summaryrefslogtreecommitdiffstats
diff options
context:
space:
mode:
authorMatthew Wilcox <mawilcox@microsoft.com>2016-12-17 08:18:17 -0500
committerMatthew Wilcox <mawilcox@microsoft.com>2017-02-13 21:44:02 -0500
commitd37cacc5adace7f3e0824e1f559192ad7299d029 (patch)
tree9932ecc9ba3010ecc53bdbf5cd299eb0842106ec
parent7ad3d4d85c7af9632055a6ac0aa15b6b6a321c6b (diff)
downloadlinux-d37cacc5adace7f3e0824e1f559192ad7299d029.tar.gz
linux-d37cacc5adace7f3e0824e1f559192ad7299d029.tar.xz
ida: Use exceptional entries for small IDAs
We can use the root entry as a bitmap and save allocating a 128 byte bitmap for an IDA that contains only a few entries (30 on a 32-bit machine, 62 on a 64-bit machine). This costs about 300 bytes of kernel text on x86-64, so as long as 3 IDAs fall into this category, this is a net win for memory consumption. Thanks to Rasmus Villemoes for his work documenting the problem and collecting statistics on IDAs. Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
-rw-r--r--lib/idr.c87
-rw-r--r--lib/radix-tree.c8
-rw-r--r--tools/testing/radix-tree/idr-test.c93
3 files changed, 181 insertions, 7 deletions
diff --git a/lib/idr.c b/lib/idr.c
index 2abd7769c430..7d25e240bc5a 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -194,6 +194,39 @@ EXPORT_SYMBOL(idr_replace);
* limitation, it should be quite straightforward to raise the maximum.
*/
+/*
+ * Developer's notes:
+ *
+ * The IDA uses the functionality provided by the IDR & radix tree to store
+ * bitmaps in each entry. The IDR_FREE tag means there is at least one bit
+ * free, unlike the IDR where it means at least one entry is free.
+ *
+ * I considered telling the radix tree that each slot is an order-10 node
+ * and storing the bit numbers in the radix tree, but the radix tree can't
+ * allow a single multiorder entry at index 0, which would significantly
+ * increase memory consumption for the IDA. So instead we divide the index
+ * by the number of bits in the leaf bitmap before doing a radix tree lookup.
+ *
+ * As an optimisation, if there are only a few low bits set in any given
+ * leaf, instead of allocating a 128-byte bitmap, we use the 'exceptional
+ * entry' functionality of the radix tree to store BITS_PER_LONG - 2 bits
+ * directly in the entry. By being really tricksy, we could store
+ * BITS_PER_LONG - 1 bits, but there're diminishing returns after optimising
+ * for 0-3 allocated IDs.
+ *
+ * We allow the radix tree 'exceptional' count to get out of date. Nothing
+ * in the IDA nor the radix tree code checks it. If it becomes important
+ * to maintain an accurate exceptional count, switch the rcu_assign_pointer()
+ * calls to radix_tree_iter_replace() which will correct the exceptional
+ * count.
+ *
+ * The IDA always requires a lock to alloc/free. If we add a 'test_bit'
+ * equivalent, it will still need locking. Going to RCU lookup would require
+ * using RCU to free bitmaps, and that's not trivial without embedding an
+ * RCU head in the bitmap, which adds a 2-pointer overhead to each 128-byte
+ * bitmap, which is excessive.
+ */
+
#define IDA_MAX (0x80000000U / IDA_BITMAP_BITS)
/**
@@ -221,11 +254,12 @@ int ida_get_new_above(struct ida *ida, int start, int *id)
struct radix_tree_iter iter;
struct ida_bitmap *bitmap;
unsigned long index;
- unsigned bit;
+ unsigned bit, ebit;
int new;
index = start / IDA_BITMAP_BITS;
bit = start % IDA_BITMAP_BITS;
+ ebit = bit + RADIX_TREE_EXCEPTIONAL_SHIFT;
slot = radix_tree_iter_init(&iter, index);
for (;;) {
@@ -240,10 +274,29 @@ int ida_get_new_above(struct ida *ida, int start, int *id)
return PTR_ERR(slot);
}
}
- if (iter.index > index)
+ if (iter.index > index) {
bit = 0;
+ ebit = RADIX_TREE_EXCEPTIONAL_SHIFT;
+ }
new = iter.index * IDA_BITMAP_BITS;
bitmap = rcu_dereference_raw(*slot);
+ if (radix_tree_exception(bitmap)) {
+ unsigned long tmp = (unsigned long)bitmap;
+ ebit = find_next_zero_bit(&tmp, BITS_PER_LONG, ebit);
+ if (ebit < BITS_PER_LONG) {
+ tmp |= 1UL << ebit;
+ rcu_assign_pointer(*slot, (void *)tmp);
+ *id = new + ebit - RADIX_TREE_EXCEPTIONAL_SHIFT;
+ return 0;
+ }
+ bitmap = this_cpu_xchg(ida_bitmap, NULL);
+ if (!bitmap)
+ return -EAGAIN;
+ memset(bitmap, 0, sizeof(*bitmap));
+ bitmap->bitmap[0] = tmp >> RADIX_TREE_EXCEPTIONAL_SHIFT;
+ rcu_assign_pointer(*slot, bitmap);
+ }
+
if (bitmap) {
bit = find_next_zero_bit(bitmap->bitmap,
IDA_BITMAP_BITS, bit);
@@ -261,6 +314,14 @@ int ida_get_new_above(struct ida *ida, int start, int *id)
new += bit;
if (new < 0)
return -ENOSPC;
+ if (ebit < BITS_PER_LONG) {
+ bitmap = (void *)((1UL << ebit) |
+ RADIX_TREE_EXCEPTIONAL_ENTRY);
+ radix_tree_iter_replace(root, &iter, slot,
+ bitmap);
+ *id = new;
+ return 0;
+ }
bitmap = this_cpu_xchg(ida_bitmap, NULL);
if (!bitmap)
return -EAGAIN;
@@ -287,6 +348,7 @@ void ida_remove(struct ida *ida, int id)
unsigned long index = id / IDA_BITMAP_BITS;
unsigned offset = id % IDA_BITMAP_BITS;
struct ida_bitmap *bitmap;
+ unsigned long *btmp;
struct radix_tree_iter iter;
void **slot;
@@ -295,12 +357,24 @@ void ida_remove(struct ida *ida, int id)
goto err;
bitmap = rcu_dereference_raw(*slot);
- if (!test_bit(offset, bitmap->bitmap))
+ if (radix_tree_exception(bitmap)) {
+ btmp = (unsigned long *)slot;
+ offset += RADIX_TREE_EXCEPTIONAL_SHIFT;
+ if (offset >= BITS_PER_LONG)
+ goto err;
+ } else {
+ btmp = bitmap->bitmap;
+ }
+ if (!test_bit(offset, btmp))
goto err;
- __clear_bit(offset, bitmap->bitmap);
+ __clear_bit(offset, btmp);
radix_tree_iter_tag_set(&ida->ida_rt, &iter, IDR_FREE);
- if (bitmap_empty(bitmap->bitmap, IDA_BITMAP_BITS)) {
+ if (radix_tree_exception(bitmap)) {
+ if (rcu_dereference_raw(*slot) ==
+ (void *)RADIX_TREE_EXCEPTIONAL_ENTRY)
+ radix_tree_iter_delete(&ida->ida_rt, &iter, slot);
+ } else if (bitmap_empty(btmp, IDA_BITMAP_BITS)) {
kfree(bitmap);
radix_tree_iter_delete(&ida->ida_rt, &iter, slot);
}
@@ -326,7 +400,8 @@ void ida_destroy(struct ida *ida)
radix_tree_for_each_slot(slot, &ida->ida_rt, &iter, 0) {
struct ida_bitmap *bitmap = rcu_dereference_raw(*slot);
- kfree(bitmap);
+ if (!radix_tree_exception(bitmap))
+ kfree(bitmap);
radix_tree_iter_delete(&ida->ida_rt, &iter, slot);
}
}
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 7b9f8515033e..14130ab197c0 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -338,6 +338,14 @@ static void dump_ida_node(void *entry, unsigned long index)
for (i = 0; i < RADIX_TREE_MAP_SIZE; i++)
dump_ida_node(node->slots[i],
index | (i << node->shift));
+ } else if (radix_tree_exceptional_entry(entry)) {
+ pr_debug("ida excp: %p offset %d indices %lu-%lu data %lx\n",
+ entry, (int)(index & RADIX_TREE_MAP_MASK),
+ index * IDA_BITMAP_BITS,
+ index * IDA_BITMAP_BITS + BITS_PER_LONG -
+ RADIX_TREE_EXCEPTIONAL_SHIFT,
+ (unsigned long)entry >>
+ RADIX_TREE_EXCEPTIONAL_SHIFT);
} else {
struct ida_bitmap *bitmap = entry;
diff --git a/tools/testing/radix-tree/idr-test.c b/tools/testing/radix-tree/idr-test.c
index 4dffad9284e0..59081122c63d 100644
--- a/tools/testing/radix-tree/idr-test.c
+++ b/tools/testing/radix-tree/idr-test.c
@@ -11,6 +11,7 @@
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
* more details.
*/
+#include <linux/bitmap.h>
#include <linux/idr.h>
#include <linux/slab.h>
#include <linux/kernel.h>
@@ -214,7 +215,7 @@ void ida_check_nomem(void)
DEFINE_IDA(ida);
int id, err;
- err = ida_get_new(&ida, &id);
+ err = ida_get_new_above(&ida, 256, &id);
assert(err == -EAGAIN);
err = ida_get_new_above(&ida, 1UL << 30, &id);
assert(err == -EAGAIN);
@@ -247,6 +248,66 @@ void ida_check_leaf(void)
}
/*
+ * Check handling of conversions between exceptional entries and full bitmaps.
+ */
+void ida_check_conv(void)
+{
+ DEFINE_IDA(ida);
+ int id;
+ unsigned long i;
+
+ for (i = 0; i < IDA_BITMAP_BITS * 2; i += IDA_BITMAP_BITS) {
+ assert(ida_pre_get(&ida, GFP_KERNEL));
+ assert(!ida_get_new_above(&ida, i + 1, &id));
+ assert(id == i + 1);
+ assert(!ida_get_new_above(&ida, i + BITS_PER_LONG, &id));
+ assert(id == i + BITS_PER_LONG);
+ ida_remove(&ida, i + 1);
+ ida_remove(&ida, i + BITS_PER_LONG);
+ assert(ida_is_empty(&ida));
+ }
+
+ assert(ida_pre_get(&ida, GFP_KERNEL));
+
+ for (i = 0; i < IDA_BITMAP_BITS * 2; i++) {
+ assert(ida_pre_get(&ida, GFP_KERNEL));
+ assert(!ida_get_new(&ida, &id));
+ assert(id == i);
+ }
+
+ for (i = IDA_BITMAP_BITS * 2; i > 0; i--) {
+ ida_remove(&ida, i - 1);
+ }
+ assert(ida_is_empty(&ida));
+
+ for (i = 0; i < IDA_BITMAP_BITS + BITS_PER_LONG - 4; i++) {
+ assert(ida_pre_get(&ida, GFP_KERNEL));
+ assert(!ida_get_new(&ida, &id));
+ assert(id == i);
+ }
+
+ for (i = IDA_BITMAP_BITS + BITS_PER_LONG - 4; i > 0; i--) {
+ ida_remove(&ida, i - 1);
+ }
+ assert(ida_is_empty(&ida));
+
+ radix_tree_cpu_dead(1);
+ for (i = 0; i < 1000000; i++) {
+ int err = ida_get_new(&ida, &id);
+ if (err == -EAGAIN) {
+ assert((i % IDA_BITMAP_BITS) == (BITS_PER_LONG - 2));
+ assert(ida_pre_get(&ida, GFP_KERNEL));
+ err = ida_get_new(&ida, &id);
+ } else {
+ assert((i % IDA_BITMAP_BITS) != (BITS_PER_LONG - 2));
+ }
+ assert(!err);
+ assert(id == i);
+ }
+ ida_destroy(&ida);
+}
+
+/*
* Check allocations up to and slightly above the maximum allowed (2^31-1) ID.
* Allocating up to 2^31-1 should succeed, and then allocating the next one
* should fail.
@@ -273,6 +334,34 @@ void ida_check_max(void)
}
}
+void ida_check_random(void)
+{
+ DEFINE_IDA(ida);
+ DECLARE_BITMAP(bitmap, 2048);
+ int id;
+ unsigned int i;
+ time_t s = time(NULL);
+
+ repeat:
+ memset(bitmap, 0, sizeof(bitmap));
+ for (i = 0; i < 100000; i++) {
+ int i = rand();
+ int bit = i & 2047;
+ if (test_bit(bit, bitmap)) {
+ __clear_bit(bit, bitmap);
+ ida_remove(&ida, bit);
+ } else {
+ __set_bit(bit, bitmap);
+ ida_pre_get(&ida, GFP_KERNEL);
+ assert(!ida_get_new_above(&ida, bit, &id));
+ assert(id == bit);
+ }
+ }
+ ida_destroy(&ida);
+ if (time(NULL) < s + 10)
+ goto repeat;
+}
+
void ida_checks(void)
{
DEFINE_IDA(ida);
@@ -337,6 +426,8 @@ void ida_checks(void)
ida_check_leaf();
ida_check_max();
+ ida_check_conv();
+ ida_check_random();
radix_tree_cpu_dead(1);
}