Eric Lee / smarc-ti-linux-kernel | Embedian Git Server

Commit 579f82901f6f41256642936d7e632f3979ad76d4

Authored by Shaohua Li 2014-02-07 04:04:21 +0800

Committed by Linus Torvalds 2014-02-07 05:48:51 +0800

Exists in master and in 16 other branches

swap: add a simple detector for inappropriate swapin readahead

This is a patch to improve swap readahead algorithm.  It's from Hugh and
I slightly changed it.

Hugh's original changelog:

swapin readahead does a blind readahead, whether or not the swapin is
sequential.  This may be ok on harddisk, because large reads have
relatively small costs, and if the readahead pages are unneeded they can
be reclaimed easily - though, what if their allocation forced reclaim of
useful pages? But on SSD devices large reads are more expensive than
small ones: if the readahead pages are unneeded, reading them in caused
significant overhead.

This patch adds very simplistic random read detection.  Stealing the
PageReadahead technique from Konstantin Khlebnikov's patch, avoiding the
vma/anon_vma sophistications of Shaohua Li's patch, swapin_nr_pages()
simply looks at readahead's current success rate, and narrows or widens
its readahead window accordingly.  There is little science to its
heuristic: it's about as stupid as can be whilst remaining effective.

The table below shows elapsed times (in centiseconds) when running a
single repetitive swapping load across a 1000MB mapping in 900MB ram
with 1GB swap (the harddisk tests had taken painfully too long when I
used mem=500M, but SSD shows similar results for that).

Vanilla is the 3.6-rc7 kernel on which I started; Shaohua denotes his
Sep 3 patch in mmotm and linux-next; HughOld denotes my Oct 1 patch
which Shaohua showed to be defective; HughNew this Nov 14 patch, with
page_cluster as usual at default of 3 (8-page reads); HughPC4 this same
patch with page_cluster 4 (16-page reads); HughPC0 with page_cluster 0
(1-page reads: no readahead).

HDD for swapping to harddisk, SSD for swapping to VertexII SSD.  Seq for
sequential access to the mapping, cycling five times around; Rand for
the same number of random touches.  Anon for a MAP_PRIVATE anon mapping;
Shmem for a MAP_SHARED anon mapping, equivalent to tmpfs.

One weakness of Shaohua's vma/anon_vma approach was that it did not
optimize Shmem: seen below.  Konstantin's approach was perhaps mistuned,
50% slower on Seq: did not compete and is not shown below.

HDD        Vanilla Shaohua HughOld HughNew HughPC4 HughPC0
Seq Anon     73921   76210   75611   76904   78191  121542
Seq Shmem    73601   73176   73855   72947   74543  118322
Rand Anon   895392  831243  871569  845197  846496  841680
Rand Shmem 1058375 1053486  827935  764955  764376  756489

SSD        Vanilla Shaohua HughOld HughNew HughPC4 HughPC0
Seq Anon     24634   24198   24673   25107   21614   70018
Seq Shmem    24959   24932   25052   25703   22030   69678
Rand Anon    43014   26146   28075   25989   26935   25901
Rand Shmem   45349   45215   28249   24268   24138   24332

These tests are, of course, two extremes of a very simple case: under
heavier mixed loads I've not yet observed any consistent improvement or
degradation, and wider testing would be welcome.

Shaohua Li:

Test shows Vanilla is slightly better in sequential workload than Hugh's
patch.  I observed with Hugh's patch sometimes the readahead size is
shrinked too fast (from 8 to 1 immediately) in sequential workload if
there is no hit.  And in such case, continuing doing readahead is good
actually.

I don't prepare a sophisticated algorithm for the sequential workload
because so far we can't guarantee sequential accessed pages are swap out
sequentially.  So I slightly change Hugh's heuristic - don't shrink
readahead size too fast.

Here is my test result (unit second, 3 runs average):
	Vanilla		Hugh		New
Seq	356		370		360
Random	4525		2447		2444

Attached graph is the swapin/swapout throughput I collected with 'vmstat
2'.  The first part is running a random workload (till around 1200 of
the x-axis) and the second part is running a sequential workload.
swapin and swapout throughput are almost identical in steady state in
both workloads.  These are expected behavior.  while in Vanilla, swapin
is much bigger than swapout especially in random workload (because wrong
readahead).

Original patches by: Shaohua Li and Konstantin Khlebnikov.

[fengguang.wu@intel.com: swapin_nr_pages() can be static]
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Showing 2 changed files with 62 additions and 5 deletions Inline Diff

include/linux/page-flags.h
mm/swap_state.c

include/linux/page-flags.h

Diff comments View file @ 579f829

mm/swap_state.c

Diff comments View file @ 579f829

 /*
  *  linux/mm/swap_state.c
  *
  *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
  *  Swap reorganised 29.12.95, Stephen Tweedie
  *
  *  Rewritten to use page cache, (C) 1998 Stephen Tweedie
  */
 #include <linux/mm.h>
 #include <linux/gfp.h>
 #include <linux/kernel_stat.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <linux/init.h>
 #include <linux/pagemap.h>
 #include <linux/backing-dev.h>
 #include <linux/blkdev.h>
 #include <linux/pagevec.h>
 #include <linux/migrate.h>
 #include <linux/page_cgroup.h>
 #include <asm/pgtable.h>
 /*
  * swapper_space is a fiction, retained to simplify the path through
  * vmscan's shrink_page_list.
  */
 static const struct address_space_operations swap_aops = {
 	.writepage	= swap_writepage,
 	.set_page_dirty	= swap_set_page_dirty,
 	.migratepage	= migrate_page,
 };
 static struct backing_dev_info swap_backing_dev_info = {
 	.name		= "swap",
 	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
 };
 struct address_space swapper_spaces[MAX_SWAPFILES] = {
 	[0 ... MAX_SWAPFILES - 1] = {
 		.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
 		.a_ops		= &swap_aops,
 		.backing_dev_info = &swap_backing_dev_info,
 	}
 };
 #define INC_CACHE_INFO(x)	do { swap_cache_info.x++; } while (0)
 static struct {
 	unsigned long add_total;
 	unsigned long del_total;
 	unsigned long find_success;
 	unsigned long find_total;
 } swap_cache_info;
 unsigned long total_swapcache_pages(void)
 {
 	int i;
 	unsigned long ret = 0;
 	for (i = 0; i < MAX_SWAPFILES; i++)
 		ret += swapper_spaces[i].nrpages;
 	return ret;
 }
+static atomic_t swapin_readahead_hits = ATOMIC_INIT(4);
 void show_swap_cache_info(void)
 {
 	printk("%lu pages in swap cache\n", total_swapcache_pages());
 	printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu\n",
 		swap_cache_info.add_total, swap_cache_info.del_total,
 		swap_cache_info.find_success, swap_cache_info.find_total);
 	printk("Free swap  = %ldkB\n",
 		get_nr_swap_pages() << (PAGE_SHIFT - 10));
 	printk("Total swap = %lukB\n", total_swap_pages << (PAGE_SHIFT - 10));
 }
 /*
  * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
  * but sets SwapCache flag and private instead of mapping and index.
  */
 int __add_to_swap_cache(struct page *page, swp_entry_t entry)
 {
 	int error;
 	struct address_space *address_space;
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(PageSwapCache(page), page);
 	VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
 	page_cache_get(page);
 	SetPageSwapCache(page);
 	set_page_private(page, entry.val);
 	address_space = swap_address_space(entry);
 	spin_lock_irq(&address_space->tree_lock);
 	error = radix_tree_insert(&address_space->page_tree,
 					entry.val, page);
 	if (likely(!error)) {
 		address_space->nrpages++;
 		__inc_zone_page_state(page, NR_FILE_PAGES);
 		INC_CACHE_INFO(add_total);
 	}
 	spin_unlock_irq(&address_space->tree_lock);
 	if (unlikely(error)) {
 		/*
 		 * Only the context which have set SWAP_HAS_CACHE flag
 		 * would call add_to_swap_cache().
 		 * So add_to_swap_cache() doesn't returns -EEXIST.
 		 */
 		VM_BUG_ON(error == -EEXIST);
 		set_page_private(page, 0UL);
 		ClearPageSwapCache(page);
 		page_cache_release(page);
 	}
 	return error;
 }
 int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
 {
 	int error;
 	error = radix_tree_maybe_preload(gfp_mask);
 	if (!error) {
 		error = __add_to_swap_cache(page, entry);
 		radix_tree_preload_end();
 	}
 	return error;
 }
 /*
  * This must be called only on pages that have
  * been verified to be in the swap cache.
  */
 void __delete_from_swap_cache(struct page *page)
 {
 	swp_entry_t entry;
 	struct address_space *address_space;
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
 	VM_BUG_ON_PAGE(PageWriteback(page), page);
 	entry.val = page_private(page);
 	address_space = swap_address_space(entry);
 	radix_tree_delete(&address_space->page_tree, page_private(page));
 	set_page_private(page, 0);
 	ClearPageSwapCache(page);
 	address_space->nrpages--;
 	__dec_zone_page_state(page, NR_FILE_PAGES);
 	INC_CACHE_INFO(del_total);
 }
 /**
  * add_to_swap - allocate swap space for a page
  * @page: page we want to move to swap
  *
  * Allocate swap space for the page and add the page to the
  * swap cache.  Caller needs to hold the page lock.
  */
 int add_to_swap(struct page *page, struct list_head *list)
 {
 	swp_entry_t entry;
 	int err;
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(!PageUptodate(page), page);
 	entry = get_swap_page();
 	if (!entry.val)
 		return 0;
 	if (unlikely(PageTransHuge(page)))
 		if (unlikely(split_huge_page_to_list(page, list))) {
 			swapcache_free(entry, NULL);
 			return 0;
 		}
 	/*
 	 * Radix-tree node allocations from PF_MEMALLOC contexts could
 	 * completely exhaust the page allocator. __GFP_NOMEMALLOC
 	 * stops emergency reserves from being allocated.
 	 *
 	 * TODO: this could cause a theoretical memory reclaim
 	 * deadlock in the swap out path.
 	 */
 	/*
 	 * Add it to the swap cache and mark it dirty
 	 */
 	err = add_to_swap_cache(page, entry,
 			__GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
 	if (!err) {	/* Success */
 		SetPageDirty(page);
 		return 1;
 	} else {	/* -ENOMEM radix-tree allocation failure */
 		/*
 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
 		 * clear SWAP_HAS_CACHE flag.
 		 */
 		swapcache_free(entry, NULL);
 		return 0;
 	}
 }
 /*
  * This must be called only on pages that have
  * been verified to be in the swap cache and locked.
  * It will never put the page into the free list,
  * the caller has a reference on the page.
  */
 void delete_from_swap_cache(struct page *page)
 {
 	swp_entry_t entry;
 	struct address_space *address_space;
 	entry.val = page_private(page);
 	address_space = swap_address_space(entry);
 	spin_lock_irq(&address_space->tree_lock);
 	__delete_from_swap_cache(page);
 	spin_unlock_irq(&address_space->tree_lock);
 	swapcache_free(entry, page);
 	page_cache_release(page);
 }
 /*
  * If we are the only user, then try to free up the swap cache.
  *
  * Its ok to check for PageSwapCache without the page lock
  * here because we are going to recheck again inside
  * try_to_free_swap() _with_ the lock.
  * 					- Marcelo
  */
 static inline void free_swap_cache(struct page *page)
 {
 	if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
 		try_to_free_swap(page);
 		unlock_page(page);
 	}
 }
 /*
  * Perform a free_page(), also freeing any swap cache associated with
  * this page if it is the last user of the page.
  */
 void free_page_and_swap_cache(struct page *page)
 {
 	free_swap_cache(page);
 	page_cache_release(page);
 }
 /*
  * Passed an array of pages, drop them all from swapcache and then release
  * them.  They are removed from the LRU and freed if this is their last use.
  */
 void free_pages_and_swap_cache(struct page **pages, int nr)
 {
 	struct page **pagep = pages;
 	lru_add_drain();
 	while (nr) {
 		int todo = min(nr, PAGEVEC_SIZE);
 		int i;
 		for (i = 0; i < todo; i++)
 			free_swap_cache(pagep[i]);
 		release_pages(pagep, todo, 0);
 		pagep += todo;
 		nr -= todo;
 	}
 }
 /*
  * Lookup a swap entry in the swap cache. A found page will be returned
  * unlocked and with its refcount incremented - we rely on the kernel
  * lock getting page table operations atomic even if we drop the page
  * lock before returning.
  */
 struct page * lookup_swap_cache(swp_entry_t entry)
 {
 	struct page *page;
 	page = find_get_page(swap_address_space(entry), entry.val);
-	if (page)
+	if (page) {
 		INC_CACHE_INFO(find_success);
+		if (TestClearPageReadahead(page))
+			atomic_inc(&swapin_readahead_hits);
+	}
 	INC_CACHE_INFO(find_total);
 	return page;
 }
 /*
  * Locate a page of swap in physical memory, reserving swap cache space
  * and reading the disk if it is not already cached.
  * A failure return means that either the page allocation failed or that
  * the swap entry is no longer in use.
  */
 struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 			struct vm_area_struct *vma, unsigned long addr)
 {
 	struct page *found_page, *new_page = NULL;
 	int err;
 	do {
 		/*
 		 * First check the swap cache.  Since this is normally
 		 * called after lookup_swap_cache() failed, re-calling
 		 * that would confuse statistics.
 		 */
 		found_page = find_get_page(swap_address_space(entry),
 					entry.val);
 		if (found_page)
 			break;
 		/*
 		 * Get a new page to read into from swap.
 		 */
 		if (!new_page) {
 			new_page = alloc_page_vma(gfp_mask, vma, addr);
 			if (!new_page)
 				break;		/* Out of memory */
 		}
 		/*
 		 * call radix_tree_preload() while we can wait.
 		 */
 		err = radix_tree_maybe_preload(gfp_mask & GFP_KERNEL);
 		if (err)
 			break;
 		/*
 		 * Swap entry may have been freed since our caller observed it.
 		 */
 		err = swapcache_prepare(entry);
 		if (err == -EEXIST) {
 			radix_tree_preload_end();
 			/*
 			 * We might race against get_swap_page() and stumble
 			 * across a SWAP_HAS_CACHE swap_map entry whose page
 			 * has not been brought into the swapcache yet, while
 			 * the other end is scheduled away waiting on discard
 			 * I/O completion at scan_swap_map().
 			 *
 			 * In order to avoid turning this transitory state
 			 * into a permanent loop around this -EEXIST case
 			 * if !CONFIG_PREEMPT and the I/O completion happens
 			 * to be waiting on the CPU waitqueue where we are now
 			 * busy looping, we just conditionally invoke the
 			 * scheduler here, if there are some more important
 			 * tasks to run.
 			 */
 			cond_resched();
 			continue;
 		}
 		if (err) {		/* swp entry is obsolete ? */
 			radix_tree_preload_end();
 			break;
 		}
 		/* May fail (-ENOMEM) if radix-tree node allocation failed. */
 		__set_page_locked(new_page);
 		SetPageSwapBacked(new_page);
 		err = __add_to_swap_cache(new_page, entry);
 		if (likely(!err)) {
 			radix_tree_preload_end();
 			/*
 			 * Initiate read into locked page and return.
 			 */
 			lru_cache_add_anon(new_page);
 			swap_readpage(new_page);
 			return new_page;
 		}
 		radix_tree_preload_end();
 		ClearPageSwapBacked(new_page);
 		__clear_page_locked(new_page);
 		/*
 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
 		 * clear SWAP_HAS_CACHE flag.
 		 */
 		swapcache_free(entry, NULL);
 	} while (err != -ENOMEM);
 	if (new_page)
 		page_cache_release(new_page);
 	return found_page;
 }
+static unsigned long swapin_nr_pages(unsigned long offset)
+{
+	static unsigned long prev_offset;
+	unsigned int pages, max_pages, last_ra;
+	static atomic_t last_readahead_pages;
+	max_pages = 1 << ACCESS_ONCE(page_cluster);
+	if (max_pages <= 1)
+		return 1;
+	/*
+	 * This heuristic has been found to work well on both sequential and
+	 * random loads, swapping to hard disk or to SSD: please don't ask
+	 * what the "+ 2" means, it just happens to work well, that's all.
+	 */
+	pages = atomic_xchg(&swapin_readahead_hits, 0) + 2;
+	if (pages == 2) {
+		/*
+		 * We can have no readahead hits to judge by: but must not get
+		 * stuck here forever, so check for an adjacent offset instead
+		 * (and don't even bother to check whether swap type is same).
+		 */
+		if (offset != prev_offset + 1 && offset != prev_offset - 1)
+			pages = 1;
+		prev_offset = offset;
+	} else {
+		unsigned int roundup = 4;
+		while (roundup < pages)
+			roundup <<= 1;
+		pages = roundup;
+	}
+	if (pages > max_pages)
+		pages = max_pages;
+	/* Don't shrink readahead too fast */
+	last_ra = atomic_read(&last_readahead_pages) / 2;
+	if (pages < last_ra)
+		pages = last_ra;
+	atomic_set(&last_readahead_pages, pages);
+	return pages;
+}
 /**
  * swapin_readahead - swap in pages in hope we need them soon
  * @entry: swap entry of this memory
  * @gfp_mask: memory allocation flags
  * @vma: user vma this address belongs to
  * @addr: target address for mempolicy
  *
  * Returns the struct page for entry and addr, after queueing swapin.
  *
  * Primitive swap readahead code. We simply read an aligned block of
  * (1 << page_cluster) entries in the swap area. This method is chosen
  * because it doesn't cost us any seek time.  We also make sure to queue
  * the 'original' request together with the readahead ones...
  *
  * This has been extended to use the NUMA policies from the mm triggering
  * the readahead.
  *
  * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
  */
 struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
 			struct vm_area_struct *vma, unsigned long addr)
 {
 	struct page *page;
-	unsigned long offset = swp_offset(entry);
+	unsigned long entry_offset = swp_offset(entry);
+	unsigned long offset = entry_offset;
 	unsigned long start_offset, end_offset;
-	unsigned long mask = (1UL << page_cluster) - 1;
+	unsigned long mask;
 	struct blk_plug plug;
+	mask = swapin_nr_pages(offset) - 1;
+	if (!mask)
+		goto skip;
 	/* Read a page_cluster sized and aligned cluster around offset. */
 	start_offset = offset & ~mask;
 	end_offset = offset | mask;
 	if (!start_offset)	/* First page is swap header. */
 		start_offset++;
 	blk_start_plug(&plug);
 	for (offset = start_offset; offset <= end_offset ; offset++) {
 		/* Ok, do the async read-ahead now */
 		page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
 						gfp_mask, vma, addr);
 		if (!page)
 			continue;
+		if (offset != entry_offset)
+			SetPageReadahead(page);
 		page_cache_release(page);
 	}
 	blk_finish_plug(&plug);
 	lru_add_drain();	/* Push any new pages onto the LRU now */
+skip:
 	return read_swap_cache_async(entry, gfp_mask, vma, addr);
 }

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1	/*	1	/*
2	* Macros for manipulating and testing page->flags	2	* Macros for manipulating and testing page->flags
3	*/	3	*/
4		4
5	#ifndef PAGE_FLAGS_H	5	#ifndef PAGE_FLAGS_H
6	#define PAGE_FLAGS_H	6	#define PAGE_FLAGS_H
7		7
8	#include <linux/types.h>	8	#include <linux/types.h>
9	#include <linux/bug.h>	9	#include <linux/bug.h>
10	#include <linux/mmdebug.h>	10	#include <linux/mmdebug.h>
11	#ifndef __GENERATING_BOUNDS_H	11	#ifndef __GENERATING_BOUNDS_H
12	#include <linux/mm_types.h>	12	#include <linux/mm_types.h>
13	#include <generated/bounds.h>	13	#include <generated/bounds.h>
14	#endif /* !__GENERATING_BOUNDS_H */	14	#endif /* !__GENERATING_BOUNDS_H */
15		15
16	/*	16	/*
17	* Various page->flags bits:	17	* Various page->flags bits:
18	*	18	*
19	* PG_reserved is set for special pages, which can never be swapped out. Some	19	* PG_reserved is set for special pages, which can never be swapped out. Some
20	* of them might not even exist (eg empty_bad_page)...	20	* of them might not even exist (eg empty_bad_page)...
21	*	21	*
22	* The PG_private bitflag is set on pagecache pages if they contain filesystem	22	* The PG_private bitflag is set on pagecache pages if they contain filesystem
23	* specific data (which is normally at page->private). It can be used by	23	* specific data (which is normally at page->private). It can be used by
24	* private allocations for its own usage.	24	* private allocations for its own usage.
25	*	25	*
26	* During initiation of disk I/O, PG_locked is set. This bit is set before I/O	26	* During initiation of disk I/O, PG_locked is set. This bit is set before I/O
27	* and cleared when writeback _starts_ or when read _completes_. PG_writeback	27	* and cleared when writeback _starts_ or when read _completes_. PG_writeback
28	* is set before writeback starts and cleared when it finishes.	28	* is set before writeback starts and cleared when it finishes.
29	*	29	*
30	* PG_locked also pins a page in pagecache, and blocks truncation of the file	30	* PG_locked also pins a page in pagecache, and blocks truncation of the file
31	* while it is held.	31	* while it is held.
32	*	32	*
33	* page_waitqueue(page) is a wait queue of all tasks waiting for the page	33	* page_waitqueue(page) is a wait queue of all tasks waiting for the page
34	* to become unlocked.	34	* to become unlocked.
35	*	35	*
36	* PG_uptodate tells whether the page's contents is valid. When a read	36	* PG_uptodate tells whether the page's contents is valid. When a read
37	* completes, the page becomes uptodate, unless a disk I/O error happened.	37	* completes, the page becomes uptodate, unless a disk I/O error happened.
38	*	38	*
39	* PG_referenced, PG_reclaim are used for page reclaim for anonymous and	39	* PG_referenced, PG_reclaim are used for page reclaim for anonymous and
40	* file-backed pagecache (see mm/vmscan.c).	40	* file-backed pagecache (see mm/vmscan.c).