struct per_cpu_pages is protected by the pagesets lock but it can be
embedded within struct per_cpu_pages at a minor cost. This is possible
because per-cpu lookups are based on offsets. Paraphrasing an explanation
from Peter Ziljstra
The whole thing relies on:
&per_cpu_ptr(msblk->stream, cpu)->lock == per_cpu_ptr(&msblk->stream->lock, cpu)
Which is true because the lhs:
(local_lock_t *)((zone->per_cpu_pages + per_cpu_offset(cpu)) + offsetof(struct per_cpu_pages, lock))
and the rhs:
(local_lock_t *)((zone->per_cpu_pages + offsetof(struct per_cpu_pages, lock)) + per_cpu_offset(cpu))
are identical, because addition is associative.
More details are included in mmzone.h. This embedding is not completely
free for three reasons.
1. As local_lock does not return a per-cpu structure, the PCP has to
be looked up twice -- first to acquire the lock and again to get the
PCP pointer.
2. For PREEMPT_RT and CONFIG_DEBUG_LOCK_ALLOC, local_lock is potentially
a spinlock or has lock-specific tracking. In both cases, it becomes
necessary to release/acquire different locks when freeing a list of
pages in free_unref_page_list.
3. For most kernel configurations, local_lock_t is empty and no storage is
required. By embedding the lock, the memory consumption on PREEMPT_RT
and CONFIG_DEBUG_LOCK_ALLOC is higher.
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
include/linux/mmzone.h | 31 ++++++++++++++++-
mm/page_alloc.c | 78 +++++++++++++++++++++++++++---------------
2 files changed, 81 insertions(+), 28 deletions(-)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index d3b8889b76ab..a1f458f939db 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -338,8 +338,37 @@ enum zone_watermarks {
#define high_wmark_pages(z) (z->_watermark[WMARK_HIGH] + z->watermark_boost)
#define wmark_pages(z, i) (z->_watermark[i] + z->watermark_boost)
-/* Fields and list protected by pagesets local_lock in page_alloc.c */
+/*
+ * struct per_cpu_pages is a per-cpu structure protected by a lock
+ * embedded within the structure. This is subtle because ordinarily
+ * that would imply the PCP structure must first be located when
+ * the task is not yet pinned to the CPU e.g.
+ *
+ * migrate_disable();
+ * pcp = this_cpu_ptr(zone->per_cpu_pageset);
+ * local_lock_irqsave(&pcp->lock, flags);
+ *
+ * However, because per-cpu addresses are calculated based on offsets,
+ * the following is true
+ *
+ * &per_cpu_ptr(zone->per_cpu_pageset, cpu)->lock == per_cpu_ptr(&zone->per_cpu_pageset->lock, cpu)
+ *
+ * This is true because the LHS is
+ *
+ * (local_lock_t *)((zone->per_cpu_pageset + per_cpu_offset(cpu)) + offsetof(struct per_cpu_pageset, lock))
+ *
+ * while the RHS is
+ *
+ * (local_lock_t *)((zone->per_cpu_pageset + offsetof(struct per_cpu_pageset, lock)) + per_cpu_offset(cpu))
+ *
+ * local_lock will first disable preempt, migration or IRQs depending on
+ * the kernel configuration prior to the per_cpu_offset is calculated.
+ * Hence, the following is safe for both PREEMPT_RT and !PREEMPT_RT.
+ *
+ * local_lock_irqsave(&zone->per_cpu_pageset->lock, flags);
+ */
struct per_cpu_pages {
+ local_lock_t lock;
int count; /* number of pages in the list */
int high; /* high watermark, emptying needed */
int batch; /* chunk size for buddy add/remove */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 44a5eb067b38..52ce688b6c73 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -112,13 +112,6 @@ typedef int __bitwise fpi_t;
static DEFINE_MUTEX(pcp_batch_high_lock);
#define MIN_PERCPU_PAGELIST_FRACTION (8)
-struct pagesets {
- local_lock_t lock;
-};
-static DEFINE_PER_CPU(struct pagesets, pagesets) = {
- .lock = INIT_LOCAL_LOCK(lock),
-};
-
#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
DEFINE_PER_CPU(int, numa_node);
EXPORT_PER_CPU_SYMBOL(numa_node);
@@ -2985,12 +2978,12 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
unsigned long flags;
int to_drain, batch;
- local_lock_irqsave(&pagesets.lock, flags);
+ local_lock_irqsave(&pcp->lock, flags);
batch = READ_ONCE(pcp->batch);
to_drain = min(pcp->count, batch);
if (to_drain > 0)
free_pcppages_bulk(zone, to_drain, pcp);
- local_unlock_irqrestore(&pagesets.lock, flags);
+ local_unlock_irqrestore(&pcp->lock, flags);
}
#endif
@@ -3006,13 +2999,13 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone)
unsigned long flags;
struct per_cpu_pages *pcp;
- local_lock_irqsave(&pagesets.lock, flags);
+ local_lock_irqsave(&zone->per_cpu_pageset->lock, flags);
pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
if (pcp->count)
free_pcppages_bulk(zone, pcp->count, pcp);
- local_unlock_irqrestore(&pagesets.lock, flags);
+ local_unlock_irqrestore(&zone->per_cpu_pageset->lock, flags);
}
/*
@@ -3232,10 +3225,9 @@ static bool free_unref_page_prepare(struct page *page, unsigned long pfn)
return true;
}
-static void free_unref_page_commit(struct page *page, unsigned long pfn,
- int migratetype)
+static void free_unref_page_commit(struct page *page, struct zone *zone,
+ unsigned long pfn, int migratetype)
{
- struct zone *zone = page_zone(page);
struct per_cpu_pages *pcp;
__count_vm_event(PGFREE);
@@ -3251,6 +3243,7 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn,
*/
void free_unref_page(struct page *page)
{
+ struct zone *zone;
unsigned long flags;
unsigned long pfn = page_to_pfn(page);
int migratetype;
@@ -3274,9 +3267,10 @@ void free_unref_page(struct page *page)
migratetype = MIGRATE_MOVABLE;
}
- local_lock_irqsave(&pagesets.lock, flags);
- free_unref_page_commit(page, pfn, migratetype);
- local_unlock_irqrestore(&pagesets.lock, flags);
+ zone = page_zone(page);
+ local_lock_irqsave(&zone->per_cpu_pageset->lock, flags);
+ free_unref_page_commit(page, zone, pfn, migratetype);
+ local_unlock_irqrestore(&zone->per_cpu_pageset->lock, flags);
}
/*
@@ -3284,11 +3278,15 @@ void free_unref_page(struct page *page)
*/
void free_unref_page_list(struct list_head *list)
{
+ struct zone *locked_zone;
struct page *page, *next;
unsigned long flags, pfn;
int batch_count = 0;
int migratetype;
+ if (list_empty(list))
+ return;
+
/* Prepare pages for freeing */
list_for_each_entry_safe(page, next, list, lru) {
pfn = page_to_pfn(page);
@@ -3311,25 +3309,49 @@ void free_unref_page_list(struct list_head *list)
set_page_private(page, pfn);
}
- local_lock_irqsave(&pagesets.lock, flags);
+ /* Acquire the lock required for the first page. */
+ page = list_first_entry(list, struct page, lru);
+ locked_zone = page_zone(page);
+ local_lock_irqsave(&locked_zone->per_cpu_pageset->lock, flags);
+
list_for_each_entry_safe(page, next, list, lru) {
+ struct zone *zone = page_zone(page);
pfn = page_private(page);
set_page_private(page, 0);
migratetype = get_pcppage_migratetype(page);
trace_mm_page_free_batched(page);
- free_unref_page_commit(page, pfn, migratetype);
+
+ /*
+ * Switch locks if the zone changes.
+ *
+ * It is only necessary to do this for PREEMPT_RT and
+ * DEBUG_LOCK_ALLOC. For !PREEMPT_RT, switching zones just
+ * enables local IRQs and disables them again which is
+ * unnecessary. On PREEMPT_RT, local_lock may be a spinlock
+ * so switching is required. For DEBUG_LOCK_ALLOC, the
+ * lock dependencies should be properly checked.
+ */
+#if defined(CONFIG_PREEMPT_RT) || defined(CONFIG_DEBUG_LOCK_ALLOC)
+ if (locked_zone != zone) {
+ local_unlock_irqrestore(&locked_zone->per_cpu_pageset->lock, flags);
+ local_lock_irqsave(&zone->per_cpu_pageset->lock, flags);
+ locked_zone = zone;
+ }
+#endif
+
+ free_unref_page_commit(page, zone, pfn, migratetype);
/*
* Guard against excessive IRQ disabled times when we get
* a large list of pages to free.
*/
if (++batch_count == SWAP_CLUSTER_MAX) {
- local_unlock_irqrestore(&pagesets.lock, flags);
+ local_unlock_irqrestore(&locked_zone->per_cpu_pageset->lock, flags);
batch_count = 0;
- local_lock_irqsave(&pagesets.lock, flags);
+ local_lock_irqsave(&locked_zone->per_cpu_pageset->lock, flags);
}
}
- local_unlock_irqrestore(&pagesets.lock, flags);
+ local_unlock_irqrestore(&locked_zone->per_cpu_pageset->lock, flags);
}
/*
@@ -3487,11 +3509,11 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
struct page *page;
unsigned long flags;
- local_lock_irqsave(&pagesets.lock, flags);
+ local_lock_irqsave(&zone->per_cpu_pageset->lock, flags);
pcp = this_cpu_ptr(zone->per_cpu_pageset);
list = &pcp->lists[migratetype];
page = __rmqueue_pcplist(zone, migratetype, alloc_flags, pcp, list);
- local_unlock_irqrestore(&pagesets.lock, flags);
+ local_unlock_irqrestore(&zone->per_cpu_pageset->lock, flags);
if (page) {
__count_zid_vm_events(PGALLOC, page_zonenum(page), 1);
zone_statistics(preferred_zone, zone, 1);
@@ -5090,7 +5112,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
goto failed;
/* Attempt the batch allocation */
- local_lock_irqsave(&pagesets.lock, flags);
+ local_lock_irqsave(&zone->per_cpu_pageset->lock, flags);
pcp = this_cpu_ptr(zone->per_cpu_pageset);
pcp_list = &pcp->lists[ac.migratetype];
@@ -5120,7 +5142,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
nr_populated++;
}
- local_unlock_irqrestore(&pagesets.lock, flags);
+ local_unlock_irqrestore(&zone->per_cpu_pageset->lock, flags);
__count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
zone_statistics(ac.preferred_zoneref->zone, zone, nr_account);
@@ -5128,7 +5150,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
return nr_populated;
failed_irq:
- local_unlock_irqrestore(&pagesets.lock, flags);
+ local_unlock_irqrestore(&zone->per_cpu_pageset->lock, flags);
failed:
page = __alloc_pages(gfp, 0, preferred_nid, nodemask);
@@ -6616,6 +6638,8 @@ static void per_cpu_pages_init(struct per_cpu_pages *pcp, struct per_cpu_zonesta
memset(pcp, 0, sizeof(*pcp));
memset(pzstats, 0, sizeof(*pzstats));
+ local_lock_init(&pcp->lock);
+
for (migratetype = 0; migratetype < MIGRATE_PCPTYPES; migratetype++)
INIT_LIST_HEAD(&pcp->lists[migratetype]);
--
2.26.2
