mm: remove per-zone hashtable of bitlock waitqueues

The per-zone waitqueues exist because of a scalability issue with the
page waitqueues on some NUMA machines, but it turns out that they hurt
normal loads, and now with the vmalloced stacks they also end up
breaking gfs2 that uses a bit_wait on a stack object:

     wait_on_bit(&gh->gh_iflags, HIF_WAIT, TASK_UNINTERRUPTIBLE)

where 'gh' can be a reference to the local variable 'mount_gh' on the
stack of fill_super().

The reason the per-zone hash table breaks for this case is that there is
no "zone" for virtual allocations, and trying to look up the physical
page to get at it will fail (with a BUG_ON()).

It turns out that I actually complained to the mm people about the
per-zone hash table for another reason just a month ago: the zone lookup
also hurts the regular use of "unlock_page()" a lot, because the zone
lookup ends up forcing several unnecessary cache misses and generates
horrible code.

As part of that earlier discussion, we had a much better solution for
the NUMA scalability issue - by just making the page lock have a
separate contention bit, the waitqueue doesn't even have to be looked at
for the normal case.

Peter Zijlstra already has a patch for that, but let's see if anybody
even notices.  In the meantime, let's fix the actual gfs2 breakage by
simplifying the bitlock waitqueues and removing the per-zone issue.
Reported-by: Andreas Gruenbacher <agruenba@redhat.com>
Tested-by: Bob Peterson <rpeterso@redhat.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -440,33 +440,7 @@ struct zone {
 	seqlock_t		span_seqlock;
 #endif
 
-	/*
-	 * wait_table		-- the array holding the hash table
-	 * wait_table_hash_nr_entries	-- the size of the hash table array
-	 * wait_table_bits	-- wait_table_size == (1 << wait_table_bits)
-	 *
-	 * The purpose of all these is to keep track of the people
-	 * waiting for a page to become available and make them
-	 * runnable again when possible. The trouble is that this
-	 * consumes a lot of space, especially when so few things
-	 * wait on pages at a given time. So instead of using
-	 * per-page waitqueues, we use a waitqueue hash table.
-	 *
-	 * The bucket discipline is to sleep on the same queue when
-	 * colliding and wake all in that wait queue when removing.
-	 * When something wakes, it must check to be sure its page is
-	 * truly available, a la thundering herd. The cost of a
-	 * collision is great, but given the expected load of the
-	 * table, they should be so rare as to be outweighed by the
-	 * benefits from the saved space.
-	 *
-	 * __wait_on_page_locked() and unlock_page() in mm/filemap.c, are the
-	 * primary users of these fields, and in mm/page_alloc.c
-	 * free_area_init_core() performs the initialization of them.
-	 */
-	wait_queue_head_t	*wait_table;
-	unsigned long		wait_table_hash_nr_entries;
-	unsigned long		wait_table_bits;
+	int initialized;
 
 	/* Write-intensive fields used from the page allocator */
 	ZONE_PADDING(_pad1_)
@@ -546,7 +520,7 @@ static inline bool zone_spans_pfn(const struct zone *zone, unsigned long pfn)
 
 static inline bool zone_is_initialized(struct zone *zone)
 {
-	return !!zone->wait_table;
+	return zone->initialized;
 }
 
 static inline bool zone_is_empty(struct zone *zone)

kernel/sched/core.c

@@ -7515,11 +7515,27 @@ static struct kmem_cache *task_group_cache __read_mostly;
 DECLARE_PER_CPU(cpumask_var_t, load_balance_mask);
 DECLARE_PER_CPU(cpumask_var_t, select_idle_mask);
 
+#define WAIT_TABLE_BITS 8
+#define WAIT_TABLE_SIZE (1 << WAIT_TABLE_BITS)
+static wait_queue_head_t bit_wait_table[WAIT_TABLE_SIZE] __cacheline_aligned;
+
+wait_queue_head_t *bit_waitqueue(void *word, int bit)
+{
+	const int shift = BITS_PER_LONG == 32 ? 5 : 6;
+	unsigned long val = (unsigned long)word << shift | bit;
+
+	return bit_wait_table + hash_long(val, WAIT_TABLE_BITS);
+}
+EXPORT_SYMBOL(bit_waitqueue);
+
 void __init sched_init(void)
 {
 	int i, j;
 	unsigned long alloc_size = 0, ptr;
 
+	for (i = 0; i < WAIT_TABLE_SIZE; i++)
+		init_waitqueue_head(bit_wait_table + i);
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	alloc_size += 2 * nr_cpu_ids * sizeof(void **);
 #endif

kernel/sched/wait.c

@@ -480,16 +480,6 @@ void wake_up_bit(void *word, int bit)
 }
 EXPORT_SYMBOL(wake_up_bit);
 
-wait_queue_head_t *bit_waitqueue(void *word, int bit)
-{
-	const int shift = BITS_PER_LONG == 32 ? 5 : 6;
-	const struct zone *zone = page_zone(virt_to_page(word));
-	unsigned long val = (unsigned long)word << shift | bit;
-
-	return &zone->wait_table[hash_long(val, zone->wait_table_bits)];
-}
-EXPORT_SYMBOL(bit_waitqueue);
-
 /*
  * Manipulate the atomic_t address to produce a better bit waitqueue table hash
  * index (we're keying off bit -1, but that would produce a horrible hash

mm/filemap.c

@@ -790,9 +790,7 @@ EXPORT_SYMBOL(__page_cache_alloc);
  */
 wait_queue_head_t *page_waitqueue(struct page *page)
 {
-	const struct zone *zone = page_zone(page);
-
-	return &zone->wait_table[hash_ptr(page, zone->wait_table_bits)];
+	return bit_waitqueue(page, 0);
 }
 EXPORT_SYMBOL(page_waitqueue);
 

mm/memory_hotplug.c

@@ -268,7 +268,6 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat)
 	unsigned long i, pfn, end_pfn, nr_pages;
 	int node = pgdat->node_id;
 	struct page *page;
-	struct zone *zone;
 
 	nr_pages = PAGE_ALIGN(sizeof(struct pglist_data)) >> PAGE_SHIFT;
 	page = virt_to_page(pgdat);
@@ -276,19 +275,6 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat)
 	for (i = 0; i < nr_pages; i++, page++)
 		get_page_bootmem(node, page, NODE_INFO);
 
-	zone = &pgdat->node_zones[0];
-	for (; zone < pgdat->node_zones + MAX_NR_ZONES - 1; zone++) {
-		if (zone_is_initialized(zone)) {
-			nr_pages = zone->wait_table_hash_nr_entries
-				* sizeof(wait_queue_head_t);
-			nr_pages = PAGE_ALIGN(nr_pages) >> PAGE_SHIFT;
-			page = virt_to_page(zone->wait_table);
-
-			for (i = 0; i < nr_pages; i++, page++)
-				get_page_bootmem(node, page, NODE_INFO);
-		}
-	}
-
 	pfn = pgdat->node_start_pfn;
 	end_pfn = pgdat_end_pfn(pgdat);
 
@@ -2158,20 +2144,6 @@ void try_offline_node(int nid)
 	 */
 	node_set_offline(nid);
 	unregister_one_node(nid);
-
-	/* free waittable in each zone */
-	for (i = 0; i < MAX_NR_ZONES; i++) {
-		struct zone *zone = pgdat->node_zones + i;
-
-		/*
-		 * wait_table may be allocated from boot memory,
-		 * here only free if it's allocated by vmalloc.
-		 */
-		if (is_vmalloc_addr(zone->wait_table)) {
-			vfree(zone->wait_table);
-			zone->wait_table = NULL;
-		}
-	}
 }
 EXPORT_SYMBOL(try_offline_node);
 

mm/page_alloc.c

@@ -4976,72 +4976,6 @@ void __ref build_all_zonelists(pg_data_t *pgdat, struct zone *zone)
 #endif
 }
 
-/*
- * Helper functions to size the waitqueue hash table.
- * Essentially these want to choose hash table sizes sufficiently
- * large so that collisions trying to wait on pages are rare.
- * But in fact, the number of active page waitqueues on typical
- * systems is ridiculously low, less than 200. So this is even
- * conservative, even though it seems large.
- *
- * The constant PAGES_PER_WAITQUEUE specifies the ratio of pages to
- * waitqueues, i.e. the size of the waitq table given the number of pages.
- */
-#define PAGES_PER_WAITQUEUE	256
-
-#ifndef CONFIG_MEMORY_HOTPLUG
-static inline unsigned long wait_table_hash_nr_entries(unsigned long pages)
-{
-	unsigned long size = 1;
-
-	pages /= PAGES_PER_WAITQUEUE;
-
-	while (size < pages)
-		size <<= 1;
-
-	/*
-	 * Once we have dozens or even hundreds of threads sleeping
-	 * on IO we've got bigger problems than wait queue collision.
-	 * Limit the size of the wait table to a reasonable size.
-	 */
-	size = min(size, 4096UL);
-
-	return max(size, 4UL);
-}
-#else
-/*
- * A zone's size might be changed by hot-add, so it is not possible to determine
- * a suitable size for its wait_table.  So we use the maximum size now.
- *
- * The max wait table size = 4096 x sizeof(wait_queue_head_t).   ie:
- *
- *    i386 (preemption config)    : 4096 x 16 = 64Kbyte.
- *    ia64, x86-64 (no preemption): 4096 x 20 = 80Kbyte.
- *    ia64, x86-64 (preemption)   : 4096 x 24 = 96Kbyte.
- *
- * The maximum entries are prepared when a zone's memory is (512K + 256) pages
- * or more by the traditional way. (See above).  It equals:
- *
- *    i386, x86-64, powerpc(4K page size) : =  ( 2G + 1M)byte.
- *    ia64(16K page size)                 : =  ( 8G + 4M)byte.
- *    powerpc (64K page size)             : =  (32G +16M)byte.
- */
-static inline unsigned long wait_table_hash_nr_entries(unsigned long pages)
-{
-	return 4096UL;
-}
-#endif
-
-/*
- * This is an integer logarithm so that shifts can be used later
- * to extract the more random high bits from the multiplicative
- * hash function before the remainder is taken.
- */
-static inline unsigned long wait_table_bits(unsigned long size)
-{
-	return ffz(~size);
-}
-
 /*
  * Initially all pages are reserved - free ones are freed
  * up by free_all_bootmem() once the early boot process is
@@ -5304,49 +5238,6 @@ void __init setup_per_cpu_pageset(void)
 			alloc_percpu(struct per_cpu_nodestat);
 }
 
-static noinline __ref
-int zone_wait_table_init(struct zone *zone, unsigned long zone_size_pages)
-{
-	int i;
-	size_t alloc_size;
-
-	/*
-	 * The per-page waitqueue mechanism uses hashed waitqueues
-	 * per zone.
-	 */
-	zone->wait_table_hash_nr_entries =
-		 wait_table_hash_nr_entries(zone_size_pages);
-	zone->wait_table_bits =
-		wait_table_bits(zone->wait_table_hash_nr_entries);
-	alloc_size = zone->wait_table_hash_nr_entries
-					* sizeof(wait_queue_head_t);
-
-	if (!slab_is_available()) {
-		zone->wait_table = (wait_queue_head_t *)
-			memblock_virt_alloc_node_nopanic(
-				alloc_size, zone->zone_pgdat->node_id);
-	} else {
-		/*
-		 * This case means that a zone whose size was 0 gets new memory
-		 * via memory hot-add.
-		 * But it may be the case that a new node was hot-added.  In
-		 * this case vmalloc() will not be able to use this new node's
-		 * memory - this wait_table must be initialized to use this new
-		 * node itself as well.
-		 * To use this new node's memory, further consideration will be
-		 * necessary.
-		 */
-		zone->wait_table = vmalloc(alloc_size);
-	}
-	if (!zone->wait_table)
-		return -ENOMEM;
-
-	for (i = 0; i < zone->wait_table_hash_nr_entries; ++i)
-		init_waitqueue_head(zone->wait_table + i);
-
-	return 0;
-}
-
 static __meminit void zone_pcp_init(struct zone *zone)
 {
 	/*
@@ -5367,10 +5258,7 @@ int __meminit init_currently_empty_zone(struct zone *zone,
 					unsigned long size)
 {
 	struct pglist_data *pgdat = zone->zone_pgdat;
-	int ret;
-	ret = zone_wait_table_init(zone, size);
-	if (ret)
-		return ret;
+
 	pgdat->nr_zones = zone_idx(zone) + 1;
 
 	zone->zone_start_pfn = zone_start_pfn;
@@ -5382,6 +5270,7 @@ int __meminit init_currently_empty_zone(struct zone *zone,
 			zone_start_pfn, (zone_start_pfn + size));
 
 	zone_init_free_lists(zone);
+	zone->initialized = 1;
 
 	return 0;
 }