[PATCH v3 12/13] mm, swap: use a global swap cluster for non-rotation devices

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Non-rotational devices (SSD / ZRAM) can tolerate fragmentation, so the
goal of the SWAP allocator is to avoid contention for clusters. It uses
a per-CPU cluster design, and each CPU will use a different cluster as
much as possible.

However, HDDs are very sensitive to fragmentation, contention is trivial
in comparison. Therefore, we use one global cluster instead. This ensures
that each order will be written to the same cluster as much as possible,
which helps make the I/O more continuous.

This ensures that the performance of the cluster allocator is as good as
that of the old allocator. Tests after this commit compared to those
before this series:

Tested using 'make -j32' with tinyconfig, a 1G memcg limit, and HDD swap:

make -j32 with tinyconfig, using 1G memcg limit and HDD swap:

Before this series:
114.44user 29.11system 39:42.90elapsed 6%CPU (0avgtext+0avgdata 157284maxresident)k
2901232inputs+0outputs (238877major+4227640minor)pagefaults

After this commit:
113.90user 23.81system 38:11.77elapsed 6%CPU (0avgtext+0avgdata 157260maxresident)k
2548728inputs+0outputs (235471major+4238110minor)pagefaults

Suggested-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Kairui Song <kasong@tencent.com>
---
 include/linux/swap.h |  2 ++
 mm/swapfile.c        | 51 ++++++++++++++++++++++++++++++++------------
 2 files changed, 39 insertions(+), 14 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 4c1d2e69689f..b13b72645db3 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -318,6 +318,8 @@ struct swap_info_struct {
 	unsigned int pages;		/* total of usable pages of swap */
 	atomic_long_t inuse_pages;	/* number of those currently in use */
 	struct percpu_cluster __percpu *percpu_cluster; /* per cpu's swap location */
+	struct percpu_cluster *global_cluster; /* Use one global cluster for rotating device */
+	spinlock_t global_cluster_lock;	/* Serialize usage of global cluster */
 	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 */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index a3d1239d944b..e57e5453a25b 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -814,7 +814,10 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
 out:
 	relocate_cluster(si, ci);
 	unlock_cluster(ci);
-	__this_cpu_write(si->percpu_cluster->next[order], next);
+	if (si->flags & SWP_SOLIDSTATE)
+		__this_cpu_write(si->percpu_cluster->next[order], next);
+	else
+		si->global_cluster->next[order] = next;
 	return found;
 }
 
@@ -875,9 +878,16 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
 	struct swap_cluster_info *ci;
 	unsigned int offset, found = 0;
 
-	/* Fast path using per CPU cluster */
-	local_lock(&si->percpu_cluster->lock);
-	offset = __this_cpu_read(si->percpu_cluster->next[order]);
+	if (si->flags & SWP_SOLIDSTATE) {
+		/* Fast path using per CPU cluster */
+		local_lock(&si->percpu_cluster->lock);
+		offset = __this_cpu_read(si->percpu_cluster->next[order]);
+	} else {
+		/* Serialize HDD SWAP allocation for each device. */
+		spin_lock(&si->global_cluster_lock);
+		offset = si->global_cluster->next[order];
+	}
+
 	if (offset) {
 		ci = lock_cluster(si, offset);
 		/* Cluster could have been used by another order */
@@ -972,8 +982,10 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
 		}
 	}
 done:
-	local_unlock(&si->percpu_cluster->lock);
-
+	if (si->flags & SWP_SOLIDSTATE)
+		local_unlock(&si->percpu_cluster->lock);
+	else
+		spin_unlock(&si->global_cluster_lock);
 	return found;
 }
 
@@ -2778,6 +2790,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	mutex_unlock(&swapon_mutex);
 	free_percpu(p->percpu_cluster);
 	p->percpu_cluster = NULL;
+	kfree(p->global_cluster);
+	p->global_cluster = NULL;
 	vfree(swap_map);
 	kvfree(zeromap);
 	kvfree(cluster_info);
@@ -3183,17 +3197,24 @@ static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
 	for (i = 0; i < nr_clusters; i++)
 		spin_lock_init(&cluster_info[i].lock);
 
-	si->percpu_cluster = alloc_percpu(struct percpu_cluster);
-	if (!si->percpu_cluster)
-		goto err_free;
+	if (si->flags & SWP_SOLIDSTATE) {
+		si->percpu_cluster = alloc_percpu(struct percpu_cluster);
+		if (!si->percpu_cluster)
+			goto err_free;
 
-	for_each_possible_cpu(cpu) {
-		struct percpu_cluster *cluster;
+		for_each_possible_cpu(cpu) {
+			struct percpu_cluster *cluster;
 
-		cluster = per_cpu_ptr(si->percpu_cluster, cpu);
+			cluster = per_cpu_ptr(si->percpu_cluster, cpu);
+			for (i = 0; i < SWAP_NR_ORDERS; i++)
+				cluster->next[i] = SWAP_ENTRY_INVALID;
+			local_lock_init(&cluster->lock);
+		}
+	} else {
+		si->global_cluster = kmalloc(sizeof(*si->global_cluster), GFP_KERNEL);
 		for (i = 0; i < SWAP_NR_ORDERS; i++)
-			cluster->next[i] = SWAP_ENTRY_INVALID;
-		local_lock_init(&cluster->lock);
+			si->global_cluster->next[i] = SWAP_ENTRY_INVALID;
+		spin_lock_init(&si->global_cluster_lock);
 	}
 
 	/*
@@ -3467,6 +3488,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 bad_swap:
 	free_percpu(si->percpu_cluster);
 	si->percpu_cluster = NULL;
+	kfree(si->global_cluster);
+	si->global_cluster = NULL;
 	inode = NULL;
 	destroy_swap_extents(si);
 	swap_cgroup_swapoff(si->type);
-- 
2.47.1