[PATCH v2] iommu/iova: Optimize alloc_iova with rbtree_augmented

From: Peng Zhang
Date: Wed Aug 24 2022 - 05:53:38 EST

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The current algorithm of alloc_iova is to scan all iovas until it finds
a gap that satisfies the condition to allocate. This can be very slow in
some scenarios. We can optimize alloc_iova() from time complexity O(n)
to O(log(n)).

We can make a test like this:
Write a module and initialize iova_domain with 4k granule.
Then using a user-mode program to call the module to allocate iova of
size 1 2^20 times within the allocation limit of 2^20. This is single
threaded and the low 4g space is full after 2^20 allocations.

Finally loop the following three steps:
1. Randomly releases an iova.

2. Allocate an iova of size 1 within the allocation limit of 2^20.

3. Allocate an iova of size 1 within the allocation limit of 2^20.
This will fail and take a very long time, because max32_alloc_size
is reset whenever an iova is released.

The data below is the result of repeating the three steps 1024 times in
a physical machine with a CPU clocked at 2.30GHz

Before improvement:
Tracing 1 functions for "alloc_iova"...
nsecs : count distbution
256 -> 511 : 1594 | |
512 -> 1023 : 1030686 |**************************************|
1024 -> 2047 : 14661 | |
2048 -> 4095 : 1730 | |
4096 -> 8191 : 634 | |
8192 -> 16383 : 20 | |
16384 -> 32767 : 2 | |
32768 -> 65535 : 2 | |
65536 -> 131071 : 3 | |
131072 -> 262143 : 6 | |
262144 -> 524287 : 8 | |
524288 -> 1048575 : 19 | |
1048576 -> 2097151 : 35 | |
2097152 -> 4194303 : 55 | |
4194304 -> 8388607 : 117 | |
8388608 -> 16777215 : 165 | |
16777216 -> 33554431 : 1112 | |
avg = 33867 nsecs, total: 35589643563 nsecs, count: 1050849

With improvement:
Tracing 1 functions for "alloc_iova"...
nsecs : count distribution
512 -> 1023 : 1033561 |****************************************|
1024 -> 2047 : 13631 | |
2048 -> 4095 : 2981 | |
4096 -> 8191 : 448 | |
8192 -> 16383 : 5 | |
16384 -> 32767 : 1 | |
avg = 696 nsecs, total: 732196323 nsecs, count: 1050627

Introduce the improved algorithm:

------------------------------------------------------------------------
| gap1 |iova1| gap2 |iova2| gap3 |iova3| gap4 |iova4| gap5 |anchor|
------------------------------------------------------------------------

let A = allocatable_size
let B = max_allocatable_size
____________
/ iova2 \ B = max( left_child->B,
| A | right_child->B,
\ B / A)
------------
/ \
/ \
____________ ____________
/ iova1 \ / iova4 \
| A | | A |
\ B / \ B /
------------ ------------
/ \
/ \
____________ ____________
/ iova3 \ / anchor \
| A | | A |
\ B / \ B /
------------ ------------

Define the gap of a iova is the gap between the iova and it's previous
iova. Such as the gap of iova3 is gap3.This gap can be used to allocate.

Add three variables to struct iova.
prev_iova:
point to the previous iova, sush as iova3->prev_iova point to
iova2.

allocatable_size:
allocatable_size is the max size can be allocated from a gap.
It is not the length of a gap because the allocated address
may need to be aligned.

max_allocatable_size:
max_allocatable_size is the max allocatable_size of all iova's
gap in the subtree.

max_allocatable_size = max( left_child->max_allocatable_size,
right_child->max_allocatable_size,
allocatable_size)

We can use rbtree_augmented to maintain max_allocatable_size in time
complexity O(log(n)).

In the rbtree, with the max_allocatable_size and allocatable_size,
searching the gap to allocate is fast and the time complexity is
O(log(n)).

Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
---
drivers/iommu/iova.c | 265 ++++++++++++++++++++++++++++++++-----------
include/linux/iova.h | 5 +-
2 files changed, 204 insertions(+), 66 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index db77aa675145..79625ac82560 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -43,6 +43,56 @@ static struct iova *to_iova(struct rb_node *node)
return rb_entry(node, struct iova, node);
}

+/*
+ * We can't judge whether it can be allocated only by a given interval length
+ * because the address may be aligned.
+ * This function computes the max allocatable size for a given interval.
+ * The time complexity of this function is O(log(n)).
+ */
+static unsigned long __compute_allocatable_size(unsigned long lo,
+ unsigned long hi)
+{
+ unsigned long allocatable_size = 0;
+
+ if (lo == 0)
+ return hi;
+ while (lo < hi) {
+ unsigned long delta = 1UL << __ffs64(lo);
+
+ if (hi - lo <= delta) {
+ allocatable_size = max(allocatable_size, hi - lo);
+ break;
+ }
+ allocatable_size = max(allocatable_size, delta);
+ lo += delta;
+ }
+ return allocatable_size;
+}
+
+static inline unsigned long prev_iova_high(struct iova *iova)
+{
+ return iova->prev_iova ? iova->prev_iova->pfn_hi + 1 : 0;
+}
+
+static inline unsigned long iova_compute_allocatable_size(struct iova *iova)
+{
+ return __compute_allocatable_size(prev_iova_high(iova), iova->pfn_lo);
+}
+
+static inline unsigned long iova_get_allocatable_size(struct iova *iova)
+{
+ return iova->allocatable_size;
+}
+
+RB_DECLARE_CALLBACKS_MAX(static, iova_gap_callbacks, struct iova, node,
+ unsigned long, max_allocatable_size,
+ iova_get_allocatable_size)
+
+static inline void iova_max_allocatable_size_update(struct iova *iova)
+{
+ iova_gap_callbacks_propagate(&iova->node, NULL);
+}
+
void
init_iova_domain(struct iova_domain *iovad, unsigned long granule,
unsigned long start_pfn)
@@ -63,8 +113,16 @@ init_iova_domain(struct iova_domain *iovad, unsigned long granule,
iovad->dma_32bit_pfn = 1UL << (32 - iova_shift(iovad));
iovad->max32_alloc_size = iovad->dma_32bit_pfn;
iovad->anchor.pfn_lo = iovad->anchor.pfn_hi = IOVA_ANCHOR;
+ iovad->anchor.prev_iova = NULL;
+ iovad->anchor.allocatable_size =
+ __compute_allocatable_size(0, IOVA_ANCHOR);
+ iovad->anchor.max_allocatable_size = iovad->anchor.allocatable_size;
+
rb_link_node(&iovad->anchor.node, NULL, &iovad->rbroot.rb_node);
rb_insert_color(&iovad->anchor.node, &iovad->rbroot);
+
+ if (start_pfn)
+ reserve_iova(iovad, 0, start_pfn - 1);
}
EXPORT_SYMBOL_GPL(init_iova_domain);

@@ -87,7 +145,8 @@ __cached_rbnode_insert_update(struct iova_domain *iovad, struct iova *new)
}

static void
-__cached_rbnode_delete_update(struct iova_domain *iovad, struct iova *free)
+__cached_rbnode_delete_update(struct iova_domain *iovad, struct iova *free,
+ struct rb_node *next)
{
struct iova *cached_iova;

@@ -95,51 +154,32 @@ __cached_rbnode_delete_update(struct iova_domain *iovad, struct iova *free)
if (free == cached_iova ||
(free->pfn_hi < iovad->dma_32bit_pfn &&
free->pfn_lo >= cached_iova->pfn_lo))
- iovad->cached32_node = rb_next(&free->node);
+ iovad->cached32_node = next;

if (free->pfn_lo < iovad->dma_32bit_pfn)
iovad->max32_alloc_size = iovad->dma_32bit_pfn;

cached_iova = to_iova(iovad->cached_node);
if (free->pfn_lo >= cached_iova->pfn_lo)
- iovad->cached_node = rb_next(&free->node);
+ iovad->cached_node = next;
}

-static struct rb_node *iova_find_limit(struct iova_domain *iovad, unsigned long limit_pfn)
+static struct rb_node *iova_find_limit(struct iova_domain *iovad,
+ unsigned long limit_pfn)
{
- struct rb_node *node, *next;
- /*
- * Ideally what we'd like to judge here is whether limit_pfn is close
- * enough to the highest-allocated IOVA that starting the allocation
- * walk from the anchor node will be quicker than this initial work to
- * find an exact starting point (especially if that ends up being the
- * anchor node anyway). This is an incredibly crude approximation which
- * only really helps the most likely case, but is at least trivially easy.
- */
- if (limit_pfn > iovad->dma_32bit_pfn)
- return &iovad->anchor.node;
-
- node = iovad->rbroot.rb_node;
- while (to_iova(node)->pfn_hi < limit_pfn)
- node = node->rb_right;
-
-search_left:
- while (node->rb_left && to_iova(node->rb_left)->pfn_lo >= limit_pfn)
- node = node->rb_left;
-
- if (!node->rb_left)
- return node;
-
- next = node->rb_left;
- while (next->rb_right) {
- next = next->rb_right;
- if (to_iova(next)->pfn_lo >= limit_pfn) {
- node = next;
- goto search_left;
- }
- }
+ struct rb_node *curr = iovad->rbroot.rb_node;

- return node;
+ while (curr) {
+ struct iova *iova = to_iova(curr);
+
+ if (limit_pfn - 1 > iova->pfn_hi)
+ curr = curr->rb_right;
+ else if (limit_pfn <= prev_iova_high(iova))
+ curr = curr->rb_left;
+ else
+ break;
+ }
+ return curr;
}

/* Insert the iova into domain rbtree by holding writer lock */
@@ -148,6 +188,7 @@ iova_insert_rbtree(struct rb_root *root, struct iova *iova,
struct rb_node *start)
{
struct rb_node **new, *parent = NULL;
+ struct iova *next_iova;

new = (start) ? &start : &(root->rb_node);
/* Figure out where to put new node */
@@ -166,61 +207,143 @@ iova_insert_rbtree(struct rb_root *root, struct iova *iova,
}
}
/* Add new node and rebalance tree. */
+
rb_link_node(&iova->node, parent, new);
- rb_insert_color(&iova->node, root);
+
+ next_iova = to_iova(rb_next(&iova->node));
+ iova->prev_iova = next_iova->prev_iova;
+ next_iova->prev_iova = iova;
+
+ iova->allocatable_size = iova_compute_allocatable_size(iova);
+ next_iova->allocatable_size = iova_compute_allocatable_size(next_iova);
+
+ /*
+ * Do't swap the following two lines, because next_iova is the ancestor
+ * of iova and updating iova first is faster.
+ */
+ iova_max_allocatable_size_update(iova);
+ iova_max_allocatable_size_update(next_iova);
+
+ rb_insert_augmented(&iova->node, root, &iova_gap_callbacks);
+}
+
+static inline bool check_interval(unsigned long lo, unsigned long hi,
+ unsigned long limit_pfn, unsigned long size,
+ unsigned long align_mask)
+{
+ hi = min(hi, limit_pfn);
+ if (lo >= hi)
+ return false;
+ if (hi >= size && ((hi - size) & align_mask) >= lo)
+ return true;
+ return false;
}

static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
unsigned long size, unsigned long limit_pfn,
struct iova *new, bool size_aligned)
{
- struct rb_node *curr, *prev;
- struct iova *curr_iova;
unsigned long flags;
- unsigned long new_pfn, retry_pfn;
+ struct rb_node *curr;
+ struct rb_node *parent;
+ struct iova *curr_iova;
unsigned long align_mask = ~0UL;
- unsigned long high_pfn = limit_pfn, low_pfn = iovad->start_pfn;
+ bool ignore = false;

if (size_aligned)
align_mask <<= fls_long(size - 1);

- /* Walk the tree backwards */
spin_lock_irqsave(&iovad->iova_rbtree_lock, flags);
+
if (limit_pfn <= iovad->dma_32bit_pfn &&
size >= iovad->max32_alloc_size)
goto iova32_full;

curr = __get_cached_rbnode(iovad, limit_pfn);
curr_iova = to_iova(curr);
- retry_pfn = curr_iova->pfn_hi + 1;

-retry:
- do {
- high_pfn = min(high_pfn, curr_iova->pfn_lo);
- new_pfn = (high_pfn - size) & align_mask;
- prev = curr;
- curr = rb_prev(curr);
- curr_iova = to_iova(curr);
- } while (curr && new_pfn <= curr_iova->pfn_hi && new_pfn >= low_pfn);
-
- if (high_pfn < size || new_pfn < low_pfn) {
- if (low_pfn == iovad->start_pfn && retry_pfn < limit_pfn) {
- high_pfn = limit_pfn;
- low_pfn = retry_pfn;
- curr = iova_find_limit(iovad, limit_pfn);
- curr_iova = to_iova(curr);
- goto retry;
+ if (limit_pfn >= curr_iova->pfn_lo &&
+ curr_iova->allocatable_size >= size)
+ goto found;
+
+ /* If limit_pfn > dma_32bit_pfn, this could be faster. */
+ if (limit_pfn > iovad->dma_32bit_pfn) {
+ curr_iova = to_iova(&iovad->anchor.node);
+
+ while (curr_iova) {
+ if (check_interval(prev_iova_high(curr_iova),
+ curr_iova->pfn_lo, limit_pfn,
+ size, align_mask))
+ goto found;
+ curr_iova = curr_iova->prev_iova;
}
iovad->max32_alloc_size = size;
goto iova32_full;
}

+ curr = iova_find_limit(iovad, limit_pfn);
+ curr_iova = to_iova(curr);
+
+ if (check_interval(prev_iova_high(curr_iova),
+ curr_iova->pfn_lo, limit_pfn,
+ size, align_mask))
+ goto found;
+
+ while (true) {
+ /* Check left subtree */
+ if (!ignore && curr->rb_left) {
+ curr_iova = to_iova(curr->rb_left);
+ if (curr_iova->max_allocatable_size >= size)
+ goto check_subtree;
+ }
+
+ parent = rb_parent(curr);
+ if (parent == NULL)
+ break;
+ /*
+ * If current node is the left child of it's parent,
+ * the parent node and the parent's right sub_tree should not
+ * to be checked because they exceed the limit_pfn.
+ */
+ ignore = parent->rb_left == curr;
+ curr = parent;
+
+ /* Check current node. */
+ if (!ignore) {
+ curr_iova = to_iova(curr);
+ if (curr_iova->allocatable_size >= size)
+ goto found;
+ }
+ }
+ if (limit_pfn >= iovad->dma_32bit_pfn)
+ iovad->max32_alloc_size = size;
+ goto iova32_full;
+
+check_subtree:
+ while (true) {
+ if (curr_iova->allocatable_size >= size)
+ goto found;
+
+ curr = &curr_iova->node;
+ if (curr->rb_right &&
+ to_iova(curr->rb_right)->max_allocatable_size >= size) {
+ curr_iova = to_iova(curr->rb_right);
+ continue;
+ }
+ WARN_ON(curr->rb_left == NULL);
+ curr_iova = to_iova(curr->rb_left);
+ }
+
+found:
/* pfn_lo will point to size aligned address if size_aligned is set */
- new->pfn_lo = new_pfn;
+ new->pfn_lo = (min(curr_iova->pfn_lo, limit_pfn) - size) & align_mask;
new->pfn_hi = new->pfn_lo + size - 1;

- /* If we have 'prev', it's a valid place to start the insertion. */
- iova_insert_rbtree(&iovad->rbroot, new, prev);
+ /*
+ * If we have 'prev' or 'next',
+ * it's a valid place to start the insertion.
+ */
+ iova_insert_rbtree(&iovad->rbroot, new, &curr_iova->node);
__cached_rbnode_insert_update(iovad, new);

spin_unlock_irqrestore(&iovad->iova_rbtree_lock, flags);
@@ -352,9 +475,18 @@ private_find_iova(struct iova_domain *iovad, unsigned long pfn)

static void remove_iova(struct iova_domain *iovad, struct iova *iova)
{
+ struct rb_node *next;
+ struct iova *next_iova;
assert_spin_locked(&iovad->iova_rbtree_lock);
- __cached_rbnode_delete_update(iovad, iova);
- rb_erase(&iova->node, &iovad->rbroot);
+
+ next = rb_next(&iova->node);
+ __cached_rbnode_delete_update(iovad, iova, next);
+
+ next_iova = to_iova(next);
+ next_iova->prev_iova = iova->prev_iova;
+ next_iova->allocatable_size = iova_compute_allocatable_size(next_iova);
+ iova_max_allocatable_size_update(next_iova);
+ rb_erase_augmented(&iova->node, &iovad->rbroot, &iova_gap_callbacks);
}

/**
@@ -554,8 +686,11 @@ static void
__adjust_overlap_range(struct iova *iova,
unsigned long *pfn_lo, unsigned long *pfn_hi)
{
- if (*pfn_lo < iova->pfn_lo)
+ if (*pfn_lo < iova->pfn_lo) {
iova->pfn_lo = *pfn_lo;
+ iova->allocatable_size = iova_compute_allocatable_size(iova);
+ iova_max_allocatable_size_update(iova);
+ }
if (*pfn_hi > iova->pfn_hi)
*pfn_lo = iova->pfn_hi + 1;
}
diff --git a/include/linux/iova.h b/include/linux/iova.h
index 320a70e40233..feb8121f104d 100644
--- a/include/linux/iova.h
+++ b/include/linux/iova.h
@@ -11,7 +11,7 @@

#include <linux/types.h>
#include <linux/kernel.h>
-#include <linux/rbtree.h>
+#include <linux/rbtree_augmented.h>
#include <linux/dma-mapping.h>

/* iova structure */
@@ -19,6 +19,9 @@ struct iova {
struct rb_node node;
unsigned long pfn_hi; /* Highest allocated pfn */
unsigned long pfn_lo; /* Lowest allocated pfn */
+ struct iova *prev_iova;
+ unsigned long allocatable_size;
+ unsigned long max_allocatable_size;
};

--
2.20.1

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