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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 /lib/idr.c
parent7ad3d4d85c7af9632055a6ac0aa15b6b6a321c6b (diff)
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>
Diffstat (limited to 'lib/idr.c')
-rw-r--r--lib/idr.c87
1 files changed, 81 insertions, 6 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+ * We allow the radix tree 'exceptional' count to get out of date.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);
}
}