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authorShaohua Li <shli@kernel.org>2013-09-11 23:20:32 +0200
committerLinus Torvalds <torvalds@linux-foundation.org>2013-09-12 00:57:17 +0200
commitebc2a1a69111eadfeda8487e577f1a5d42ef0dae (patch)
tree8a1d08bc6c0a1eb7e1bcd93056141614c22a7d40
parentswap: fix races exposed by swap discard (diff)
downloadlinux-ebc2a1a69111eadfeda8487e577f1a5d42ef0dae.tar.xz
linux-ebc2a1a69111eadfeda8487e577f1a5d42ef0dae.zip
swap: make cluster allocation per-cpu
swap cluster allocation is to get better request merge to improve performance. But the cluster is shared globally, if multiple tasks are doing swap, this will cause interleave disk access. While multiple tasks swap is quite common, for example, each numa node has a kswapd thread doing swap and multiple threads/processes doing direct page reclaim. ioscheduler can't help too much here, because tasks don't send swapout IO down to block layer in the meantime. Block layer does merge some IOs, but a lot not, depending on how many tasks are doing swapout concurrently. In practice, I've seen a lot of small size IO in swapout workloads. We makes the cluster allocation per-cpu here. The interleave disk access issue goes away. All tasks swapout to their own cluster, so swapout will become sequential, which can be easily merged to big size IO. If one CPU can't get its per-cpu cluster (for example, there is no free cluster anymore in the swap), it will fallback to scan swap_map. The CPU can still continue swap. We don't need recycle free swap entries of other CPUs. In my test (swap to a 2-disk raid0 partition), this improves around 10% swapout throughput, and request size is increased significantly. How does this impact swap readahead is uncertain though. On one side, page reclaim always isolates and swaps several adjancent pages, this will make page reclaim write the pages sequentially and benefit readahead. On the other side, several CPU write pages interleave means the pages don't live _sequentially_ but relatively _near_. In the per-cpu allocation case, if adjancent pages are written by different cpus, they will live relatively _far_. So how this impacts swap readahead depends on how many pages page reclaim isolates and swaps one time. If the number is big, this patch will benefit swap readahead. Of course, this is about sequential access pattern. The patch has no impact for random access pattern, because the new cluster allocation algorithm is just for SSD. Alternative solution is organizing swap layout to be per-mm instead of this per-cpu approach. In the per-mm layout, we allocate a disk range for each mm, so pages of one mm live in swap disk adjacently. per-mm layout has potential issues of lock contention if multiple reclaimers are swap pages from one mm. For a sequential workload, per-mm layout is better to implement swap readahead, because pages from the mm are adjacent in disk. But per-cpu layout isn't very bad in this workload, as page reclaim always isolates and swaps several pages one time, such pages will still live in disk sequentially and readahead can utilize this. For a random workload, per-mm layout isn't beneficial of request merge, because it's quite possible pages from different mm are swapout in the meantime and IO can't be merged in per-mm layout. while with per-cpu layout we can merge requests from any mm. Considering random workload is more popular in workloads with swap (and per-cpu approach isn't too bad for sequential workload too), I'm choosing per-cpu layout. [akpm@linux-foundation.org: coding-style fixes] Signed-off-by: Shaohua Li <shli@fusionio.com> Cc: Rik van Riel <riel@redhat.com> Cc: Minchan Kim <minchan@kernel.org> Cc: Kyungmin Park <kmpark@infradead.org> Cc: Hugh Dickins <hughd@google.com> Cc: Rafael Aquini <aquini@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/swap.h11
-rw-r--r--mm/swapfile.c125
2 files changed, 102 insertions, 34 deletions
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 8a3c4a1caa14..24db9142e93b 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -199,6 +199,16 @@ struct swap_cluster_info {
#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
/*
+ * We assign a cluster to each CPU, so each CPU can allocate swap entry from
+ * its own cluster and swapout sequentially. The purpose is to optimize swapout
+ * throughput.
+ */
+struct percpu_cluster {
+ struct swap_cluster_info index; /* Current cluster index */
+ unsigned int next; /* Likely next allocation offset */
+};
+
+/*
* The in-memory structure used to track swap areas.
*/
struct swap_info_struct {
@@ -217,6 +227,7 @@ struct swap_info_struct {
unsigned int inuse_pages; /* number of those currently in use */
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 block_device *bdev; /* swap device or bdev of swap file */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 98e52e373bd8..3963fc24fcc1 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -392,13 +392,78 @@ static void dec_cluster_info_page(struct swap_info_struct *p,
* It's possible scan_swap_map() uses a free cluster in the middle of free
* cluster list. Avoiding such abuse to avoid list corruption.
*/
-static inline bool scan_swap_map_recheck_cluster(struct swap_info_struct *si,
+static bool
+scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
unsigned long offset)
{
+ struct percpu_cluster *percpu_cluster;
+ bool conflict;
+
offset /= SWAPFILE_CLUSTER;
- return !cluster_is_null(&si->free_cluster_head) &&
+ conflict = !cluster_is_null(&si->free_cluster_head) &&
offset != cluster_next(&si->free_cluster_head) &&
cluster_is_free(&si->cluster_info[offset]);
+
+ if (!conflict)
+ return false;
+
+ percpu_cluster = this_cpu_ptr(si->percpu_cluster);
+ cluster_set_null(&percpu_cluster->index);
+ return true;
+}
+
+/*
+ * Try to get a swap entry from current cpu's swap entry pool (a cluster). This
+ * might involve allocating a new cluster for current CPU too.
+ */
+static void scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
+ unsigned long *offset, unsigned long *scan_base)
+{
+ struct percpu_cluster *cluster;
+ bool found_free;
+ unsigned long tmp;
+
+new_cluster:
+ cluster = this_cpu_ptr(si->percpu_cluster);
+ if (cluster_is_null(&cluster->index)) {
+ if (!cluster_is_null(&si->free_cluster_head)) {
+ cluster->index = si->free_cluster_head;
+ cluster->next = cluster_next(&cluster->index) *
+ SWAPFILE_CLUSTER;
+ } else if (!cluster_is_null(&si->discard_cluster_head)) {
+ /*
+ * we don't have free cluster but have some clusters in
+ * discarding, do discard now and reclaim them
+ */
+ swap_do_scheduled_discard(si);
+ *scan_base = *offset = si->cluster_next;
+ goto new_cluster;
+ } else
+ return;
+ }
+
+ found_free = false;
+
+ /*
+ * Other CPUs can use our cluster if they can't find a free cluster,
+ * check if there is still free entry in the cluster
+ */
+ tmp = cluster->next;
+ while (tmp < si->max && tmp < (cluster_next(&cluster->index) + 1) *
+ SWAPFILE_CLUSTER) {
+ if (!si->swap_map[tmp]) {
+ found_free = true;
+ break;
+ }
+ tmp++;
+ }
+ if (!found_free) {
+ cluster_set_null(&cluster->index);
+ goto new_cluster;
+ }
+ cluster->next = tmp + 1;
+ *offset = tmp;
+ *scan_base = tmp;
}
static unsigned long scan_swap_map(struct swap_info_struct *si,
@@ -423,41 +488,17 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,
si->flags += SWP_SCANNING;
scan_base = offset = si->cluster_next;
+ /* SSD algorithm */
+ if (si->cluster_info) {
+ scan_swap_map_try_ssd_cluster(si, &offset, &scan_base);
+ goto checks;
+ }
+
if (unlikely(!si->cluster_nr--)) {
if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER) {
si->cluster_nr = SWAPFILE_CLUSTER - 1;
goto checks;
}
-check_cluster:
- if (!cluster_is_null(&si->free_cluster_head)) {
- offset = cluster_next(&si->free_cluster_head) *
- SWAPFILE_CLUSTER;
- last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
- si->cluster_next = offset;
- si->cluster_nr = SWAPFILE_CLUSTER - 1;
- goto checks;
- } else if (si->cluster_info) {
- /*
- * we don't have free cluster but have some clusters in
- * discarding, do discard now and reclaim them
- */
- if (!cluster_is_null(&si->discard_cluster_head)) {
- si->cluster_nr = 0;
- swap_do_scheduled_discard(si);
- scan_base = offset = si->cluster_next;
- if (!si->cluster_nr)
- goto check_cluster;
- si->cluster_nr--;
- goto checks;
- }
-
- /*
- * Checking free cluster is fast enough, we can do the
- * check every time
- */
- si->cluster_nr = 0;
- goto checks;
- }
spin_unlock(&si->lock);
@@ -516,8 +557,10 @@ check_cluster:
}
checks:
- if (scan_swap_map_recheck_cluster(si, offset))
- goto check_cluster;
+ if (si->cluster_info) {
+ while (scan_swap_map_ssd_cluster_conflict(si, offset))
+ scan_swap_map_try_ssd_cluster(si, &offset, &scan_base);
+ }
if (!(si->flags & SWP_WRITEOK))
goto no_page;
if (!si->highest_bit)
@@ -1884,6 +1927,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
spin_unlock(&swap_lock);
frontswap_invalidate_area(type);
mutex_unlock(&swapon_mutex);
+ free_percpu(p->percpu_cluster);
+ p->percpu_cluster = NULL;
vfree(swap_map);
vfree(cluster_info);
vfree(frontswap_map);
@@ -2403,6 +2448,16 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
error = -ENOMEM;
goto bad_swap;
}
+ p->percpu_cluster = alloc_percpu(struct percpu_cluster);
+ if (!p->percpu_cluster) {
+ error = -ENOMEM;
+ goto bad_swap;
+ }
+ for_each_possible_cpu(i) {
+ struct percpu_cluster *cluster;
+ cluster = per_cpu_ptr(p->percpu_cluster, i);
+ cluster_set_null(&cluster->index);
+ }
}
error = swap_cgroup_swapon(p->type, maxpages);
@@ -2475,6 +2530,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
error = 0;
goto out;
bad_swap:
+ free_percpu(p->percpu_cluster);
+ p->percpu_cluster = NULL;
if (inode && S_ISBLK(inode->i_mode) && p->bdev) {
set_blocksize(p->bdev, p->old_block_size);
blkdev_put(p->bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);