From: Aaron Lu <ziqian.lzq@xxxxxxxxxx>
swap_extent is used to map swap page offset to backing device's block
offset. For a continuous block range, one swap_extent is used and all
these swap_extents are managed in a linked list.
These swap_extents are used by map_swap_entry() during swap's read and
write path. To find out the backing device's block offset for a page
offset, the swap_extent list will be traversed linearly, with
curr_swap_extent being used as a cache to speed up the search.
This works well as long as swap_extents are not huge or when the number
of processes that access swap device are few, but when the swap device
has many extents and there are a number of processes accessing the swap
device concurrently, it can be a problem. On one of our servers, the
disk's remaining size is tight:
$df -h
Filesystem Size Used Avail Use% Mounted on
... ...
/dev/nvme0n1p1 1.8T 1.3T 504G 72% /home/t4
When creating a 80G swapfile there, there are as many as 84656 swap
extents. The end result is, kernel spends abou 30% time in map_swap_entry()
and swap throughput is only 70MB/s. As a comparison, when I used smaller
sized swapfile, like 4G whose swap_extent dropped to 2000, swap throughput
is back to 400-500MB/s and map_swap_entry() is about 3%.
One downside of using rbtree for swap_extent is, 'struct rbtree' takes
24 bytes while 'struct list_head' takes 16 bytes, that's 8 bytes more
for each swap_extent. For a swapfile that has 80k swap_extents, that
means 625KiB more memory consumed.
Test:
Since it's not possible to reboot that server, I can not test this patch
diretly there. Instead, I tested it on another server with NVMe disk.
I created a 20G swapfile on an NVMe backed XFS fs. By default, the
filesystem is quite clean and the created swapfile has only 2 extents.
Testing vanilla and this patch shows no obvious performance difference
when swapfile is not fragmented.
To see the patch's effects, I used some tweaks to manually fragment the
swapfile by breaking the extent at 1M boundary. This made the swapfile
have 20K extents.
nr_task=4
kernel swapout(KB/s) map_swap_entry(perf) swapin(KB/s) map_swap_entry(perf)
vanilla 165191 90.77% 171798 90.21%
patched 858993 +420% 2.16% 715827 +317% 0.77%
nr_task=8
kernel swapout(KB/s) map_swap_entry(perf) swapin(KB/s) map_swap_entry(perf)
vanilla 306783 92.19% 318145 87.76%
patched 954437 +211% 2.35% 1073741 +237% 1.57%
swapout: the throughput of swap out, in KB/s, higher is better
1st map_swap_entry: cpu cycles percent sampled by perf
swapin: the throughput of swap in, in KB/s, higher is better.
2nd map_swap_entry: cpu cycles percent sampled by perf
nr_task=1 doesn't show any difference, this is due to the
curr_swap_extent can be effectively used to cache the correct swap
extent for single task workload.
Signed-off-by: Aaron Lu <ziqian.lzq@xxxxxxxxxx>
---
include/linux/swap.h | 5 +-
mm/page_io.c | 2 +-
mm/swapfile.c | 137 +++++++++++++++++++++++--------------------
3 files changed, 78 insertions(+), 66 deletions(-)
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 4bfb5c4ac108..a229c6273781 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -148,7 +148,7 @@ struct zone;
* We always assume that blocks are of size PAGE_SIZE.
*/
struct swap_extent {
- struct list_head list;
+ struct rb_node rb_node;
pgoff_t start_page;
pgoff_t nr_pages;
sector_t start_block;
@@ -247,8 +247,7 @@ struct swap_info_struct {
unsigned int cluster_next; /* likely index for next allocation */
unsigned int cluster_nr; /* countdown to next cluster search */
struct percpu_cluster __percpu *percpu_cluster; /* per cpu's swap location */
- struct swap_extent *curr_swap_extent;
- struct swap_extent first_swap_extent;
+ struct rb_root swap_extent_root;/* root of the swap extent rbtree */
struct block_device *bdev; /* swap device or bdev of swap file */
struct file *swap_file; /* seldom referenced */
unsigned int old_block_size; /* seldom referenced */
diff --git a/mm/page_io.c b/mm/page_io.c
index 2e8019d0e048..24ad2d43c11d 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -164,7 +164,7 @@ int generic_swapfile_activate(struct swap_info_struct *sis,
blocks_per_page = PAGE_SIZE >> blkbits;
/*
- * Map all the blocks into the extent list. This code doesn't try
+ * Map all the blocks into the extent tree. This code doesn't try
* to be very smart.
*/
probe_block = 0;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 596ac98051c5..82b96750ad45 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -152,6 +152,18 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
return ret;
}
+static inline struct swap_extent *first_se(struct swap_info_struct *sis)
+{
+ struct rb_node *rb = rb_first(&sis->swap_extent_root);
+ return rb_entry(rb, struct swap_extent, rb_node);
+}
+
+static inline struct swap_extent *next_se(struct swap_extent *se)
+{
+ struct rb_node *rb = rb_next(&se->rb_node);
+ return rb ? rb_entry(rb, struct swap_extent, rb_node) : NULL;
+}
+
/*
* swapon tell device that all the old swap contents can be discarded,
* to allow the swap device to optimize its wear-levelling.
@@ -164,7 +176,7 @@ static int discard_swap(struct swap_info_struct *si)
int err = 0;
/* Do not discard the swap header page! */
- se = &si->first_swap_extent;
+ se = first_se(si);
start_block = (se->start_block + 1) << (PAGE_SHIFT - 9);
nr_blocks = ((sector_t)se->nr_pages - 1) << (PAGE_SHIFT - 9);
if (nr_blocks) {
@@ -175,7 +187,7 @@ static int discard_swap(struct swap_info_struct *si)
cond_resched();
}
- list_for_each_entry(se, &si->first_swap_extent.list, list) {
+ for (se = next_se(se); se; se = next_se(se)) {
start_block = se->start_block << (PAGE_SHIFT - 9);
nr_blocks = (sector_t)se->nr_pages << (PAGE_SHIFT - 9);
@@ -189,6 +201,26 @@ static int discard_swap(struct swap_info_struct *si)
return err; /* That will often be -EOPNOTSUPP */
}
+static struct swap_extent *
+offset_to_swap_extent(struct swap_info_struct *sis, unsigned long offset)
+{
+ struct swap_extent *se;
+ struct rb_node *rb;
+
+ rb = sis->swap_extent_root.rb_node;
+ while (rb) {
+ se = rb_entry(rb, struct swap_extent, rb_node);
+ if (offset < se->start_page)
+ rb = rb->rb_left;
+ else if (offset >= se->start_page + se->nr_pages)
+ rb = rb->rb_right;
+ else
+ return se;
+ }
+ /* It *must* be present */
+ BUG_ON(1);
+}
+
/*
* swap allocation tell device that a cluster of swap can now be discarded,
* to allow the swap device to optimize its wear-levelling.
@@ -196,32 +228,25 @@ static int discard_swap(struct swap_info_struct *si)
static void discard_swap_cluster(struct swap_info_struct *si,
pgoff_t start_page, pgoff_t nr_pages)
{
- struct swap_extent *se = si->curr_swap_extent;
- int found_extent = 0;
+ struct swap_extent *se = offset_to_swap_extent(si, start_page);
while (nr_pages) {
- if (se->start_page <= start_page &&
- start_page < se->start_page + se->nr_pages) {
- pgoff_t offset = start_page - se->start_page;
- sector_t start_block = se->start_block + offset;
- sector_t nr_blocks = se->nr_pages - offset;
-
- if (nr_blocks > nr_pages)
- nr_blocks = nr_pages;
- start_page += nr_blocks;
- nr_pages -= nr_blocks;
-
- if (!found_extent++)
- si->curr_swap_extent = se;
-
- start_block <<= PAGE_SHIFT - 9;
- nr_blocks <<= PAGE_SHIFT - 9;
- if (blkdev_issue_discard(si->bdev, start_block,
- nr_blocks, GFP_NOIO, 0))
- break;
- }
+ pgoff_t offset = start_page - se->start_page;
+ sector_t start_block = se->start_block + offset;
+ sector_t nr_blocks = se->nr_pages - offset;
+
+ if (nr_blocks > nr_pages)
+ nr_blocks = nr_pages;
+ start_page += nr_blocks;
+ nr_pages -= nr_blocks;
+
+ start_block <<= PAGE_SHIFT - 9;
+ nr_blocks <<= PAGE_SHIFT - 9;
+ if (blkdev_issue_discard(si->bdev, start_block,
+ nr_blocks, GFP_NOIO, 0))
+ break;
- se = list_next_entry(se, list);
+ se = next_se(se);
}
}
@@ -1684,7 +1709,7 @@ int swap_type_of(dev_t device, sector_t offset, struct block_device **bdev_p)
return type;
}
if (bdev == sis->bdev) {
- struct swap_extent *se = &sis->first_swap_extent;
+ struct swap_extent *se = first_se(sis);
if (se->start_block == offset) {
if (bdev_p)
@@ -2161,7 +2186,6 @@ static void drain_mmlist(void)
static sector_t map_swap_entry(swp_entry_t entry, struct block_device **bdev)
{
struct swap_info_struct *sis;
- struct swap_extent *start_se;
struct swap_extent *se;
pgoff_t offset;
@@ -2169,18 +2193,8 @@ static sector_t map_swap_entry(swp_entry_t entry, struct block_device **bdev)
*bdev = sis->bdev;
offset = swp_offset(entry);
- start_se = sis->curr_swap_extent;
- se = start_se;
-
- for ( ; ; ) {
- if (se->start_page <= offset &&
- offset < (se->start_page + se->nr_pages)) {
- return se->start_block + (offset - se->start_page);
- }
- se = list_next_entry(se, list);
- sis->curr_swap_extent = se;
- BUG_ON(se == start_se); /* It *must* be present */
- }
+ se = offset_to_swap_extent(sis, offset);
+ return se->start_block + (offset - se->start_page);
}
/*
@@ -2198,12 +2212,11 @@ sector_t map_swap_page(struct page *page, struct block_device **bdev)
*/
static void destroy_swap_extents(struct swap_info_struct *sis)
{
- while (!list_empty(&sis->first_swap_extent.list)) {
- struct swap_extent *se;
+ while (!RB_EMPTY_ROOT(&sis->swap_extent_root)) {
+ struct rb_node *rb = sis->swap_extent_root.rb_node;
+ struct swap_extent *se = rb_entry(rb, struct swap_extent, rb_node);
- se = list_first_entry(&sis->first_swap_extent.list,
- struct swap_extent, list);
- list_del(&se->list);
+ rb_erase(rb, &sis->swap_extent_root);
kfree(se);
}
@@ -2219,7 +2232,7 @@ static void destroy_swap_extents(struct swap_info_struct *sis)
/*
* Add a block range (and the corresponding page range) into this swapdev's
- * extent list. The extent list is kept sorted in page order.
+ * extent tree.
*
* This function rather assumes that it is called in ascending page order.
*/
@@ -2227,20 +2240,21 @@ int
add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
unsigned long nr_pages, sector_t start_block)
{
+ struct rb_node **link = &sis->swap_extent_root.rb_node, *parent = NULL;
struct swap_extent *se;
struct swap_extent *new_se;
- struct list_head *lh;
-
- if (start_page == 0) {
- se = &sis->first_swap_extent;
- sis->curr_swap_extent = se;
- se->start_page = 0;
- se->nr_pages = nr_pages;
- se->start_block = start_block;
- return 1;
- } else {
- lh = sis->first_swap_extent.list.prev; /* Highest extent */
- se = list_entry(lh, struct swap_extent, list);
+
+ /*
+ * place the new node at the right most since the
+ * function is called in ascending page order.
+ */
+ while (*link) {
+ parent = *link;
+ link = &parent->rb_right;
+ }
+
+ if (parent) {
+ se = rb_entry(parent, struct swap_extent, rb_node);
BUG_ON(se->start_page + se->nr_pages != start_page);
if (se->start_block + se->nr_pages == start_block) {
/* Merge it */
@@ -2249,9 +2263,7 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
}
}
- /*
- * No merge. Insert a new extent, preserving ordering.
- */
+ /* No merge, insert a new extent. */
new_se = kmalloc(sizeof(*se), GFP_KERNEL);
if (new_se == NULL)
return -ENOMEM;
@@ -2259,7 +2271,8 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
new_se->nr_pages = nr_pages;
new_se->start_block = start_block;
- list_add_tail(&new_se->list, &sis->first_swap_extent.list);
+ rb_link_node(&new_se->rb_node, parent, link);
+ rb_insert_color(&new_se->rb_node, &sis->swap_extent_root);
return 1;
}
EXPORT_SYMBOL_GPL(add_swap_extent);
@@ -2749,7 +2762,7 @@ static struct swap_info_struct *alloc_swap_info(void)
* would be relying on p->type to remain valid.
*/
}
- INIT_LIST_HEAD(&p->first_swap_extent.list);
+ p->swap_extent_root = RB_ROOT;
plist_node_init(&p->list, 0);
for_each_node(i)
plist_node_init(&p->avail_lists[i], 0);
--
2.19.1.3.ge56e4f7
