mm/swap: add cluster lock

This patch is to reduce the lock contention of swap_info_struct->lock
via using a more fine grained lock in swap_cluster_info for some swap
operations.  swap_info_struct->lock is heavily contended if multiple
processes reclaim pages simultaneously.  Because there is only one lock
for each swap device.  While in common configuration, there is only one
or several swap devices in the system.  The lock protects almost all
swap related operations.

In fact, many swap operations only access one element of
swap_info_struct->swap_map array.  And there is no dependency between
different elements of swap_info_struct->swap_map.  So a fine grained
lock can be used to allow parallel access to the different elements of
swap_info_struct->swap_map.

In this patch, a spinlock is added to swap_cluster_info to protect the
elements of swap_info_struct->swap_map in the swap cluster and the
fields of swap_cluster_info.  This reduced locking contention for
swap_info_struct->swap_map access greatly.

Because of the added spinlock, the size of swap_cluster_info increases
from 4 bytes to 8 bytes on the 64 bit and 32 bit system.  This will use
additional 4k RAM for every 1G swap space.

Because the size of swap_cluster_info is much smaller than the size of
the cache line (8 vs 64 on x86_64 architecture), there may be false
cache line sharing between spinlocks in swap_cluster_info.  To avoid the
false sharing in the first round of the swap cluster allocation, the
order of the swap clusters in the free clusters list is changed.  So
that, the swap_cluster_info sharing the same cache line will be placed
as far as possible.  After the first round of allocation, the order of
the clusters in free clusters list is expected to be random.  So the
false sharing should be not serious.

Compared with a previous implementation using bit_spin_lock, the
sequential swap out throughput improved about 3.2%.  Test was done on a
Xeon E5 v3 system.  The swap device used is a RAM simulated PMEM
(persistent memory) device.  To test the sequential swapping out, the
test case created 32 processes, which sequentially allocate and write to
the anonymous pages until the RAM and part of the swap device is used.

[ying.huang@intel.com: v5]
  Link: http://lkml.kernel.org/r/878tqeuuic.fsf_-_@yhuang-dev.intel.com
[minchan@kernel.org: initialize spinlock for swap_cluster_info]
  Link: http://lkml.kernel.org/r/1486434945-29753-1-git-send-email-minchan@kernel.org
[hughd@google.com: annotate nested locking for cluster lock]
  Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1702161050540.21773@eggly.anvils
Link: http://lkml.kernel.org/r/dbb860bbd825b1aaba18988015e8963f263c3f0d.1484082593.git.tim.c.chen@linux.intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Aaron Lu <aaron.lu@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net> escreveu:
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap.h

@@ -176,6 +176,12 @@ enum {
  * protected by swap_info_struct.lock.
  */
 struct swap_cluster_info {
+	spinlock_t lock;	/*
+				 * Protect swap_cluster_info fields
+				 * and swap_info_struct->swap_map
+				 * elements correspond to the swap
+				 * cluster
+				 */
 	unsigned int data:24;
 	unsigned int flags:8;
 };

mm/swapfile.c

@@ -257,6 +257,47 @@ static inline void cluster_set_null(struct swap_cluster_info *info)
 	info->data = 0;
 }
 
+static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
+						     unsigned long offset)
+{
+	struct swap_cluster_info *ci;
+
+	ci = si->cluster_info;
+	if (ci) {
+		ci += offset / SWAPFILE_CLUSTER;
+		spin_lock(&ci->lock);
+	}
+	return ci;
+}
+
+static inline void unlock_cluster(struct swap_cluster_info *ci)
+{
+	if (ci)
+		spin_unlock(&ci->lock);
+}
+
+static inline struct swap_cluster_info *lock_cluster_or_swap_info(
+	struct swap_info_struct *si,
+	unsigned long offset)
+{
+	struct swap_cluster_info *ci;
+
+	ci = lock_cluster(si, offset);
+	if (!ci)
+		spin_lock(&si->lock);
+
+	return ci;
+}
+
+static inline void unlock_cluster_or_swap_info(struct swap_info_struct *si,
+					       struct swap_cluster_info *ci)
+{
+	if (ci)
+		unlock_cluster(ci);
+	else
+		spin_unlock(&si->lock);
+}
+
 static inline bool cluster_list_empty(struct swap_cluster_list *list)
 {
 	return cluster_is_null(&list->head);
@@ -281,9 +322,17 @@ static void cluster_list_add_tail(struct swap_cluster_list *list,
 		cluster_set_next_flag(&list->head, idx, 0);
 		cluster_set_next_flag(&list->tail, idx, 0);
 	} else {
+		struct swap_cluster_info *ci_tail;
 		unsigned int tail = cluster_next(&list->tail);
 
-		cluster_set_next(&ci[tail], idx);
+		/*
+		 * Nested cluster lock, but both cluster locks are
+		 * only acquired when we held swap_info_struct->lock
+		 */
+		ci_tail = ci + tail;
+		spin_lock_nested(&ci_tail->lock, SINGLE_DEPTH_NESTING);
+		cluster_set_next(ci_tail, idx);
+		unlock_cluster(ci_tail);
 		cluster_set_next_flag(&list->tail, idx, 0);
 	}
 }
@@ -328,7 +377,7 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
 */
 static void swap_do_scheduled_discard(struct swap_info_struct *si)
 {
-	struct swap_cluster_info *info;
+	struct swap_cluster_info *info, *ci;
 	unsigned int idx;
 
 	info = si->cluster_info;
@@ -341,10 +390,14 @@ static void swap_do_scheduled_discard(struct swap_info_struct *si)
 				SWAPFILE_CLUSTER);
 
 		spin_lock(&si->lock);
-		cluster_set_flag(&info[idx], CLUSTER_FLAG_FREE);
+		ci = lock_cluster(si, idx * SWAPFILE_CLUSTER);
+		cluster_set_flag(ci, CLUSTER_FLAG_FREE);
+		unlock_cluster(ci);
 		cluster_list_add_tail(&si->free_clusters, info, idx);
+		ci = lock_cluster(si, idx * SWAPFILE_CLUSTER);
 		memset(si->swap_map + idx * SWAPFILE_CLUSTER,
 				0, SWAPFILE_CLUSTER);
+		unlock_cluster(ci);
 	}
 }
 
@@ -447,8 +500,9 @@ static void scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
 	unsigned long *offset, unsigned long *scan_base)
 {
 	struct percpu_cluster *cluster;
+	struct swap_cluster_info *ci;
 	bool found_free;
-	unsigned long tmp;
+	unsigned long tmp, max;
 
 new_cluster:
 	cluster = this_cpu_ptr(si->percpu_cluster);
@@ -476,14 +530,21 @@ new_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) {
+	max = min_t(unsigned long, si->max,
+		    (cluster_next(&cluster->index) + 1) * SWAPFILE_CLUSTER);
+	if (tmp >= max) {
+		cluster_set_null(&cluster->index);
+		goto new_cluster;
+	}
+	ci = lock_cluster(si, tmp);
+	while (tmp < max) {
 		if (!si->swap_map[tmp]) {
 			found_free = true;
 			break;
 		}
 		tmp++;
 	}
+	unlock_cluster(ci);
 	if (!found_free) {
 		cluster_set_null(&cluster->index);
 		goto new_cluster;
@@ -496,6 +557,7 @@ new_cluster:
 static unsigned long scan_swap_map(struct swap_info_struct *si,
 				   unsigned char usage)
 {
+	struct swap_cluster_info *ci;
 	unsigned long offset;
 	unsigned long scan_base;
 	unsigned long last_in_cluster = 0;
@@ -572,9 +634,11 @@ checks:
 	if (offset > si->highest_bit)
 		scan_base = offset = si->lowest_bit;
 
+	ci = lock_cluster(si, offset);
 	/* reuse swap entry of cache-only swap if not busy. */
 	if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
 		int swap_was_freed;
+		unlock_cluster(ci);
 		spin_unlock(&si->lock);
 		swap_was_freed = __try_to_reclaim_swap(si, offset);
 		spin_lock(&si->lock);
@@ -584,8 +648,10 @@ checks:
 		goto scan; /* check next one */
 	}
 
-	if (si->swap_map[offset])
+	if (si->swap_map[offset]) {
+		unlock_cluster(ci);
 		goto scan;
+	}
 
 	if (offset == si->lowest_bit)
 		si->lowest_bit++;
@@ -601,6 +667,7 @@ checks:
 	}
 	si->swap_map[offset] = usage;
 	inc_cluster_info_page(si, si->cluster_info, offset);
+	unlock_cluster(ci);
 	si->cluster_next = offset + 1;
 	si->flags -= SWP_SCANNING;
 
@@ -731,7 +798,7 @@ swp_entry_t get_swap_page_of_type(int type)
 	return (swp_entry_t) {0};
 }
 
-static struct swap_info_struct *swap_info_get(swp_entry_t entry)
+static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
 {
 	struct swap_info_struct *p;
 	unsigned long offset, type;
@@ -749,7 +816,6 @@ static struct swap_info_struct *swap_info_get(swp_entry_t entry)
 		goto bad_offset;
 	if (!p->swap_map[offset])
 		goto bad_free;
-	spin_lock(&p->lock);
 	return p;
 
 bad_free:
@@ -767,14 +833,45 @@ out:
 	return NULL;
 }
 
+static struct swap_info_struct *swap_info_get(swp_entry_t entry)
+{
+	struct swap_info_struct *p;
+
+	p = _swap_info_get(entry);
+	if (p)
+		spin_lock(&p->lock);
+	return p;
+}
+
 static unsigned char swap_entry_free(struct swap_info_struct *p,
-				     swp_entry_t entry, unsigned char usage)
+				     swp_entry_t entry, unsigned char usage,
+				     bool swap_info_locked)
 {
+	struct swap_cluster_info *ci;
 	unsigned long offset = swp_offset(entry);
 	unsigned char count;
 	unsigned char has_cache;
+	bool lock_swap_info = false;
+
+	if (!swap_info_locked) {
+		count = p->swap_map[offset];
+		if (!p->cluster_info || count == usage || count == SWAP_MAP_SHMEM) {
+lock_swap_info:
+			swap_info_locked = true;
+			lock_swap_info = true;
+			spin_lock(&p->lock);
+		}
+	}
+
+	ci = lock_cluster(p, offset);
 
 	count = p->swap_map[offset];
+
+	if (!swap_info_locked && (count == usage || count == SWAP_MAP_SHMEM)) {
+		unlock_cluster(ci);
+		goto lock_swap_info;
+	}
+
 	has_cache = count & SWAP_HAS_CACHE;
 	count &= ~SWAP_HAS_CACHE;
 
@@ -800,10 +897,15 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
 	usage = count | has_cache;
 	p->swap_map[offset] = usage;
 
+	unlock_cluster(ci);
+
 	/* free if no reference */
 	if (!usage) {
+		VM_BUG_ON(!swap_info_locked);
 		mem_cgroup_uncharge_swap(entry);
+		ci = lock_cluster(p, offset);
 		dec_cluster_info_page(p, p->cluster_info, offset);
+		unlock_cluster(ci);
 		if (offset < p->lowest_bit)
 			p->lowest_bit = offset;
 		if (offset > p->highest_bit) {
@@ -829,6 +931,9 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
 		}
 	}
 
+	if (lock_swap_info)
+		spin_unlock(&p->lock);
+
 	return usage;
 }
 
@@ -840,11 +945,9 @@ void swap_free(swp_entry_t entry)
 {
 	struct swap_info_struct *p;
 
-	p = swap_info_get(entry);
-	if (p) {
-		swap_entry_free(p, entry, 1);
-		spin_unlock(&p->lock);
-	}
+	p = _swap_info_get(entry);
+	if (p)
+		swap_entry_free(p, entry, 1, false);
 }
 
 /*
@@ -854,11 +957,9 @@ void swapcache_free(swp_entry_t entry)
 {
 	struct swap_info_struct *p;
 
-	p = swap_info_get(entry);
-	if (p) {
-		swap_entry_free(p, entry, SWAP_HAS_CACHE);
-		spin_unlock(&p->lock);
-	}
+	p = _swap_info_get(entry);
+	if (p)
+		swap_entry_free(p, entry, SWAP_HAS_CACHE, false);
 }
 
 /*
@@ -870,13 +971,17 @@ int page_swapcount(struct page *page)
 {
 	int count = 0;
 	struct swap_info_struct *p;
+	struct swap_cluster_info *ci;
 	swp_entry_t entry;
+	unsigned long offset;
 
 	entry.val = page_private(page);
-	p = swap_info_get(entry);
+	p = _swap_info_get(entry);
 	if (p) {
-		count = swap_count(p->swap_map[swp_offset(entry)]);
-		spin_unlock(&p->lock);
+		offset = swp_offset(entry);
+		ci = lock_cluster_or_swap_info(p, offset);
+		count = swap_count(p->swap_map[offset]);
+		unlock_cluster_or_swap_info(p, ci);
 	}
 	return count;
 }
@@ -889,22 +994,26 @@ int swp_swapcount(swp_entry_t entry)
 {
 	int count, tmp_count, n;
 	struct swap_info_struct *p;
+	struct swap_cluster_info *ci;
 	struct page *page;
 	pgoff_t offset;
 	unsigned char *map;
 
-	p = swap_info_get(entry);
+	p = _swap_info_get(entry);
 	if (!p)
 		return 0;
 
-	count = swap_count(p->swap_map[swp_offset(entry)]);
+	offset = swp_offset(entry);
+
+	ci = lock_cluster_or_swap_info(p, offset);
+
+	count = swap_count(p->swap_map[offset]);
 	if (!(count & COUNT_CONTINUED))
 		goto out;
 
 	count &= ~COUNT_CONTINUED;
 	n = SWAP_MAP_MAX + 1;
 
-	offset = swp_offset(entry);
 	page = vmalloc_to_page(p->swap_map + offset);
 	offset &= ~PAGE_MASK;
 	VM_BUG_ON(page_private(page) != SWP_CONTINUED);
@@ -919,7 +1028,7 @@ int swp_swapcount(swp_entry_t entry)
 		n *= (SWAP_CONT_MAX + 1);
 	} while (tmp_count & COUNT_CONTINUED);
 out:
-	spin_unlock(&p->lock);
+	unlock_cluster_or_swap_info(p, ci);
 	return count;
 }
 
@@ -1017,7 +1126,7 @@ int free_swap_and_cache(swp_entry_t entry)
 
 	p = swap_info_get(entry);
 	if (p) {
-		if (swap_entry_free(p, entry, 1) == SWAP_HAS_CACHE) {
+		if (swap_entry_free(p, entry, 1, true) == SWAP_HAS_CACHE) {
 			page = find_get_page(swap_address_space(entry),
 					     swp_offset(entry));
 			if (page && !trylock_page(page)) {
@@ -2298,6 +2407,9 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
 	return maxpages;
 }
 
+#define SWAP_CLUSTER_COLS						\
+	DIV_ROUND_UP(L1_CACHE_BYTES, sizeof(struct swap_cluster_info))
+
 static int setup_swap_map_and_extents(struct swap_info_struct *p,
 					union swap_header *swap_header,
 					unsigned char *swap_map,
@@ -2305,11 +2417,12 @@ static int setup_swap_map_and_extents(struct swap_info_struct *p,
 					unsigned long maxpages,
 					sector_t *span)
 {
-	int i;
+	unsigned int j, k;
 	unsigned int nr_good_pages;
 	int nr_extents;
 	unsigned long nr_clusters = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
-	unsigned long idx = p->cluster_next / SWAPFILE_CLUSTER;
+	unsigned long col = p->cluster_next / SWAPFILE_CLUSTER % SWAP_CLUSTER_COLS;
+	unsigned long i, idx;
 
 	nr_good_pages = maxpages - 1;	/* omit header page */
 
@@ -2357,15 +2470,20 @@ static int setup_swap_map_and_extents(struct swap_info_struct *p,
 	if (!cluster_info)
 		return nr_extents;
 
-	for (i = 0; i < nr_clusters; i++) {
-		if (!cluster_count(&cluster_info[idx])) {
+
+	/* Reduce false cache line sharing between cluster_info */
+	for (k = 0; k < SWAP_CLUSTER_COLS; k++) {
+		j = (k + col) % SWAP_CLUSTER_COLS;
+		for (i = 0; i < DIV_ROUND_UP(nr_clusters, SWAP_CLUSTER_COLS); i++) {
+			idx = i * SWAP_CLUSTER_COLS + j;
+			if (idx >= nr_clusters)
+				continue;
+			if (cluster_count(&cluster_info[idx]))
+				continue;
 			cluster_set_flag(&cluster_info[idx], CLUSTER_FLAG_FREE);
 			cluster_list_add_tail(&p->free_clusters, cluster_info,
 					      idx);
 		}
-		idx++;
-		if (idx == nr_clusters)
-			idx = 0;
 	}
 	return nr_extents;
 }
@@ -2468,6 +2586,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 
 	if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
 		int cpu;
+		unsigned long ci, nr_cluster;
 
 		p->flags |= SWP_SOLIDSTATE;
 		/*
@@ -2475,13 +2594,17 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		 * SSD
 		 */
 		p->cluster_next = 1 + (prandom_u32() % p->highest_bit);
+		nr_cluster = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
 
-		cluster_info = vzalloc(DIV_ROUND_UP(maxpages,
-			SWAPFILE_CLUSTER) * sizeof(*cluster_info));
+		cluster_info = vzalloc(nr_cluster * sizeof(*cluster_info));
 		if (!cluster_info) {
 			error = -ENOMEM;
 			goto bad_swap;
 		}
+
+		for (ci = 0; ci < nr_cluster; ci++)
+			spin_lock_init(&((cluster_info + ci)->lock));
+
 		p->percpu_cluster = alloc_percpu(struct percpu_cluster);
 		if (!p->percpu_cluster) {
 			error = -ENOMEM;
@@ -2627,6 +2750,7 @@ void si_swapinfo(struct sysinfo *val)
 static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 {
 	struct swap_info_struct *p;
+	struct swap_cluster_info *ci;
 	unsigned long offset, type;
 	unsigned char count;
 	unsigned char has_cache;
@@ -2640,10 +2764,10 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 		goto bad_file;
 	p = swap_info[type];
 	offset = swp_offset(entry);
-
-	spin_lock(&p->lock);
 	if (unlikely(offset >= p->max))
-		goto unlock_out;
+		goto out;
+
+	ci = lock_cluster_or_swap_info(p, offset);
 
 	count = p->swap_map[offset];
 
@@ -2686,7 +2810,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
 	p->swap_map[offset] = count | has_cache;
 
 unlock_out:
-	spin_unlock(&p->lock);
+	unlock_cluster_or_swap_info(p, ci);
 out:
 	return err;
 
@@ -2775,6 +2899,7 @@ EXPORT_SYMBOL_GPL(__page_file_index);
 int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
 {
 	struct swap_info_struct *si;
+	struct swap_cluster_info *ci;
 	struct page *head;
 	struct page *page;
 	struct page *list_page;
@@ -2798,6 +2923,9 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
 	}
 
 	offset = swp_offset(entry);
+
+	ci = lock_cluster(si, offset);
+
 	count = si->swap_map[offset] & ~SWAP_HAS_CACHE;
 
 	if ((count & ~COUNT_CONTINUED) != SWAP_MAP_MAX) {
@@ -2810,6 +2938,7 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
 	}
 
 	if (!page) {
+		unlock_cluster(ci);
 		spin_unlock(&si->lock);
 		return -ENOMEM;
 	}
@@ -2858,6 +2987,7 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
 	list_add_tail(&page->lru, &head->lru);
 	page = NULL;			/* now it's attached, don't free it */
 out:
+	unlock_cluster(ci);
 	spin_unlock(&si->lock);
 outer:
 	if (page)
@@ -2871,7 +3001,8 @@ outer:
  * into, carry if so, or else fail until a new continuation page is allocated;
  * when the original swap_map count is decremented from 0 with continuation,
  * borrow from the continuation and report whether it still holds more.
- * Called while __swap_duplicate() or swap_entry_free() holds swap_lock.
+ * Called while __swap_duplicate() or swap_entry_free() holds swap or cluster
+ * lock.
  */
 static bool swap_count_continued(struct swap_info_struct *si,
 				 pgoff_t offset, unsigned char count)