxxxxxxx>
> Cc: Vlad Buslov <vladbu@xxxxxxxxxxxx>; kernel-team@xxxxxx;
> linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: [PATCH 12/12] percpu: use chunk scan_hint to skip some scanning
> 
> Just like blocks, chunks now maintain a scan_hint. This can be used to skip
> some scanning by promoting the scan_hint to be the contig_hint.
> The chunk's scan_hint is primarily updated on the backside and relies on full
> scanning when a block becomes free or the free region spans across blocks.
> 
> Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx>
> ---
>  mm/percpu.c | 36 +++++++++++++++++++++++++++---------
>  1 file changed, 27 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 197479f2c489..40d49d7fb286 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -711,20 +711,31 @@ static void pcpu_block_update_scan(struct
> pcpu_chunk *chunk, int bit_off,
>  /**
>   * pcpu_chunk_refresh_hint - updates metadata about a chunk
>   * @chunk: chunk of interest
> + * @full_scan: if we should scan from the beginning
>   *
>   * Iterates over the metadata blocks to find the largest contig area.
> - * It also counts the populated pages and uses the delta to update the
> - * global count.
> + * A full scan can be avoided on the allocation path as this is
> + triggered
> + * if we broke the contig_hint.  In doing so, the scan_hint will be
> + before
> + * the contig_hint or after if the scan_hint == contig_hint.  This
> + cannot
> + * be prevented on freeing as we want to find the largest area possibly
> + * spanning blocks.
>   */
> -static void pcpu_chunk_refresh_hint(struct pcpu_chunk *chunk)
> +static void pcpu_chunk_refresh_hint(struct pcpu_chunk *chunk, bool
> +full_scan)
>  {
>  	struct pcpu_block_md *chunk_md = &chunk->chunk_md;
>  	int bit_off, bits;
> 
> -	/* clear metadata */
> -	chunk_md->contig_hint = 0;
> +	/* promote scan_hint to contig_hint */
> +	if (!full_scan && chunk_md->scan_hint) {
> +		bit_off = chunk_md->scan_hint_start + chunk_md->scan_hint;
> +		chunk_md->contig_hint_start = chunk_md->scan_hint_start;
> +		chunk_md->contig_hint = chunk_md->scan_hint;
> +		chunk_md->scan_hint = 0;
> +	} else {
> +		bit_off = chunk_md->first_free;
> +		chunk_md->contig_hint = 0;
> +	}
> 
> -	bit_off = chunk_md->first_free;
>  	bits = 0;
>  	pcpu_for_each_md_free_region(chunk, bit_off, bits) {
>  		pcpu_block_update(chunk_md, bit_off, bit_off + bits); @@ -884,6
> +895,13 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk
> *chunk, int bit_off,
>  	if (nr_empty_pages)
>  		pcpu_update_empty_pages(chunk, -1 * nr_empty_pages);
> 
> +	if (pcpu_region_overlap(chunk_md->scan_hint_start,
> +				chunk_md->scan_hint_start +
> +				chunk_md->scan_hint,
> +				bit_off,
> +				bit_off + bits))
> +		chunk_md->scan_hint = 0;
> +
>  	/*
>  	 * The only time a full chunk scan is required is if the chunk
>  	 * contig hint is broken.  Otherwise, it means a smaller space @@
> -894,7 +912,7 @@ static void pcpu_block_update_hint_alloc(struct
> pcpu_chunk *chunk, int bit_off,
>  				chunk_md->contig_hint,
>  				bit_off,
>  				bit_off + bits))
> -		pcpu_chunk_refresh_hint(chunk);
> +		pcpu_chunk_refresh_hint(chunk, false);
>  }
> 
>  /**
> @@ -1005,7 +1023,7 @@ static void pcpu_block_update_hint_free(struct
> pcpu_chunk *chunk, int bit_off,
>  	 * the else condition below.
>  	 */
>  	if (((end - start) >= PCPU_BITMAP_BLOCK_BITS) || s_index != e_index)
> -		pcpu_chunk_refresh_hint(chunk);
> +		pcpu_chunk_refresh_hint(chunk, true);
>  	else
>  		pcpu_block_update(&chunk->chunk_md,
>  				  pcpu_block_off_to_off(s_index, start), @@ -1078,7
> +1096,7 @@ static int pcpu_find_block_fit(struct pcpu_chunk *chunk, int
> alloc_bits,
>  	if (bit_off + alloc_bits > chunk_md->contig_hint)
>  		return -1;
> 
> -	bit_off = chunk_md->first_free;
> +	bit_off = pcpu_next_hint(chunk_md, alloc_bits);
>  	bits = 0;
>  	pcpu_for_each_fit_region(chunk, alloc_bits, align, bit_off, bits) {
>  		if (!pop_only || pcpu_is_populated(chunk, bit_off, bits,

Reviewed-by: Peng Fan <peng.fan@xxxxxxx>

> --
> 2.17.1