On Tue, Mar 26, 2013 at 01:38:43PM +0800, Shaohua Li wrote:
> 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 or multiple
> threads/processes do direct page reclaim.
>
> We makes the cluster allocation per-cpu here. The interleave disk access issue
> goes away. All tasks will do sequential swap.
>
> If one CPU can't get its per-cpu cluster, 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.
>
> Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> ---
Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
> include/linux/swap.h | 6 ++
> mm/swapfile.c | 110 +++++++++++++++++++++++++++++++++++++--------------
> 2 files changed, 87 insertions(+), 29 deletions(-)
>
> Index: linux/include/linux/swap.h
> ===================================================================
> --- linux.orig/include/linux/swap.h 2013-03-22 17:23:56.000000000 +0800
> +++ linux/include/linux/swap.h 2013-03-22 17:44:16.877775720 +0800
> @@ -175,6 +175,11 @@ enum {
> #define COUNT_CONTINUED 0x80 /* See swap_map continuation for full count */
> #define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs, in first swap_map */
>
> +struct percpu_cluster {
> + unsigned int index; /* Current cluster index */
> + unsigned int next; /* Likely next allocation offset */
> +};
> +
> /*
> * The in-memory structure used to track swap areas.
> */
> @@ -194,6 +199,7 @@ 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;
> struct swap_extent *curr_swap_extent;
> struct swap_extent first_swap_extent;
> struct block_device *bdev; /* swap device or bdev of swap file */
> Index: linux/mm/swapfile.c
> ===================================================================
> --- linux.orig/mm/swapfile.c 2013-03-22 17:40:51.000000000 +0800
> +++ linux/mm/swapfile.c 2013-03-26 11:03:09.225915386 +0800
> @@ -353,13 +353,71 @@ static inline void dec_cluster_info_page
> * 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 si->free_cluster_head != CLUSTER_NULL &&
> + conflict = si->free_cluster_head != CLUSTER_NULL &&
> offset != si->free_cluster_head &&
> cluster_is_free(si->cluster_info[offset]);
> +
> + if (!conflict)
> + return false;
> +
> + percpu_cluster = this_cpu_ptr(si->percpu_cluster);
> + percpu_cluster->index = CLUSTER_NULL;
> + return true;
> +}
> +
> +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->index == CLUSTER_NULL) {
> + if (si->free_cluster_head != CLUSTER_NULL) {
> + cluster->index = si->free_cluster_head;
> + cluster->next = cluster->index * SWAPFILE_CLUSTER;
> + } else if (si->discard_cluster_head != CLUSTER_NULL) {
> + /*
> + * we don't have free cluster but have some clusters in
> + * discarding, do discard now and reclaim them
> + */
> + swap_do_scheduled_discard(si);
> + 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->index + 1) * SWAPFILE_CLUSTER) {
> + if (!si->swap_map[tmp]) {
> + found_free = true;
> + break;
> + }
> + tmp++;
> + }
> + if (!found_free) {
> + cluster->index = CLUSTER_NULL;
> + goto new_cluster;
> + }
> + cluster->next = tmp + 1;
> + *offset = tmp;
> + *scan_base = tmp;
> }
>
> static unsigned long scan_swap_map(struct swap_info_struct *si,
> @@ -384,36 +442,17 @@ static unsigned long scan_swap_map(struc
> 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 (si->free_cluster_head != CLUSTER_NULL) {
> - offset = 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 (si->discard_cluster_head != CLUSTER_NULL) {
> - swap_do_scheduled_discard(si);
> - goto check_cluster;
> - }
> -
> - /*
> - * Checking free cluster is fast enough, we can do the
> - * check every time
> - */
> - si->cluster_nr = 0;
> - goto checks;
> - }
> -
> spin_unlock(&si->lock);
>
> /*
> @@ -471,8 +510,11 @@ 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)
> @@ -1813,6 +1855,8 @@ SYSCALL_DEFINE1(swapoff, const char __us
> spin_unlock(&p->lock);
> spin_unlock(&swap_lock);
> mutex_unlock(&swapon_mutex);
> + free_percpu(p->percpu_cluster);
> + p->percpu_cluster = NULL;
> vfree(swap_map);
> vfree(cluster_info);
> vfree(frontswap_map_get(p));
> @@ -2310,6 +2354,12 @@ SYSCALL_DEFINE2(swapon, const char __use
> error = -ENOMEM;
> goto bad_swap;
> }
> + /* It's fine to initialize percpu_cluster to 0 */
> + p->percpu_cluster = alloc_percpu(struct percpu_cluster);
> + if (!p->percpu_cluster) {
> + error = -ENOMEM;
> + goto bad_swap;
> + }
> }
>
> error = swap_cgroup_swapon(p->type, maxpages);
> @@ -2353,6 +2403,8 @@ SYSCALL_DEFINE2(swapon, const char __use
> 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);
>
> --
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