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percpu: add block level scan_hint

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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>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
This commit is contained in:
Dennis Zhou 2019-02-25 13:41:45 -08:00
parent b239f7daf5
commit 382b88e961
2 changed files with 103 additions and 7 deletions
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9
mm/percpu-internal.h
View File

@ -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 */

101
mm/percpu.c
View File

@ -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;
*bit_off = pcpu_block_off_to_off(i, start);
return;
}
/* reset to satisfy the second predicate above */
@ -628,12 +658,57 @@ 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;
}
}
}
@ -652,7 +727,7 @@ 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 */
@ -709,6 +784,12 @@ 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,
@ -745,6 +826,9 @@ 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);
@ -759,6 +843,7 @@ 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;
@ -869,6 +954,7 @@ 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;
@ -1080,6 +1166,7 @@ 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;