Re: [patch 1/4 v3]swap: change block allocation algorithm for SSD

[Rafael Aquini <aquini@redhat.com>]

On Thu, Feb 21, 2013 at 10:17:10AM +0800, Shaohua Li wrote:
> I'm using a fast SSD to do swap. scan_swap_map() sometimes uses up to 20~30%
> CPU time (when cluster is hard to find, the CPU time can be up to 80%), which
> becomes a bottleneck.  scan_swap_map() scans a byte array to search a 256 page
> cluster, which is very slow.
> 
> Here I introduced a simple algorithm to search cluster. Since we only care
> about 256 pages cluster, we can just use a counter to track if a cluster is
> free. Every 256 pages use one int to store the counter. If the counter of a
> cluster is 0, the cluster is free. All free clusters will be added to a list,
> so searching cluster is very efficient. With this, scap_swap_map() overhead
> disappears.
>

Nice work! I've been testing your series with the attached test-prog, as it was
triggering a swapin_readahead() softlockup storm due to a race window 
scan_swap_map() was opening when discarding clusters and swap device was
saturated by the swap thrashing going on. Your patch 3 closes that window, and
your series is still playing very well as I'm writing this message.

/me just thinks you could use some numbers of yours to actually show the benefits
instead of just claim them on the commit message.

Other than that, I have little to say about your work, and as long as Hugh is
happy with it, you can surely stick my Acked-by on your series.

Regards!

Rafael
 
> Since searching cluster with a list is easy, we can easily implement a per-cpu
> cluster algorithm to do block allocation, which can make swapout more
> efficient. This is in my TODO list.
> 
> This might help low end SD card swap too. Because if the cluster is aligned, SD
> firmware can do flash erase more efficiently.
> 
> We only enable the algorithm for SSD. Hard disk swap isn't fast enough and has
> downside with the algorithm which might introduce regression (see below).
> 
> The patch slightly changes which cluster is choosen. It always adds free
> cluster to list tail. This can help wear leveling for low end SSD too. And if
> no cluster found, the scan_swap_map() will do search from the end of last
> cluster. So if no cluster found, the scan_swap_map() will do search from the
> end of last free cluster, which is random. For SSD, this isn't a problem at
> all.
> 
> Another downside is the cluster must be aligned to 256 pages, which will reduce
> the chance to find a cluster. I would expect this isn't a big problem for SSD
> because of the non-seek penality. (And this is the reason I only enable the
> algorithm for SSD).
> 
> V2 -> V3:
> rebase to latest linux-next
> 
> V1 -> V2:
> 1. free cluster is added to a list, which makes searching cluster more efficient
> 2. only enable the algorithm for SSD.
> 
> Signed-off-by: Shaohua Li <shli@fusionio.com>
> ---
>  include/linux/swap.h |    3 
>  mm/swapfile.c        |  181 +++++++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 172 insertions(+), 12 deletions(-)
> 
> Index: linux/include/linux/swap.h
> ===================================================================
> --- linux.orig/include/linux/swap.h	2013-02-18 15:06:06.000000000 +0800
> +++ linux/include/linux/swap.h	2013-02-18 15:21:09.285317914 +0800
> @@ -185,6 +185,9 @@ struct swap_info_struct {
>  	signed char	next;		/* next type on the swap list */
>  	unsigned int	max;		/* extent of the swap_map */
>  	unsigned char *swap_map;	/* vmalloc'ed array of usage counts */
> +	unsigned int *cluster_info;	/* cluster info. Only for SSD */
> +	unsigned int free_cluster_head;
> +	unsigned int free_cluster_tail;
>  	unsigned int lowest_bit;	/* index of first free in swap_map */
>  	unsigned int highest_bit;	/* index of last free in swap_map */
>  	unsigned int pages;		/* total of usable pages of swap */
> Index: linux/mm/swapfile.c
> ===================================================================
> --- linux.orig/mm/swapfile.c	2013-02-18 15:06:06.000000000 +0800
> +++ linux/mm/swapfile.c	2013-02-18 15:21:09.285317914 +0800
> @@ -184,6 +184,85 @@ static int wait_for_discard(void *word)
>  #define SWAPFILE_CLUSTER	256
>  #define LATENCY_LIMIT		256
>  
> +/*
> + * cluster info is a unsigned int, the highest 8 bits stores flags, the low 24
> + * bits stores next cluster if the cluster is free or cluster counter otherwise
> + */
> +#define CLUSTER_FLAG_FREE (1 << 0)
> +#define CLUSTER_FLAG_NEXT_NULL (1 << 1)
> +#define CLUSTER_NULL (CLUSTER_FLAG_NEXT_NULL << 24)
> +#define cluster_flag(info) ((info) >> 24)
> +#define cluster_set_flag(info, flag) \
> +	do { info = ((info) & 0xffffff) | ((flag) << 24); } while (0)
> +#define cluster_count(info) ((info) & 0xffffff)
> +#define cluster_set_count(info, c) \
> +	do { info = (cluster_flag(info) << 24) | (c); } while (0)
> +#define cluster_next(info) ((info) & 0xffffff)
> +#define cluster_set_next(info, n) \
> +	do { info = (cluster_flag(info) << 24) | (n); } while (0)
> +#define cluster_is_free(info) (cluster_flag(info) & CLUSTER_FLAG_FREE)
> +
> +static inline void inc_cluster_info_page(struct swap_info_struct *p,
> +	unsigned int *cluster_info, unsigned long page_nr)
> +{
> +	unsigned long idx = page_nr / SWAPFILE_CLUSTER;
> +
> +	if (!cluster_info)
> +		return;
> +	if (cluster_is_free(cluster_info[idx])) {
> +		VM_BUG_ON(p->free_cluster_head != idx);
> +		p->free_cluster_head = cluster_next(cluster_info[idx]);
> +		if (p->free_cluster_tail == idx) {
> +			p->free_cluster_tail = CLUSTER_NULL;
> +			p->free_cluster_head = CLUSTER_NULL;
> +		}
> +		cluster_set_flag(cluster_info[idx], 0);
> +		cluster_set_count(cluster_info[idx], 0);
> +	}
> +
> +	VM_BUG_ON(cluster_count(cluster_info[idx]) >= SWAPFILE_CLUSTER);
> +	cluster_set_count(cluster_info[idx],
> +		cluster_count(cluster_info[idx]) + 1);
> +}
> +
> +static inline void dec_cluster_info_page(struct swap_info_struct *p,
> +	unsigned int *cluster_info, unsigned long page_nr)
> +{
> +	unsigned long idx = page_nr / SWAPFILE_CLUSTER;
> +
> +	if (!cluster_info)
> +		return;
> +
> +	VM_BUG_ON(cluster_count(cluster_info[idx]) == 0);
> +	cluster_set_count(cluster_info[idx],
> +		cluster_count(cluster_info[idx]) - 1);
> +
> +	if (cluster_count(cluster_info[idx]) == 0) {
> +		cluster_set_flag(cluster_info[idx], CLUSTER_FLAG_FREE);
> +		if (p->free_cluster_head == CLUSTER_NULL) {
> +			p->free_cluster_head = idx;
> +			p->free_cluster_tail = idx;
> +		} else {
> +			cluster_set_next(cluster_info[p->free_cluster_tail],
> +				idx);
> +			p->free_cluster_tail = idx;
> +		}
> +	}
> +}
> +
> +/*
> + * 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,
> +	unsigned long offset)
> +{
> +	offset /= SWAPFILE_CLUSTER;
> +	return si->free_cluster_head != CLUSTER_NULL &&
> +		offset != si->free_cluster_head &&
> +		cluster_is_free(si->cluster_info[offset]);
> +}
> +
>  static unsigned long scan_swap_map(struct swap_info_struct *si,
>  				   unsigned char usage)
>  {
> @@ -225,6 +304,24 @@ static unsigned long scan_swap_map(struc
>  			si->lowest_alloc = si->max;
>  			si->highest_alloc = 0;
>  		}
> +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;
> +			found_free_cluster = 1;
> +			goto checks;
> +		} else if (si->cluster_info) {
> +			/*
> +			 * Checking free cluster is fast enough, we can do the
> +			 * check every time
> +			 */
> +			si->cluster_nr = 0;
> +			si->lowest_alloc = 0;
> +			goto checks;
> +		}
> +
>  		spin_unlock(&si->lock);
>  
>  		/*
> @@ -285,6 +382,8 @@ static unsigned long scan_swap_map(struc
>  	}
>  
>  checks:
> +	if (scan_swap_map_recheck_cluster(si, offset))
> +		goto check_cluster;
>  	if (!(si->flags & SWP_WRITEOK))
>  		goto no_page;
>  	if (!si->highest_bit)
> @@ -317,6 +416,7 @@ checks:
>  		si->highest_bit = 0;
>  	}
>  	si->swap_map[offset] = usage;
> +	inc_cluster_info_page(si, si->cluster_info, offset);
>  	si->cluster_next = offset + 1;
>  	si->flags -= SWP_SCANNING;
>  
> @@ -600,6 +700,7 @@ static unsigned char swap_entry_free(str
>  
>  	/* free if no reference */
>  	if (!usage) {
> +		dec_cluster_info_page(p, p->cluster_info, offset);
>  		if (offset < p->lowest_bit)
>  			p->lowest_bit = offset;
>  		if (offset > p->highest_bit)
> @@ -1497,6 +1598,7 @@ static int setup_swap_extents(struct swa
>  
>  static void _enable_swap_info(struct swap_info_struct *p, int prio,
>  				unsigned char *swap_map,
> +				unsigned int *cluster_info,
>  				unsigned long *frontswap_map)
>  {
>  	int i, prev;
> @@ -1506,6 +1608,7 @@ static void _enable_swap_info(struct swa
>  	else
>  		p->prio = --least_priority;
>  	p->swap_map = swap_map;
> +	p->cluster_info = cluster_info;
>  	frontswap_map_set(p, frontswap_map);
>  	p->flags |= SWP_WRITEOK;
>  	atomic_long_add(p->pages, &nr_swap_pages);
> @@ -1527,11 +1630,12 @@ static void _enable_swap_info(struct swa
>  
>  static void enable_swap_info(struct swap_info_struct *p, int prio,
>  				unsigned char *swap_map,
> +				unsigned int *cluster_info,
>  				unsigned long *frontswap_map)
>  {
>  	spin_lock(&swap_lock);
>  	spin_lock(&p->lock);
> -	_enable_swap_info(p, prio, swap_map, frontswap_map);
> +	_enable_swap_info(p, prio, swap_map, cluster_info, frontswap_map);
>  	frontswap_init(p->type);
>  	spin_unlock(&p->lock);
>  	spin_unlock(&swap_lock);
> @@ -1541,7 +1645,8 @@ static void reinsert_swap_info(struct sw
>  {
>  	spin_lock(&swap_lock);
>  	spin_lock(&p->lock);
> -	_enable_swap_info(p, p->prio, p->swap_map, frontswap_map_get(p));
> +	_enable_swap_info(p, p->prio, p->swap_map, p->cluster_info,
> +					frontswap_map_get(p));
>  	spin_unlock(&p->lock);
>  	spin_unlock(&swap_lock);
>  }
> @@ -1550,6 +1655,7 @@ SYSCALL_DEFINE1(swapoff, const char __us
>  {
>  	struct swap_info_struct *p = NULL;
>  	unsigned char *swap_map;
> +	unsigned int *cluster_info;
>  	struct file *swap_file, *victim;
>  	struct address_space *mapping;
>  	struct inode *inode;
> @@ -1648,12 +1754,15 @@ SYSCALL_DEFINE1(swapoff, const char __us
>  	p->max = 0;
>  	swap_map = p->swap_map;
>  	p->swap_map = NULL;
> +	cluster_info = p->cluster_info;
> +	p->cluster_info = NULL;
>  	p->flags = 0;
>  	frontswap_invalidate_area(type);
>  	spin_unlock(&p->lock);
>  	spin_unlock(&swap_lock);
>  	mutex_unlock(&swapon_mutex);
>  	vfree(swap_map);
> +	vfree(cluster_info);
>  	vfree(frontswap_map_get(p));
>  	/* Destroy swap account informatin */
>  	swap_cgroup_swapoff(type);
> @@ -1966,15 +2075,21 @@ static unsigned long read_swap_header(st
>  static int setup_swap_map_and_extents(struct swap_info_struct *p,
>  					union swap_header *swap_header,
>  					unsigned char *swap_map,
> +					unsigned int *cluster_info,
>  					unsigned long maxpages,
>  					sector_t *span)
>  {
>  	int i;
>  	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;
>  
>  	nr_good_pages = maxpages - 1;	/* omit header page */
>  
> +	p->free_cluster_head = CLUSTER_NULL;
> +	p->free_cluster_tail = CLUSTER_NULL;
> +
>  	for (i = 0; i < swap_header->info.nr_badpages; i++) {
>  		unsigned int page_nr = swap_header->info.badpages[i];
>  		if (page_nr == 0 || page_nr > swap_header->info.last_page)
> @@ -1982,11 +2097,25 @@ static int setup_swap_map_and_extents(st
>  		if (page_nr < maxpages) {
>  			swap_map[page_nr] = SWAP_MAP_BAD;
>  			nr_good_pages--;
> +			/*
> +			 * Not mark the cluster free yet, no list
> +			 * operation involved
> +			 */
> +			inc_cluster_info_page(p, cluster_info, page_nr);
>  		}
>  	}
>  
> +	/* Not mark the cluster free yet, no list operation involved */
> +	for (i = maxpages; i < round_up(maxpages, SWAPFILE_CLUSTER); i++)
> +		inc_cluster_info_page(p, cluster_info, i);
> +
>  	if (nr_good_pages) {
>  		swap_map[0] = SWAP_MAP_BAD;
> +		/*
> +		 * Not mark the cluster free yet, no list
> +		 * operation involved
> +		 */
> +		inc_cluster_info_page(p, cluster_info, 0);
>  		p->max = maxpages;
>  		p->pages = nr_good_pages;
>  		nr_extents = setup_swap_extents(p, span);
> @@ -1999,6 +2128,27 @@ static int setup_swap_map_and_extents(st
>  		return -EINVAL;
>  	}
>  
> +	if (!cluster_info)
> +		return nr_extents;
> +
> +	for (i = 0; i < nr_clusters; i++) {
> +		if (!cluster_count(cluster_info[idx])) {
> +			cluster_set_flag(cluster_info[idx], CLUSTER_FLAG_FREE);
> +			if (p->free_cluster_head == CLUSTER_NULL) {
> +				p->free_cluster_head = idx;
> +				p->free_cluster_tail = idx;
> +			} else {
> +				cluster_set_next(
> +					cluster_info[p->free_cluster_tail],
> +					idx);
> +				p->free_cluster_tail = idx;
> +			}
> +		}
> +		idx++;
> +		if (idx == nr_clusters)
> +			idx = 0;
> +	}
> +
>  	return nr_extents;
>  }
>  
> @@ -2016,6 +2166,7 @@ SYSCALL_DEFINE2(swapon, const char __use
>  	sector_t span;
>  	unsigned long maxpages;
>  	unsigned char *swap_map = NULL;
> +	unsigned int *cluster_info = NULL;
>  	unsigned long *frontswap_map = NULL;
>  	struct page *page = NULL;
>  	struct inode *inode = NULL;
> @@ -2089,13 +2240,24 @@ SYSCALL_DEFINE2(swapon, const char __use
>  		error = -ENOMEM;
>  		goto bad_swap;
>  	}
> +	if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
> +		p->flags |= SWP_SOLIDSTATE;
> +		p->cluster_next = 1 + (random32() % p->highest_bit);
> +
> +		cluster_info = vzalloc(DIV_ROUND_UP(maxpages,
> +			SWAPFILE_CLUSTER) * sizeof(*cluster_info));
> +		if (!cluster_info) {
> +			error = -ENOMEM;
> +			goto bad_swap;
> +		}
> +	}
>  
>  	error = swap_cgroup_swapon(p->type, maxpages);
>  	if (error)
>  		goto bad_swap;
>  
>  	nr_extents = setup_swap_map_and_extents(p, swap_header, swap_map,
> -		maxpages, &span);
> +		cluster_info, maxpages, &span);
>  	if (unlikely(nr_extents < 0)) {
>  		error = nr_extents;
>  		goto bad_swap;
> @@ -2104,21 +2266,15 @@ SYSCALL_DEFINE2(swapon, const char __use
>  	if (frontswap_enabled)
>  		frontswap_map = vzalloc(maxpages / sizeof(long));
>  
> -	if (p->bdev) {
> -		if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
> -			p->flags |= SWP_SOLIDSTATE;
> -			p->cluster_next = 1 + (random32() % p->highest_bit);
> -		}
> -		if ((swap_flags & SWAP_FLAG_DISCARD) && discard_swap(p) == 0)
> -			p->flags |= SWP_DISCARDABLE;
> -	}
> +	if (p->bdev && (swap_flags & SWAP_FLAG_DISCARD) && discard_swap(p) == 0)
> +		p->flags |= SWP_DISCARDABLE;
>  
>  	mutex_lock(&swapon_mutex);
>  	prio = -1;
>  	if (swap_flags & SWAP_FLAG_PREFER)
>  		prio =
>  		  (swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT;
> -	enable_swap_info(p, prio, swap_map, frontswap_map);
> +	enable_swap_info(p, prio, swap_map, cluster_info, frontswap_map);
>  
>  	printk(KERN_INFO "Adding %uk swap on %s.  "
>  			"Priority:%d extents:%d across:%lluk %s%s%s\n",
> @@ -2148,6 +2304,7 @@ bad_swap:
>  	p->flags = 0;
>  	spin_unlock(&swap_lock);
>  	vfree(swap_map);
> +	vfree(cluster_info);
>  	if (swap_file) {
>  		if (inode && S_ISREG(inode->i_mode)) {
>  			mutex_unlock(&inode->i_mutex);
> 
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