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Subject[PATCH 06/10] percpu: remove unit_size power-of-2 restriction
Date
Impact: allow unit_size to be arbitrary multiple of PAGE_SIZE

In dynamic percpu allocator, there is no reason the unit size should
be power of two. Remove the restriction.

As non-power-of-two unit size means that empty chunks fall into the
same slot index as lightly occupied chunks which is bad for reclaming.
Reserve an extra slot for empty chunks.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
mm/percpu.c | 33 +++++++++++++++++++--------------
1 files changed, 19 insertions(+), 14 deletions(-)
diff --git a/mm/percpu.c b/mm/percpu.c
index 41e7a5f..d9e6e5d 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -67,7 +67,7 @@
#include <asm/cacheflush.h>
#include <asm/tlbflush.h>

-#define PCPU_MIN_UNIT_PAGES_SHIFT 4 /* also max alloc size */
+#define PCPU_MIN_UNIT_PAGES 16 /* max alloc size in pages */
#define PCPU_SLOT_BASE_SHIFT 5 /* 1-31 shares the same slot */
#define PCPU_DFL_MAP_ALLOC 16 /* start a map with 16 ents */

@@ -83,9 +83,7 @@ struct pcpu_chunk {
struct page *page[]; /* #cpus * UNIT_PAGES */
};

-static int pcpu_unit_pages_shift;
static int pcpu_unit_pages;
-static int pcpu_unit_shift;
static int pcpu_unit_size;
static int pcpu_chunk_size;
static int pcpu_nr_slots;
@@ -117,12 +115,19 @@ static DEFINE_MUTEX(pcpu_mutex);
static struct list_head *pcpu_slot; /* chunk list slots */
static struct rb_root pcpu_addr_root = RB_ROOT; /* chunks by address */

-static int pcpu_size_to_slot(int size)
+static int __pcpu_size_to_slot(int size)
{
int highbit = fls(size); /* size is in bytes */
return max(highbit - PCPU_SLOT_BASE_SHIFT + 2, 1);
}

+static int pcpu_size_to_slot(int size)
+{
+ if (size == pcpu_unit_size)
+ return pcpu_nr_slots - 1;
+ return __pcpu_size_to_slot(size);
+}
+
static int pcpu_chunk_slot(const struct pcpu_chunk *chunk)
{
if (chunk->free_size < sizeof(int) || chunk->contig_hint < sizeof(int))
@@ -133,7 +138,7 @@ static int pcpu_chunk_slot(const struct pcpu_chunk *chunk)

static int pcpu_page_idx(unsigned int cpu, int page_idx)
{
- return (cpu << pcpu_unit_pages_shift) + page_idx;
+ return cpu * pcpu_unit_pages + page_idx;
}

static struct page **pcpu_chunk_pagep(struct pcpu_chunk *chunk,
@@ -659,7 +664,7 @@ static int pcpu_populate_chunk(struct pcpu_chunk *chunk, int off, int size)
goto err;

for_each_possible_cpu(cpu)
- memset(chunk->vm->addr + (cpu << pcpu_unit_shift) + off, 0,
+ memset(chunk->vm->addr + cpu * pcpu_unit_size + off, 0,
size);

return 0;
@@ -722,7 +727,7 @@ void *__alloc_percpu(size_t size, size_t align)
struct pcpu_chunk *chunk;
int slot, off;

- if (unlikely(!size || size > PAGE_SIZE << PCPU_MIN_UNIT_PAGES_SHIFT ||
+ if (unlikely(!size || size > PCPU_MIN_UNIT_PAGES * PAGE_SIZE ||
align > PAGE_SIZE)) {
WARN(true, "illegal size (%zu) or align (%zu) for "
"percpu allocation\n", size, align);
@@ -840,19 +845,19 @@ size_t __init pcpu_setup_static(pcpu_populate_pte_fn_t populate_pte_fn,
unsigned int cpu;
int err, i;

- pcpu_unit_pages_shift = max_t(int, PCPU_MIN_UNIT_PAGES_SHIFT,
- order_base_2(cpu_size) - PAGE_SHIFT);
+ pcpu_unit_pages = max_t(int, PCPU_MIN_UNIT_PAGES, PFN_UP(cpu_size));

pcpu_static_size = cpu_size;
- pcpu_unit_pages = 1 << pcpu_unit_pages_shift;
- pcpu_unit_shift = PAGE_SHIFT + pcpu_unit_pages_shift;
- pcpu_unit_size = 1 << pcpu_unit_shift;
+ pcpu_unit_size = pcpu_unit_pages << PAGE_SHIFT;
pcpu_chunk_size = num_possible_cpus() * pcpu_unit_size;
- pcpu_nr_slots = pcpu_size_to_slot(pcpu_unit_size) + 1;
pcpu_chunk_struct_size = sizeof(struct pcpu_chunk)
+ num_possible_cpus() * pcpu_unit_pages * sizeof(struct page *);

- /* allocate chunk slots */
+ /*
+ * Allocate chunk slots. The additional last slot is for
+ * empty chunks.
+ */
+ pcpu_nr_slots = __pcpu_size_to_slot(pcpu_unit_size) + 2;
pcpu_slot = alloc_bootmem(pcpu_nr_slots * sizeof(pcpu_slot[0]));
for (i = 0; i < pcpu_nr_slots; i++)
INIT_LIST_HEAD(&pcpu_slot[i]);
--
1.6.0.2


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