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Subject[PATCH 06/12] percpu: set PCPU_BITMAP_BLOCK_SIZE to PAGE_SIZE
Date
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 piggybacks 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 <dennis@kernel.org>
---
include/linux/percpu.h | 12 ++---
mm/percpu-km.c | 2 +-
mm/percpu.c | 111 +++++++++++++++++------------------------
3 files changed, 49 insertions(+), 76 deletions(-)

diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 70b7123f38c7..9909dc0e273a 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -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 >> \
diff --git a/mm/percpu-km.c b/mm/percpu-km.c
index 0f643dc2dc65..c10bf7466596 100644
--- a/mm/percpu-km.c
+++ b/mm/percpu-km.c
@@ -70,7 +70,7 @@ static struct pcpu_chunk *pcpu_create_chunk(gfp_t gfp)
chunk->base_addr = page_address(pages) - pcpu_group_offsets[0];

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();
diff --git a/mm/percpu.c b/mm/percpu.c
index 3d7deece9556..967c9cc3a928 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -527,37 +527,21 @@ 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 pcpu_block_md 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;
}

/*
@@ -611,36 +595,19 @@ 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;
}

/**
@@ -712,6 +679,7 @@ 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 */
@@ -736,6 +704,9 @@ 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),
@@ -763,6 +734,9 @@ 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.
@@ -787,6 +761,7 @@ 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->contig_hint = 0;
block->left_free = 0;
@@ -794,6 +769,9 @@ 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);
+
/*
* The only time a full chunk scan is required is if the chunk
* contig hint is broken. Otherwise, it means a smaller space
@@ -826,6 +804,7 @@ 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 */
@@ -879,14 +858,19 @@ 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;
@@ -896,15 +880,16 @@ 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),
@@ -1164,9 +1149,7 @@ 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;
@@ -1261,7 +1244,6 @@ 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
@@ -1271,7 +1253,7 @@ 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;

@@ -1281,10 +1263,7 @@ 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);
}

/**
@@ -1306,9 +1285,9 @@ 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, -1 * nr);
}

/*
@@ -1523,7 +1502,7 @@ 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);
}

@@ -1722,7 +1701,7 @@ 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;
--
2.17.1
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