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SubjectRE: [PATCH 07/12] percpu: add block level scan_hint
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Hi Dennis

> -----Original Message-----
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On
> Behalf Of Dennis Zhou
> Sent: 2019年2月28日 10:19
> To: Dennis Zhou <dennis@kernel.org>; Tejun Heo <tj@kernel.org>; Christoph
> Lameter <cl@linux.com>
> Cc: Vlad Buslov <vladbu@mellanox.com>; kernel-team@fb.com;
> linux-mm@kvack.org; linux-kernel@vger.kernel.org
> 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@kernel.org>
> ---
> 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@nxp.com>

> --
> 2.17.1

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