percpu: set PCPU_BITMAP_BLOCK_SIZE to PAGE_SIZE

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 <dennis@kernel.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>

diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 70b7123f38c7c9044bb262a2cecc96ab415b1f4d..9909dc0e273adb1e7bf8eac0eae95862e9b35210 100644 (file)
--- 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 b68d5df147317295514e61ccae2b9f65f5311895..3a2ff5c9192c60728068c91defc32a026e235d05 100644 (file)
--- 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);
 
        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 2c1a9a2ca13bb6f9098aef832e1e392757fdce92..0e98616501b3b4f2126ad126ebdfe507d7e590d6 100644 (file)
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -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;
 }
 
 /*
@@ -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;
 }
 
 /**
@@ -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 */
@@ -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),
@@ -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.
@@ -784,6 +757,7 @@ 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;
@@ -791,6 +765,9 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
                }
        }
 
+       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
@@ -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 */
@@ -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;
@@ -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),
@@ -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;
@@ -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
@@ -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;
 
@@ -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);
 }
 
 /**
@@ -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);
 }
 
 /*
@@ -1537,7 +1515,7 @@ area_found:
                                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);
                }
 
@@ -1736,7 +1714,7 @@ retry_pop:
                        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;