Re: [PATCH UPDATED] percpu: use dynamic percpu allocator as the default percpu allocator

Christoph Lameter
2009-03-31T16:19:26Z

Needs additional feedback and review by Tejun I would think. Just compile
tested so far. The removal of the rbtree also relaxes locking restrictions
for pcpu_chunk_address_search (which is not really searching anymore).

Subject: dynamic percpu allocator: Remove rbtree

The rbtree is used to determine the chunk from the virtual address. However,
we can already determine the page struct from a virtual address and there
are several unused fields in page struct used by vmalloc. Use the index
field to store a pointer to the chunk. Then there is no need anymore for
an rbtree.

Signed-off-by: Christoph Lameter <cl@linux.com>

---
mm/percpu.c | 101 +++++++++++-------------------------------------------------
1 file changed, 19 insertions(+), 82 deletions(-)

Index: linux-2.6/mm/percpu.c
===================================================================
--- linux-2.6.orig/mm/percpu.c 2009-03-31 11:02:01.000000000 -0500
+++ linux-2.6/mm/percpu.c 2009-03-31 11:04:04.000000000 -0500
@@ -23,7 +23,7 @@
* Allocation is done in offset-size areas of single unit space. Ie,
* an area of 512 bytes at 6k in c1 occupies 512 bytes at 6k of c1:u0,
* c1:u1, c1:u2 and c1:u3. Percpu access can be done by configuring
- * percpu base registers UNIT_SIZE apart.
+ * percpu base registers pcpu_unit_size apart.
*
* There are usually many small percpu allocations many of them as
* small as 4 bytes. The allocator organizes chunks into lists
@@ -38,8 +38,8 @@
* region and negative allocated. Allocation inside a chunk is done
* by scanning this map sequentially and serving the first matching
* entry. This is mostly copied from the percpu_modalloc() allocator.
- * Chunks are also linked into a rb tree to ease address to chunk
- * mapping during free.
+ * Chunks can be determined from the address using the index field
+ * in the page struct. The index field contains a pointer to the chunk.
*
* To use this allocator, arch code should do the followings.
*
@@ -61,7 +61,6 @@
#include <linux/mutex.h>
#include <linux/percpu.h>
#include <linux/pfn.h>
-#include <linux/rbtree.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/vmalloc.h>
@@ -88,7 +87,6 @@

struct pcpu_chunk {
struct list_head list; /* linked to pcpu_slot lists */
- struct rb_node rb_node; /* key is chunk->vm->addr */
int free_size; /* free bytes in the chunk */
int contig_hint; /* max contiguous size hint */
struct vm_struct *vm; /* mapped vmalloc region */
@@ -121,7 +119,7 @@ static int pcpu_reserved_chunk_limit;
* There are two locks - pcpu_alloc_mutex and pcpu_lock. The former
* protects allocation/reclaim paths, chunks and chunk->page arrays.
* The latter is a spinlock and protects the index data structures -
- * chunk slots, rbtree, chunks and area maps in chunks.
+ * chunk slots, chunks and area maps in chunks.
*
* During allocation, pcpu_alloc_mutex is kept locked all the time and
* pcpu_lock is grabbed and released as necessary. All actual memory
@@ -140,7 +138,6 @@ static DEFINE_MUTEX(pcpu_alloc_mutex); /
static DEFINE_SPINLOCK(pcpu_lock); /* protects index data structures */

static struct list_head *pcpu_slot; /* chunk list slots */
-static struct rb_root pcpu_addr_root = RB_ROOT; /* chunks by address */

/* reclaim work to release fully free chunks, scheduled from free path */
static void pcpu_reclaim(struct work_struct *work);
@@ -257,49 +254,27 @@ static void pcpu_chunk_relocate(struct p
}
}

-static struct rb_node **pcpu_chunk_rb_search(void *addr,
- struct rb_node **parentp)
+/* Set the pointer to a chunk in a page struct */
+static inline void set_chunk(struct page *page, struct pcpu_chunk *pcpu)
{
- struct rb_node **p = &pcpu_addr_root.rb_node;
- struct rb_node *parent = NULL;
- struct pcpu_chunk *chunk;
-
- while (*p) {
- parent = *p;
- chunk = rb_entry(parent, struct pcpu_chunk, rb_node);
-
- if (addr < chunk->vm->addr)
- p = &(*p)->rb_left;
- else if (addr > chunk->vm->addr)
- p = &(*p)->rb_right;
- else
- break;
- }
+ page->index = (unsigned long)pcpu;
+}

- if (parentp)
- *parentp = parent;
- return p;
+/* Obtain pointer to a chunk from a page struct */
+static inline struct pcpu_chunk*get_chunk(struct page *page)
+{
+ return (struct pcpu_chunk *)page->index;
}

/**
- * pcpu_chunk_addr_search - search for chunk containing specified address
- * @addr: address to search for
- *
- * Look for chunk which might contain @addr. More specifically, it
- * searchs for the chunk with the highest start address which isn't
- * beyond @addr.
- *
- * CONTEXT:
- * pcpu_lock.
+ * pcpu_chunk_addr_search - determine chunk containing specified address
+ * @addr: address for which the chunk needs to be determined.
*
* RETURNS:
* The address of the found chunk.
*/
static struct pcpu_chunk *pcpu_chunk_addr_search(void *addr)
{
- struct rb_node *n, *parent;
- struct pcpu_chunk *chunk;
-
/* is it in the reserved chunk? */
if (pcpu_reserved_chunk) {
void *start = pcpu_reserved_chunk->vm->addr;
@@ -308,42 +283,7 @@ static struct pcpu_chunk *pcpu_chunk_add
return pcpu_reserved_chunk;
}

- /* nah... search the regular ones */
- n = *pcpu_chunk_rb_search(addr, &parent);
- if (!n) {
- /* no exactly matching chunk, the parent is the closest */
- n = parent;
- BUG_ON(!n);
- }
- chunk = rb_entry(n, struct pcpu_chunk, rb_node);
-
- if (addr < chunk->vm->addr) {
- /* the parent was the next one, look for the previous one */
- n = rb_prev(n);
- BUG_ON(!n);
- chunk = rb_entry(n, struct pcpu_chunk, rb_node);
- }
-
- return chunk;
-}
-
-/**
- * pcpu_chunk_addr_insert - insert chunk into address rb tree
- * @new: chunk to insert
- *
- * Insert @new into address rb tree.
- *
- * CONTEXT:
- * pcpu_lock.
- */
-static void pcpu_chunk_addr_insert(struct pcpu_chunk *new)
-{
- struct rb_node **p, *parent;
-
- p = pcpu_chunk_rb_search(new->vm->addr, &parent);
- BUG_ON(*p);
- rb_link_node(&new->rb_node, parent, p);
- rb_insert_color(&new->rb_node, &pcpu_addr_root);
+ return get_chunk(vmalloc_to_page(addr));
}

/**
@@ -755,6 +695,7 @@ static int pcpu_populate_chunk(struct pc
alloc_mask, 0);
if (!*pagep)
goto err;
+ set_chunk(*pagep, chunk);
}
}

@@ -879,7 +820,6 @@ restart:

spin_lock_irq(&pcpu_lock);
pcpu_chunk_relocate(chunk, -1);
- pcpu_chunk_addr_insert(chunk);
goto restart;

area_found:
@@ -968,7 +908,6 @@ static void pcpu_reclaim(struct work_str
if (chunk == list_first_entry(head, struct pcpu_chunk, list))
continue;

- rb_erase(&chunk->rb_node, &pcpu_addr_root);
list_move(&chunk->list, &todo);
}

@@ -1202,6 +1141,7 @@ size_t __init pcpu_setup_first_chunk(pcp
if (!page)
break;
*pcpu_chunk_pagep(schunk, cpu, i) = page;
+ set_chunk(page, schunk);
}

BUG_ON(i < PFN_UP(static_size));
@@ -1226,13 +1166,10 @@ size_t __init pcpu_setup_first_chunk(pcp
}

/* link the first chunk in */
- if (!dchunk) {
+ if (!dchunk)
pcpu_chunk_relocate(schunk, -1);
- pcpu_chunk_addr_insert(schunk);
- } else {
+ else
pcpu_chunk_relocate(dchunk, -1);
- pcpu_chunk_addr_insert(dchunk);
- }

/* we're done */
pcpu_base_addr = (void *)pcpu_chunk_addr(schunk, 0, 0);