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Subject[PATCH 4/4] mm, page_alloc: Add a bulk page allocator
Date
This patch adds a new page allocator interface via alloc_pages_bulk,
__alloc_pages_bulk and __alloc_pages_bulk_nodemask. A caller requests a
number of pages to be allocated and added to a list. They can be freed in
bulk using free_pages_bulk(). Note that it would theoretically be possible
to use free_hot_cold_page_list for faster frees if the symbol was exported,
the refcounts were 0 and the caller guaranteed it was not in an interrupt.
This would be significantly faster in the free path but also more unsafer
and a harder API to use.

The API is not guaranteed to return the requested number of pages and
may fail if the preferred allocation zone has limited free memory, the
cpuset changes during the allocation or page debugging decides to fail
an allocation. It's up to the caller to request more pages in batch if
necessary.

The following compares the allocation cost per page for different batch
sizes. The baseline is allocating them one at a time and it compares with
the performance when using the new allocation interface.

pagealloc
4.10.0-rc2 4.10.0-rc2
one-at-a-time bulk-v2
Amean alloc-odr0-1 259.54 ( 0.00%) 106.62 ( 58.92%)
Amean alloc-odr0-2 193.38 ( 0.00%) 76.38 ( 60.50%)
Amean alloc-odr0-4 162.38 ( 0.00%) 57.23 ( 64.76%)
Amean alloc-odr0-8 144.31 ( 0.00%) 48.77 ( 66.20%)
Amean alloc-odr0-16 134.08 ( 0.00%) 45.38 ( 66.15%)
Amean alloc-odr0-32 128.62 ( 0.00%) 42.77 ( 66.75%)
Amean alloc-odr0-64 126.00 ( 0.00%) 41.00 ( 67.46%)
Amean alloc-odr0-128 125.00 ( 0.00%) 40.08 ( 67.94%)
Amean alloc-odr0-256 136.62 ( 0.00%) 56.00 ( 59.01%)
Amean alloc-odr0-512 152.00 ( 0.00%) 69.00 ( 54.61%)
Amean alloc-odr0-1024 158.00 ( 0.00%) 76.23 ( 51.75%)
Amean alloc-odr0-2048 163.00 ( 0.00%) 81.15 ( 50.21%)
Amean alloc-odr0-4096 169.77 ( 0.00%) 85.92 ( 49.39%)
Amean alloc-odr0-8192 170.00 ( 0.00%) 88.00 ( 48.24%)
Amean alloc-odr0-16384 170.00 ( 0.00%) 89.00 ( 47.65%)
Amean free-odr0-1 88.69 ( 0.00%) 55.69 ( 37.21%)
Amean free-odr0-2 66.00 ( 0.00%) 49.38 ( 25.17%)
Amean free-odr0-4 54.23 ( 0.00%) 45.38 ( 16.31%)
Amean free-odr0-8 48.23 ( 0.00%) 44.23 ( 8.29%)
Amean free-odr0-16 47.00 ( 0.00%) 45.00 ( 4.26%)
Amean free-odr0-32 44.77 ( 0.00%) 43.92 ( 1.89%)
Amean free-odr0-64 44.00 ( 0.00%) 43.00 ( 2.27%)
Amean free-odr0-128 43.00 ( 0.00%) 43.00 ( 0.00%)
Amean free-odr0-256 60.69 ( 0.00%) 60.46 ( 0.38%)
Amean free-odr0-512 79.23 ( 0.00%) 76.00 ( 4.08%)
Amean free-odr0-1024 86.00 ( 0.00%) 85.38 ( 0.72%)
Amean free-odr0-2048 91.00 ( 0.00%) 91.23 ( -0.25%)
Amean free-odr0-4096 94.85 ( 0.00%) 95.62 ( -0.81%)
Amean free-odr0-8192 97.00 ( 0.00%) 97.00 ( 0.00%)
Amean free-odr0-16384 98.00 ( 0.00%) 97.46 ( 0.55%)
Amean total-odr0-1 348.23 ( 0.00%) 162.31 ( 53.39%)
Amean total-odr0-2 259.38 ( 0.00%) 125.77 ( 51.51%)
Amean total-odr0-4 216.62 ( 0.00%) 102.62 ( 52.63%)
Amean total-odr0-8 192.54 ( 0.00%) 93.00 ( 51.70%)
Amean total-odr0-16 181.08 ( 0.00%) 90.38 ( 50.08%)
Amean total-odr0-32 173.38 ( 0.00%) 86.69 ( 50.00%)
Amean total-odr0-64 170.00 ( 0.00%) 84.00 ( 50.59%)
Amean total-odr0-128 168.00 ( 0.00%) 83.08 ( 50.55%)
Amean total-odr0-256 197.31 ( 0.00%) 116.46 ( 40.97%)
Amean total-odr0-512 231.23 ( 0.00%) 145.00 ( 37.29%)
Amean total-odr0-1024 244.00 ( 0.00%) 161.62 ( 33.76%)
Amean total-odr0-2048 254.00 ( 0.00%) 172.38 ( 32.13%)
Amean total-odr0-4096 264.62 ( 0.00%) 181.54 ( 31.40%)
Amean total-odr0-8192 267.00 ( 0.00%) 185.00 ( 30.71%)
Amean total-odr0-16384 268.00 ( 0.00%) 186.46 ( 30.42%)

It shows a roughly 50-60% reduction in the cost of allocating pages.
The free paths are not improved as much but relatively little can be batched
there. It's not quite as fast as it could be but taking further shortcuts
would require making a lot of assumptions about the state of the page and
the context of the caller.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
include/linux/gfp.h | 24 +++++++++++
mm/page_alloc.c | 116 +++++++++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 139 insertions(+), 1 deletion(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 4175dca4ac39..b2fe171ee1c4 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -433,6 +433,29 @@ __alloc_pages(gfp_t gfp_mask, unsigned int order,
return __alloc_pages_nodemask(gfp_mask, order, zonelist, NULL);
}

+unsigned long
+__alloc_pages_bulk_nodemask(gfp_t gfp_mask, unsigned int order,
+ struct zonelist *zonelist, nodemask_t *nodemask,
+ unsigned long nr_pages, struct list_head *alloc_list);
+
+static inline unsigned long
+__alloc_pages_bulk(gfp_t gfp_mask, unsigned int order,
+ struct zonelist *zonelist, unsigned long nr_pages,
+ struct list_head *list)
+{
+ return __alloc_pages_bulk_nodemask(gfp_mask, order, zonelist, NULL,
+ nr_pages, list);
+}
+
+static inline unsigned long
+alloc_pages_bulk(gfp_t gfp_mask, unsigned int order,
+ unsigned long nr_pages, struct list_head *list)
+{
+ int nid = numa_mem_id();
+ return __alloc_pages_bulk(gfp_mask, order,
+ node_zonelist(nid, gfp_mask), nr_pages, list);
+}
+
/*
* Allocate pages, preferring the node given as nid. The node must be valid and
* online. For more general interface, see alloc_pages_node().
@@ -504,6 +527,7 @@ extern void __free_pages(struct page *page, unsigned int order);
extern void free_pages(unsigned long addr, unsigned int order);
extern void free_hot_cold_page(struct page *page, bool cold);
extern void free_hot_cold_page_list(struct list_head *list, bool cold);
+extern void free_pages_bulk(struct list_head *list);

struct page_frag_cache;
extern void __page_frag_drain(struct page *page, unsigned int order,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 232cadbe9231..4f142270fbf0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2485,7 +2485,7 @@ void free_hot_cold_page(struct page *page, bool cold)
}

/*
- * Free a list of 0-order pages
+ * Free a list of 0-order pages whose reference count is already zero.
*/
void free_hot_cold_page_list(struct list_head *list, bool cold)
{
@@ -2495,7 +2495,28 @@ void free_hot_cold_page_list(struct list_head *list, bool cold)
trace_mm_page_free_batched(page, cold);
free_hot_cold_page(page, cold);
}
+
+ INIT_LIST_HEAD(list);
+}
+
+/* Drop reference counts and free pages from a list */
+void free_pages_bulk(struct list_head *list)
+{
+ struct page *page, *next;
+ bool free_percpu = !in_interrupt();
+
+ list_for_each_entry_safe(page, next, list, lru) {
+ trace_mm_page_free_batched(page, 0);
+ if (put_page_testzero(page)) {
+ list_del(&page->lru);
+ if (free_percpu)
+ free_hot_cold_page(page, false);
+ else
+ __free_pages_ok(page, 0);
+ }
+ }
}
+EXPORT_SYMBOL_GPL(free_pages_bulk);

/*
* split_page takes a non-compound higher-order page, and splits it into
@@ -3887,6 +3908,99 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
EXPORT_SYMBOL(__alloc_pages_nodemask);

/*
+ * This is a batched version of the page allocator that attempts to
+ * allocate nr_pages quickly from the preferred zone and add them to list.
+ * Note that there is no guarantee that nr_pages will be allocated although
+ * every effort will be made to allocate at least one. Unlike the core
+ * allocator, no special effort is made to recover from transient
+ * failures caused by changes in cpusets. It should only be used from !IRQ
+ * context. An attempt to allocate a batch of patches from an interrupt
+ * will allocate a single page.
+ */
+unsigned long
+__alloc_pages_bulk_nodemask(gfp_t gfp_mask, unsigned int order,
+ struct zonelist *zonelist, nodemask_t *nodemask,
+ unsigned long nr_pages, struct list_head *alloc_list)
+{
+ struct page *page;
+ unsigned long alloced = 0;
+ unsigned int alloc_flags = ALLOC_WMARK_LOW;
+ struct zone *zone;
+ struct per_cpu_pages *pcp;
+ struct list_head *pcp_list;
+ int migratetype;
+ gfp_t alloc_mask = gfp_mask; /* The gfp_t that was actually used for allocation */
+ struct alloc_context ac = { };
+ bool cold = ((gfp_mask & __GFP_COLD) != 0);
+
+ /* If there are already pages on the list, don't bother */
+ if (!list_empty(alloc_list))
+ return 0;
+
+ /* Only handle bulk allocation of order-0 */
+ if (order || in_interrupt())
+ goto failed;
+
+ gfp_mask &= gfp_allowed_mask;
+ if (!prepare_alloc_pages(gfp_mask, order, zonelist, nodemask, &ac, &alloc_mask, &alloc_flags))
+ return 0;
+
+ finalise_ac(gfp_mask, order, &ac);
+ if (!ac.preferred_zoneref)
+ return 0;
+
+ /*
+ * Only attempt a batch allocation if watermarks on the preferred zone
+ * are safe.
+ */
+ zone = ac.preferred_zoneref->zone;
+ if (!zone_watermark_fast(zone, order, zone->watermark[ALLOC_WMARK_HIGH] + nr_pages,
+ zonelist_zone_idx(ac.preferred_zoneref), alloc_flags))
+ goto failed;
+
+ /* Attempt the batch allocation */
+ migratetype = ac.migratetype;
+
+ preempt_disable();
+ pcp = &this_cpu_ptr(zone->pageset)->pcp;
+ pcp_list = &pcp->lists[migratetype];
+
+ while (nr_pages) {
+ page = __rmqueue_pcplist(zone, order, gfp_mask, migratetype,
+ cold, pcp, pcp_list);
+ if (!page)
+ break;
+
+ prep_new_page(page, order, gfp_mask, 0);
+ nr_pages--;
+ alloced++;
+ list_add(&page->lru, alloc_list);
+ }
+
+ if (!alloced) {
+ preempt_enable_no_resched();
+ goto failed;
+ }
+
+ __count_zid_vm_events(PGALLOC, zone_idx(zone), alloced);
+ zone_statistics(zone, zone, gfp_mask);
+
+ preempt_enable_no_resched();
+
+ return alloced;
+
+failed:
+ page = __alloc_pages_nodemask(gfp_mask, order, zonelist, nodemask);
+ if (page) {
+ alloced++;
+ list_add(&page->lru, alloc_list);
+ }
+
+ return alloced;
+}
+EXPORT_SYMBOL(__alloc_pages_bulk_nodemask);
+
+/*
* Common helper functions.
*/
unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
--
2.11.0
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