[PATCH v4 5/9] mm/swap: Allocate swap slots in batches

Tim Chen
2016-12-09T21:11:26Z

Currently, the swap slots are allocated one page at a time,
causing contention to the swap_info lock protecting the swap partition
on every page being swapped.

This patch adds new functions get_swap_pages and scan_swap_map_slots
to request multiple swap slots at once. This will reduces the lock
contention on the swap_info lock. Also scan_swap_map_slots can operate
more efficiently as swap slots often occurs in clusters close to each
other on a swap device and it is quicker to allocate them together.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Co-developed-by: "Huang, Ying" <ying.huang@intel.com>
---
include/linux/swap.h | 2 +
mm/swapfile.c | 138 +++++++++++++++++++++++++++++++++++++++++----------
2 files changed, 115 insertions(+), 25 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 6b7a48b..68d2ea5 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -27,6 +27,7 @@ struct bio;
#define SWAP_FLAGS_VALID (SWAP_FLAG_PRIO_MASK | SWAP_FLAG_PREFER | \
SWAP_FLAG_DISCARD | SWAP_FLAG_DISCARD_ONCE | \
SWAP_FLAG_DISCARD_PAGES)
+#define SWAP_BATCH 64

static inline int current_is_kswapd(void)
{
@@ -412,6 +413,7 @@ static inline long get_nr_swap_pages(void)
extern void si_swapinfo(struct sysinfo *);
extern swp_entry_t get_swap_page(void);
extern swp_entry_t get_swap_page_of_type(int);
+extern int get_swap_pages(int n, swp_entry_t swp_entries[]);
extern int add_swap_count_continuation(swp_entry_t, gfp_t);
extern void swap_shmem_alloc(swp_entry_t);
extern int swap_duplicate(swp_entry_t);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 136c9dd..8b2a97e 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -501,7 +501,7 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
* 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,
+static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
unsigned long *offset, unsigned long *scan_base)
{
struct percpu_cluster *cluster;
@@ -525,7 +525,7 @@ static void scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
*scan_base = *offset = si->cluster_next;
goto new_cluster;
} else
- return;
+ return false;
}

found_free = false;
@@ -557,16 +557,22 @@ static void scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
cluster->next = tmp + 1;
*offset = tmp;
*scan_base = tmp;
+ return found_free;
}

-static unsigned long scan_swap_map(struct swap_info_struct *si,
- unsigned char usage)
+static int scan_swap_map_slots(struct swap_info_struct *si,
+ unsigned char usage, int nr,
+ swp_entry_t slots[])
{
struct swap_cluster_info *ci;
unsigned long offset;
unsigned long scan_base;
unsigned long last_in_cluster = 0;
int latency_ration = LATENCY_LIMIT;
+ int n_ret = 0;
+
+ if (nr > SWAP_BATCH)
+ nr = SWAP_BATCH;

/*
* We try to cluster swap pages by allocating them sequentially
@@ -584,8 +590,10 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,

/* SSD algorithm */
if (si->cluster_info) {
- scan_swap_map_try_ssd_cluster(si, &offset, &scan_base);
- goto checks;
+ if (scan_swap_map_try_ssd_cluster(si, &offset, &scan_base))
+ goto checks;
+ else
+ goto scan;
}

if (unlikely(!si->cluster_nr--)) {
@@ -629,8 +637,14 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,

checks:
if (si->cluster_info) {
- while (scan_swap_map_ssd_cluster_conflict(si, offset))
- scan_swap_map_try_ssd_cluster(si, &offset, &scan_base);
+ while (scan_swap_map_ssd_cluster_conflict(si, offset)) {
+ /* take a break if we already got some slots */
+ if (n_ret)
+ goto done;
+ if (!scan_swap_map_try_ssd_cluster(si, &offset,
+ &scan_base))
+ goto scan;
+ }
}
if (!(si->flags & SWP_WRITEOK))
goto no_page;
@@ -655,7 +669,10 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,

if (si->swap_map[offset]) {
unlock_cluster(ci);
- goto scan;
+ if (!n_ret)
+ goto scan;
+ else
+ goto done;
}

if (offset == si->lowest_bit)
@@ -674,9 +691,43 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,
inc_cluster_info_page(si, si->cluster_info, offset);
unlock_cluster(ci);
si->cluster_next = offset + 1;
- si->flags -= SWP_SCANNING;
+ slots[n_ret++] = swp_entry(si->type, offset);
+
+ /* got enough slots or reach max slots? */
+ if ((n_ret == nr) || (offset >= si->highest_bit))
+ goto done;

- return offset;
+ /* search for next available slot */
+
+ /* time to take a break? */
+ if (unlikely(--latency_ration < 0)) {
+ if (n_ret)
+ goto done;
+ spin_unlock(&si->lock);
+ cond_resched();
+ spin_lock(&si->lock);
+ latency_ration = LATENCY_LIMIT;
+ }
+
+ /* try to get more slots in cluster */
+ if (si->cluster_info) {
+ if (scan_swap_map_try_ssd_cluster(si, &offset, &scan_base))
+ goto checks;
+ else
+ goto done;
+ }
+ /* non-ssd case */
+ ++offset;
+
+ /* non-ssd case, still more slots in cluster? */
+ if (si->cluster_nr && !si->swap_map[offset]) {
+ --si->cluster_nr;
+ goto checks;
+ }
+
+done:
+ si->flags -= SWP_SCANNING;
+ return n_ret;

scan:
spin_unlock(&si->lock);
@@ -714,17 +765,43 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,

no_page:
si->flags -= SWP_SCANNING;
- return 0;
+ return n_ret;
}

-swp_entry_t get_swap_page(void)
+static unsigned long scan_swap_map(struct swap_info_struct *si,
+ unsigned char usage)
+{
+ swp_entry_t entry;
+ int n_ret;
+
+ n_ret = scan_swap_map_slots(si, usage, 1, &entry);
+
+ if (n_ret)
+ return swp_offset(entry);
+ else
+ return 0;
+
+}
+
+int get_swap_pages(int n_goal, swp_entry_t swp_entries[])
{
struct swap_info_struct *si, *next;
- pgoff_t offset;
+ long avail_pgs;
+ int n_ret = 0;

- if (atomic_long_read(&nr_swap_pages) <= 0)
+ avail_pgs = atomic_long_read(&nr_swap_pages);
+ if (avail_pgs <= 0)
goto noswap;
- atomic_long_dec(&nr_swap_pages);
+
+ swp_entries[0] = (swp_entry_t) {0};
+
+ if (n_goal > SWAP_BATCH)
+ n_goal = SWAP_BATCH;
+
+ if (n_goal > avail_pgs)
+ n_goal = avail_pgs;
+
+ atomic_long_sub(n_goal, &nr_swap_pages);

spin_lock(&swap_avail_lock);

@@ -750,14 +827,14 @@ swp_entry_t get_swap_page(void)
spin_unlock(&si->lock);
goto nextsi;
}
-
- /* This is called for allocating swap entry for cache */
- offset = scan_swap_map(si, SWAP_HAS_CACHE);
+ n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
+ n_goal, swp_entries);
spin_unlock(&si->lock);
- if (offset)
- return swp_entry(si->type, offset);
+ if (n_ret)
+ goto check_out;
pr_debug("scan_swap_map of si %d failed to find offset\n",
- si->type);
+ si->type);
+
spin_lock(&swap_avail_lock);
nextsi:
/*
@@ -768,7 +845,8 @@ swp_entry_t get_swap_page(void)
* up between us dropping swap_avail_lock and taking si->lock.
* Since we dropped the swap_avail_lock, the swap_avail_head
* list may have been modified; so if next is still in the
- * swap_avail_head list then try it, otherwise start over.
+ * swap_avail_head list then try it, otherwise start over
+ * if we have not gotten any slots.
*/
if (plist_node_empty(&next->avail_list))
goto start_over;
@@ -776,9 +854,19 @@ swp_entry_t get_swap_page(void)

spin_unlock(&swap_avail_lock);

- atomic_long_inc(&nr_swap_pages);
+check_out:
+ if (n_ret < n_goal)
+ atomic_long_add((long) (n_goal-n_ret), &nr_swap_pages);
noswap:
- return (swp_entry_t) {0};
+ return n_ret;
+}
+
+swp_entry_t get_swap_page(void)
+{
+ swp_entry_t entry;
+
+ get_swap_pages(1, &entry);
+ return entry;
}

/* The only caller of this function is now suspend routine */
--
2.5.5