RE: [PATCH 07/12] percpu: add block level scan_hint

[Peng Fan]

Hi Dennis

> -----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 07/12] percpu: add block level scan_hint
> 
> Fragmentation can cause both blocks and chunks to have an early first_firee
> bit available, but only able to satisfy allocations much later on. This patch
> introduces a scan_hint to help mitigate some unnecessary scanning.
> 
> The scan_hint remembers the largest area prior to the contig_hint. If the
> contig_hint == scan_hint, then scan_hint_start > contig_hint_start.
> This is necessary for scan_hint discovery when refreshing a block.
> 
> Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx>
> ---
>  mm/percpu-internal.h |   9 ++++
>  mm/percpu.c          | 101
> ++++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 103 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/percpu-internal.h b/mm/percpu-internal.h index
> b1739dc06b73..ec58b244545d 100644
> --- a/mm/percpu-internal.h
> +++ b/mm/percpu-internal.h
> @@ -9,8 +9,17 @@
>   * pcpu_block_md is the metadata block struct.
>   * Each chunk's bitmap is split into a number of full blocks.
>   * All units are in terms of bits.
> + *
> + * The scan hint is the largest known contiguous area before the contig hint.
> + * It is not necessarily the actual largest contig hint though.  There
> + is an
> + * invariant that the scan_hint_start > contig_hint_start iff
> + * scan_hint == contig_hint.  This is necessary because when scanning
> + forward,
> + * we don't know if a new contig hint would be better than the current one.
>   */
>  struct pcpu_block_md {
> +	int			scan_hint;	/* scan hint for block */
> +	int			scan_hint_start; /* block relative starting
> +						    position of the scan hint */
>  	int                     contig_hint;    /* contig hint for block */
>  	int                     contig_hint_start; /* block relative starting
>  						      position of the contig hint */ diff --git
> a/mm/percpu.c b/mm/percpu.c index 967c9cc3a928..df1aacf58ac8 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -320,6 +320,34 @@ static unsigned long pcpu_block_off_to_off(int index,
> int off)
>  	return index * PCPU_BITMAP_BLOCK_BITS + off;  }
> 
> +/*
> + * pcpu_next_hint - determine which hint to use
> + * @block: block of interest
> + * @alloc_bits: size of allocation
> + *
> + * This determines if we should scan based on the scan_hint or first_free.
> + * In general, we want to scan from first_free to fulfill allocations
> +by
> + * first fit.  However, if we know a scan_hint at position
> +scan_hint_start
> + * cannot fulfill an allocation, we can begin scanning from there
> +knowing
> + * the contig_hint will be our fallback.
> + */
> +static int pcpu_next_hint(struct pcpu_block_md *block, int alloc_bits)
> +{
> +	/*
> +	 * The three conditions below determine if we can skip past the
> +	 * scan_hint.  First, does the scan hint exist.  Second, is the
> +	 * contig_hint after the scan_hint (possibly not true iff
> +	 * contig_hint == scan_hint).  Third, is the allocation request
> +	 * larger than the scan_hint.
> +	 */
> +	if (block->scan_hint &&
> +	    block->contig_hint_start > block->scan_hint_start &&
> +	    alloc_bits > block->scan_hint)
> +		return block->scan_hint_start + block->scan_hint;
> +
> +	return block->first_free;
> +}
> +
>  /**
>   * pcpu_next_md_free_region - finds the next hint free area
>   * @chunk: chunk of interest
> @@ -415,9 +443,11 @@ static void pcpu_next_fit_region(struct pcpu_chunk
> *chunk, int alloc_bits,
>  		if (block->contig_hint &&
>  		    block->contig_hint_start >= block_off &&
>  		    block->contig_hint >= *bits + alloc_bits) {
> +			int start = pcpu_next_hint(block, alloc_bits);
> +
>  			*bits += alloc_bits + block->contig_hint_start -
> -				 block->first_free;
> -			*bit_off = pcpu_block_off_to_off(i, block->first_free);
> +				 start;

This might not relevant to this patch.
Not sure it is intended or not.
For `alloc_bits + block->contig_hink_start - [block->first_free or start]`
If the reason is to let pcpu_is_populated return a proper next_off when pcpu_is_populated
fail, it makes sense. If not, why not just use *bits += alloc_bits.

> +			*bit_off = pcpu_block_off_to_off(i, start);
>  			return;
>  		}
>  		/* reset to satisfy the second predicate above */ @@ -632,12
> +662,57 @@ static void pcpu_block_update(struct pcpu_block_md *block, int
> start, int end)
>  		block->right_free = contig;
> 
>  	if (contig > block->contig_hint) {
> +		/* promote the old contig_hint to be the new scan_hint */
> +		if (start > block->contig_hint_start) {
> +			if (block->contig_hint > block->scan_hint) {
> +				block->scan_hint_start =
> +					block->contig_hint_start;
> +				block->scan_hint = block->contig_hint;
> +			} else if (start < block->scan_hint_start) {
> +				/*
> +				 * The old contig_hint == scan_hint.  But, the
> +				 * new contig is larger so hold the invariant
> +				 * scan_hint_start < contig_hint_start.
> +				 */
> +				block->scan_hint = 0;
> +			}
> +		} else {
> +			block->scan_hint = 0;
> +		}
>  		block->contig_hint_start = start;
>  		block->contig_hint = contig;
> -	} else if (block->contig_hint_start && contig == block->contig_hint &&
> -		   (!start || __ffs(start) > __ffs(block->contig_hint_start))) {
> -		/* use the start with the best alignment */
> -		block->contig_hint_start = start;
> +	} else if (contig == block->contig_hint) {
> +		if (block->contig_hint_start &&
> +		    (!start ||
> +		     __ffs(start) > __ffs(block->contig_hint_start))) {
> +			/* start has a better alignment so use it */
> +			block->contig_hint_start = start;
> +			if (start < block->scan_hint_start &&
> +			    block->contig_hint > block->scan_hint)
> +				block->scan_hint = 0;
> +		} else if (start > block->scan_hint_start ||
> +			   block->contig_hint > block->scan_hint) {
> +			/*
> +			 * Knowing contig == contig_hint, update the scan_hint
> +			 * if it is farther than or larger than the current
> +			 * scan_hint.
> +			 */
> +			block->scan_hint_start = start;
> +			block->scan_hint = contig;
> +		}
> +	} else {
> +		/*
> +		 * The region is smaller than the contig_hint.  So only update
> +		 * the scan_hint if it is larger than or equal and farther than
> +		 * the current scan_hint.
> +		 */
> +		if ((start < block->contig_hint_start &&
> +		     (contig > block->scan_hint ||
> +		      (contig == block->scan_hint &&
> +		       start > block->scan_hint_start)))) {
> +			block->scan_hint_start = start;
> +			block->scan_hint = contig;
> +		}
>  	}
>  }
> 
> @@ -656,7 +731,7 @@ static void pcpu_block_refresh_hint(struct
> pcpu_chunk *chunk, int index)
>  	int rs, re;	/* region start, region end */
> 
>  	/* clear hints */
> -	block->contig_hint = 0;
> +	block->contig_hint = block->scan_hint = 0;
>  	block->left_free = block->right_free = 0;
> 
>  	/* iterate over free areas and update the contig hints */ @@ -713,6
> +788,12 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk
> *chunk, int bit_off,
>  					PCPU_BITMAP_BLOCK_BITS,
>  					s_off + bits);
> 
> +	if (pcpu_region_overlap(s_block->scan_hint_start,
> +				s_block->scan_hint_start + s_block->scan_hint,
> +				s_off,
> +				s_off + bits))
> +		s_block->scan_hint = 0;
> +
>  	if (pcpu_region_overlap(s_block->contig_hint_start,
>  				s_block->contig_hint_start +
>  				s_block->contig_hint,
> @@ -749,6 +830,9 @@ static void pcpu_block_update_hint_alloc(struct
> pcpu_chunk *chunk, int bit_off,
>  			/* reset the block */
>  			e_block++;
>  		} else {
> +			if (e_off > e_block->scan_hint_start)
> +				e_block->scan_hint = 0;
> +
>  			if (e_off > e_block->contig_hint_start) {
>  				/* contig hint is broken - scan to fix it */
>  				pcpu_block_refresh_hint(chunk, e_index); @@ -763,6
> +847,7 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk
> *chunk, int bit_off,
>  		/* update in-between md_blocks */
>  		nr_empty_pages += (e_index - s_index - 1);
>  		for (block = s_block + 1; block < e_block; block++) {
> +			block->scan_hint = 0;
>  			block->contig_hint = 0;
>  			block->left_free = 0;
>  			block->right_free = 0;
> @@ -873,6 +958,7 @@ static void pcpu_block_update_hint_free(struct
> pcpu_chunk *chunk, int bit_off,
>  		nr_empty_pages += (e_index - s_index - 1);
>  		for (block = s_block + 1; block < e_block; block++) {
>  			block->first_free = 0;
> +			block->scan_hint = 0;
>  			block->contig_hint_start = 0;
>  			block->contig_hint = PCPU_BITMAP_BLOCK_BITS;
>  			block->left_free = PCPU_BITMAP_BLOCK_BITS; @@ -1084,6
> +1170,7 @@ static void pcpu_init_md_blocks(struct pcpu_chunk *chunk)
>  	for (md_block = chunk->md_blocks;
>  	     md_block != chunk->md_blocks + pcpu_chunk_nr_blocks(chunk);
>  	     md_block++) {
> +		md_block->scan_hint = 0;
>  		md_block->contig_hint = PCPU_BITMAP_BLOCK_BITS;
>  		md_block->left_free = PCPU_BITMAP_BLOCK_BITS;
>  		md_block->right_free = PCPU_BITMAP_BLOCK_BITS;

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

> --
> 2.17.1