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percpu: set PCPU_BITMAP_BLOCK_SIZE to PAGE_SIZE
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Previously, block size was flexible based on the constraint that the
GCD(PCPU_BITMAP_BLOCK_SIZE, PAGE_SIZE) > 1. However, this carried the
overhead that keeping a floating number of populated free pages required
scanning over the free regions of a chunk.

Setting the block size to be fixed at PAGE_SIZE lets us know when an
empty page becomes used as we will break a full contig_hint of a block.
This means we no longer have to scan the whole chunk upon breaking a
contig_hint which empty page management piggybacked off. A later patch
takes advantage of this to optimize the allocation path by only scanning
forward using the scan_hint introduced later too.

Signed-off-by: Dennis Zhou <[email protected]>
Reviewed-by: Peng Fan <[email protected]>
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@dennisszhou
dennisszhou committed Mar 13, 2019
1 parent 8744d85 commit b239f7d
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Showing 3 changed files with 48 additions and 76 deletions.
12 changes: 3 additions & 9 deletions include/linux/percpu.h
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Original file line number Diff line number Diff line change
Expand Up @@ -26,16 +26,10 @@
#define PCPU_MIN_ALLOC_SHIFT 2
#define PCPU_MIN_ALLOC_SIZE (1 << PCPU_MIN_ALLOC_SHIFT)

/* number of bits per page, used to trigger a scan if blocks are > PAGE_SIZE */
#define PCPU_BITS_PER_PAGE (PAGE_SIZE >> PCPU_MIN_ALLOC_SHIFT)

/*
* This determines the size of each metadata block. There are several subtle
* constraints around this constant. The reserved region must be a multiple of
* PCPU_BITMAP_BLOCK_SIZE. Additionally, PCPU_BITMAP_BLOCK_SIZE must be a
* multiple of PAGE_SIZE or PAGE_SIZE must be a multiple of
* PCPU_BITMAP_BLOCK_SIZE to align with the populated page map. The unit_size
* also has to be a multiple of PCPU_BITMAP_BLOCK_SIZE to ensure full blocks.
* The PCPU_BITMAP_BLOCK_SIZE must be the same size as PAGE_SIZE as the
* updating of hints is used to manage the nr_empty_pop_pages in both
* the chunk and globally.
*/
#define PCPU_BITMAP_BLOCK_SIZE PAGE_SIZE
#define PCPU_BITMAP_BLOCK_BITS (PCPU_BITMAP_BLOCK_SIZE >> \
Expand Down
2 changes: 1 addition & 1 deletion mm/percpu-km.c
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Original file line number Diff line number Diff line change
Expand Up @@ -70,7 +70,7 @@ static struct pcpu_chunk *pcpu_create_chunk(gfp_t gfp)
chunk->base_addr = page_address(pages);

spin_lock_irqsave(&pcpu_lock, flags);
pcpu_chunk_populated(chunk, 0, nr_pages, false);
pcpu_chunk_populated(chunk, 0, nr_pages);
spin_unlock_irqrestore(&pcpu_lock, flags);

pcpu_stats_chunk_alloc();
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110 changes: 44 additions & 66 deletions mm/percpu.c
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Original file line number Diff line number Diff line change
Expand Up @@ -527,37 +527,20 @@ static void pcpu_chunk_relocate(struct pcpu_chunk *chunk, int oslot)
__pcpu_chunk_move(chunk, nslot, oslot < nslot);
}

/**
* pcpu_cnt_pop_pages- counts populated backing pages in range
/*
* pcpu_update_empty_pages - update empty page counters
* @chunk: chunk of interest
* @bit_off: start offset
* @bits: size of area to check
* @nr: nr of empty pages
*
* Calculates the number of populated pages in the region
* [page_start, page_end). This keeps track of how many empty populated
* pages are available and decide if async work should be scheduled.
*
* RETURNS:
* The nr of populated pages.
* This is used to keep track of the empty pages now based on the premise
* a md_block covers a page. The hint update functions recognize if a block
* is made full or broken to calculate deltas for keeping track of free pages.
*/
static inline int pcpu_cnt_pop_pages(struct pcpu_chunk *chunk, int bit_off,
int bits)
static inline void pcpu_update_empty_pages(struct pcpu_chunk *chunk, int nr)
{
int page_start = PFN_UP(bit_off * PCPU_MIN_ALLOC_SIZE);
int page_end = PFN_DOWN((bit_off + bits) * PCPU_MIN_ALLOC_SIZE);

if (page_start >= page_end)
return 0;

/*
* bitmap_weight counts the number of bits set in a bitmap up to
* the specified number of bits. This is counting the populated
* pages up to page_end and then subtracting the populated pages
* up to page_start to count the populated pages in
* [page_start, page_end).
*/
return bitmap_weight(chunk->populated, page_end) -
bitmap_weight(chunk->populated, page_start);
chunk->nr_empty_pop_pages += nr;
if (chunk != pcpu_reserved_chunk)
pcpu_nr_empty_pop_pages += nr;
}

/*
Expand Down Expand Up @@ -608,36 +591,19 @@ static void pcpu_chunk_update(struct pcpu_chunk *chunk, int bit_off, int bits)
* Updates:
* chunk->contig_bits
* chunk->contig_bits_start
* nr_empty_pop_pages (chunk and global)
*/
static void pcpu_chunk_refresh_hint(struct pcpu_chunk *chunk)
{
int bit_off, bits, nr_empty_pop_pages;
int bit_off, bits;

/* clear metadata */
chunk->contig_bits = 0;

bit_off = chunk->first_bit;
bits = nr_empty_pop_pages = 0;
bits = 0;
pcpu_for_each_md_free_region(chunk, bit_off, bits) {
pcpu_chunk_update(chunk, bit_off, bits);

nr_empty_pop_pages += pcpu_cnt_pop_pages(chunk, bit_off, bits);
}

/*
* Keep track of nr_empty_pop_pages.
*
* The chunk maintains the previous number of free pages it held,
* so the delta is used to update the global counter. The reserved
* chunk is not part of the free page count as they are populated
* at init and are special to serving reserved allocations.
*/
if (chunk != pcpu_reserved_chunk)
pcpu_nr_empty_pop_pages +=
(nr_empty_pop_pages - chunk->nr_empty_pop_pages);

chunk->nr_empty_pop_pages = nr_empty_pop_pages;
}

/**
Expand Down Expand Up @@ -709,6 +675,7 @@ static void pcpu_block_refresh_hint(struct pcpu_chunk *chunk, int index)
static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
int bits)
{
int nr_empty_pages = 0;
struct pcpu_block_md *s_block, *e_block, *block;
int s_index, e_index; /* block indexes of the freed allocation */
int s_off, e_off; /* block offsets of the freed allocation */
Expand All @@ -733,6 +700,9 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
* If the allocation breaks the contig_hint, a scan is required to
* restore this hint.
*/
if (s_block->contig_hint == PCPU_BITMAP_BLOCK_BITS)
nr_empty_pages++;

if (s_off == s_block->first_free)
s_block->first_free = find_next_zero_bit(
pcpu_index_alloc_map(chunk, s_index),
Expand Down Expand Up @@ -760,6 +730,9 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
* Update e_block.
*/
if (s_index != e_index) {
if (e_block->contig_hint == PCPU_BITMAP_BLOCK_BITS)
nr_empty_pages++;

/*
* When the allocation is across blocks, the end is along
* the left part of the e_block.
Expand All @@ -784,13 +757,17 @@ 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->contig_hint = 0;
block->left_free = 0;
block->right_free = 0;
}
}

if (nr_empty_pages)
pcpu_update_empty_pages(chunk, -nr_empty_pages);

/*
* The only time a full chunk scan is required is if the chunk
* contig hint is broken. Otherwise, it means a smaller space
Expand Down Expand Up @@ -823,6 +800,7 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
static void pcpu_block_update_hint_free(struct pcpu_chunk *chunk, int bit_off,
int bits)
{
int nr_empty_pages = 0;
struct pcpu_block_md *s_block, *e_block, *block;
int s_index, e_index; /* block indexes of the freed allocation */
int s_off, e_off; /* block offsets of the freed allocation */
Expand Down Expand Up @@ -876,14 +854,19 @@ static void pcpu_block_update_hint_free(struct pcpu_chunk *chunk, int bit_off,

/* update s_block */
e_off = (s_index == e_index) ? end : PCPU_BITMAP_BLOCK_BITS;
if (!start && e_off == PCPU_BITMAP_BLOCK_BITS)
nr_empty_pages++;
pcpu_block_update(s_block, start, e_off);

/* freeing in the same block */
if (s_index != e_index) {
/* update e_block */
if (end == PCPU_BITMAP_BLOCK_BITS)
nr_empty_pages++;
pcpu_block_update(e_block, 0, end);

/* reset md_blocks in the middle */
nr_empty_pages += (e_index - s_index - 1);
for (block = s_block + 1; block < e_block; block++) {
block->first_free = 0;
block->contig_hint_start = 0;
Expand All @@ -893,15 +876,16 @@ static void pcpu_block_update_hint_free(struct pcpu_chunk *chunk, int bit_off,
}
}

if (nr_empty_pages)
pcpu_update_empty_pages(chunk, nr_empty_pages);

/*
* Refresh chunk metadata when the free makes a page free, a block
* free, or spans across blocks. The contig hint may be off by up to
* a page, but if the hint is contained in a block, it will be accurate
* with the else condition below.
* Refresh chunk metadata when the free makes a block free or spans
* across blocks. The contig_hint may be off by up to a page, but if
* the contig_hint is contained in a block, it will be accurate with
* the else condition below.
*/
if ((ALIGN_DOWN(end, min(PCPU_BITS_PER_PAGE, PCPU_BITMAP_BLOCK_BITS)) >
ALIGN(start, min(PCPU_BITS_PER_PAGE, PCPU_BITMAP_BLOCK_BITS))) ||
s_index != e_index)
if (((end - start) >= PCPU_BITMAP_BLOCK_BITS) || s_index != e_index)
pcpu_chunk_refresh_hint(chunk);
else
pcpu_chunk_update(chunk, pcpu_block_off_to_off(s_index, start),
Expand Down Expand Up @@ -1178,9 +1162,7 @@ static struct pcpu_chunk * __init pcpu_alloc_first_chunk(unsigned long tmp_addr,
chunk->immutable = true;
bitmap_fill(chunk->populated, chunk->nr_pages);
chunk->nr_populated = chunk->nr_pages;
chunk->nr_empty_pop_pages =
pcpu_cnt_pop_pages(chunk, start_offset / PCPU_MIN_ALLOC_SIZE,
map_size / PCPU_MIN_ALLOC_SIZE);
chunk->nr_empty_pop_pages = chunk->nr_pages;

chunk->contig_bits = map_size / PCPU_MIN_ALLOC_SIZE;
chunk->free_bytes = map_size;
Expand Down Expand Up @@ -1275,7 +1257,6 @@ static void pcpu_free_chunk(struct pcpu_chunk *chunk)
* @chunk: pcpu_chunk which got populated
* @page_start: the start page
* @page_end: the end page
* @for_alloc: if this is to populate for allocation
*
* Pages in [@page_start,@page_end) have been populated to @chunk. Update
* the bookkeeping information accordingly. Must be called after each
Expand All @@ -1285,7 +1266,7 @@ static void pcpu_free_chunk(struct pcpu_chunk *chunk)
* is to serve an allocation in that area.
*/
static void pcpu_chunk_populated(struct pcpu_chunk *chunk, int page_start,
int page_end, bool for_alloc)
int page_end)
{
int nr = page_end - page_start;

Expand All @@ -1295,10 +1276,7 @@ static void pcpu_chunk_populated(struct pcpu_chunk *chunk, int page_start,
chunk->nr_populated += nr;
pcpu_nr_populated += nr;

if (!for_alloc) {
chunk->nr_empty_pop_pages += nr;
pcpu_nr_empty_pop_pages += nr;
}
pcpu_update_empty_pages(chunk, nr);
}

/**
Expand All @@ -1320,9 +1298,9 @@ static void pcpu_chunk_depopulated(struct pcpu_chunk *chunk,

bitmap_clear(chunk->populated, page_start, nr);
chunk->nr_populated -= nr;
chunk->nr_empty_pop_pages -= nr;
pcpu_nr_empty_pop_pages -= nr;
pcpu_nr_populated -= nr;

pcpu_update_empty_pages(chunk, -nr);
}

/*
Expand Down Expand Up @@ -1537,7 +1515,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
err = "failed to populate";
goto fail_unlock;
}
pcpu_chunk_populated(chunk, rs, re, true);
pcpu_chunk_populated(chunk, rs, re);
spin_unlock_irqrestore(&pcpu_lock, flags);
}

Expand Down Expand Up @@ -1736,7 +1714,7 @@ static void pcpu_balance_workfn(struct work_struct *work)
if (!ret) {
nr_to_pop -= nr;
spin_lock_irq(&pcpu_lock);
pcpu_chunk_populated(chunk, rs, rs + nr, false);
pcpu_chunk_populated(chunk, rs, rs + nr);
spin_unlock_irq(&pcpu_lock);
} else {
nr_to_pop = 0;
Expand Down

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