[PATCH 5/7] mm/page_alloc: Protect PCP lists with a spinlock

Mel Gorman
2022-06-13T17:31:59Z

Currently the PCP lists are protected by using local_lock_irqsave to
prevent migration and IRQ reentrancy but this is inconvenient. Remote
draining of the lists is impossible and a workqueue is required and every
task allocation/free must disable then enable interrupts which is
expensive.

As preparation for dealing with both of those problems, protect the lists
with a spinlock. The IRQ-unsafe version of the lock is used because IRQs
are already disabled by local_lock_irqsave. spin_trylock is used in
preparation for a time when local_lock could be used instead of
lock_lock_irqsave.

The per_cpu_pages still fits within the same number of cache lines after
this patch relative to before the series.

struct per_cpu_pages {
spinlock_t lock; /* 0 4 */
int count; /* 4 4 */
int high; /* 8 4 */
int batch; /* 12 4 */
short int free_factor; /* 16 2 */
short int expire; /* 18 2 */

/* XXX 4 bytes hole, try to pack */

struct list_head lists[13]; /* 24 208 */

/* size: 256, cachelines: 4, members: 7 */
/* sum members: 228, holes: 1, sum holes: 4 */
/* padding: 24 */
} __attribute__((__aligned__(64)));

There is overhead in the fast path due to acquiring the spinlock even
though the spinlock is per-cpu and uncontended in the common case. Page
Fault Test (PFT) running on a 1-socket reported the following results on a
1 socket machine.

5.18.0-rc1 5.18.0-rc1
vanilla mm-pcpdrain-v2r1
Hmean faults/sec-1 886331.5718 ( 0.00%) 885462.7479 ( -0.10%)
Hmean faults/sec-3 2337706.1583 ( 0.00%) 2332130.4909 * -0.24%*
Hmean faults/sec-5 2851594.2897 ( 0.00%) 2844123.9307 ( -0.26%)
Hmean faults/sec-7 3543251.5507 ( 0.00%) 3516889.0442 * -0.74%*
Hmean faults/sec-8 3947098.0024 ( 0.00%) 3916162.8476 * -0.78%*
Stddev faults/sec-1 2302.9105 ( 0.00%) 2065.0845 ( 10.33%)
Stddev faults/sec-3 7275.2442 ( 0.00%) 6033.2620 ( 17.07%)
Stddev faults/sec-5 24726.0328 ( 0.00%) 12525.1026 ( 49.34%)
Stddev faults/sec-7 9974.2542 ( 0.00%) 9543.9627 ( 4.31%)
Stddev faults/sec-8 9468.0191 ( 0.00%) 7958.2607 ( 15.95%)
CoeffVar faults/sec-1 0.2598 ( 0.00%) 0.2332 ( 10.24%)
CoeffVar faults/sec-3 0.3112 ( 0.00%) 0.2587 ( 16.87%)
CoeffVar faults/sec-5 0.8670 ( 0.00%) 0.4404 ( 49.21%)
CoeffVar faults/sec-7 0.2815 ( 0.00%) 0.2714 ( 3.60%)
CoeffVar faults/sec-8 0.2399 ( 0.00%) 0.2032 ( 15.28%)

There is a small hit in the number of faults per second but given that the
results are more stable, it's borderline noise.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Tested-by: Minchan Kim <minchan@kernel.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
---
include/linux/mmzone.h | 1 +
mm/page_alloc.c | 167 +++++++++++++++++++++++++++++++++++------
2 files changed, 147 insertions(+), 21 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 4e0352cc2fcb..299259cfe462 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -382,6 +382,7 @@ enum zone_watermarks {

/* Fields and list protected by pagesets local_lock in page_alloc.c */
struct per_cpu_pages {
+ spinlock_t lock; /* Protects lists field */
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 7fb262eeec2f..533fc5527582 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -133,6 +133,20 @@ static DEFINE_PER_CPU(struct pagesets, pagesets) = {
.lock = INIT_LOCAL_LOCK(lock),
};

+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
+/*
+ * On SMP, spin_trylock is sufficient protection.
+ * On PREEMPT_RT, spin_trylock is equivalent on both SMP and UP.
+ */
+#define pcp_trylock_prepare(flags) do { } while (0)
+#define pcp_trylock_finish(flag) do { } while (0)
+#else
+
+/* UP spin_trylock always succeeds so disable IRQs to prevent re-entrancy. */
+#define pcp_trylock_prepare(flags) local_irq_save(flags)
+#define pcp_trylock_finish(flags) local_irq_restore(flags)
+#endif
+
#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
DEFINE_PER_CPU(int, numa_node);
EXPORT_PER_CPU_SYMBOL(numa_node);
@@ -3097,15 +3111,22 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
*/
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);
batch = READ_ONCE(pcp->batch);
to_drain = min(pcp->count, batch);
- if (to_drain > 0)
+ if (to_drain > 0) {
+ unsigned long flags;
+
+ /*
+ * free_pcppages_bulk expects IRQs disabled for zone->lock
+ * so even though pcp->lock is not intended to be IRQ-safe,
+ * it's needed in this context.
+ */
+ spin_lock_irqsave(&pcp->lock, flags);
free_pcppages_bulk(zone, to_drain, pcp, 0);
- local_unlock_irqrestore(&pagesets.lock, flags);
+ spin_unlock_irqrestore(&pcp->lock, flags);
+ }
}
#endif

@@ -3118,16 +3139,17 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
*/
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);
-
pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
- if (pcp->count)
- free_pcppages_bulk(zone, pcp->count, pcp, 0);
+ if (pcp->count) {
+ unsigned long flags;

- local_unlock_irqrestore(&pagesets.lock, flags);
+ /* See drain_zone_pages on why this is disabling IRQs */
+ spin_lock_irqsave(&pcp->lock, flags);
+ free_pcppages_bulk(zone, pcp->count, pcp, 0);
+ spin_unlock_irqrestore(&pcp->lock, flags);
+ }
}

/*
@@ -3395,18 +3417,30 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
return min(READ_ONCE(pcp->batch) << 2, high);
}

-static void free_unref_page_commit(struct page *page, int migratetype,
- unsigned int order)
+/* Returns true if the page was committed to the per-cpu list. */
+static bool free_unref_page_commit(struct page *page, int migratetype,
+ unsigned int order, bool locked)
{
struct zone *zone = page_zone(page);
struct per_cpu_pages *pcp;
int high;
int pindex;
bool free_high;
+ unsigned long __maybe_unused UP_flags;

__count_vm_event(PGFREE);
pcp = this_cpu_ptr(zone->per_cpu_pageset);
pindex = order_to_pindex(migratetype, order);
+
+ if (!locked) {
+ /* Protect against a parallel drain. */
+ pcp_trylock_prepare(UP_flags);
+ if (!spin_trylock(&pcp->lock)) {
+ pcp_trylock_finish(UP_flags);
+ return false;
+ }
+ }
+
list_add(&page->pcp_list, &pcp->lists[pindex]);
pcp->count += 1 << order;

@@ -3424,6 +3458,13 @@ static void free_unref_page_commit(struct page *page, int migratetype,

free_pcppages_bulk(zone, nr_pcp_free(pcp, high, batch, free_high), pcp, pindex);
}
+
+ if (!locked) {
+ spin_unlock(&pcp->lock);
+ pcp_trylock_finish(UP_flags);
+ }
+
+ return true;
}

/*
@@ -3434,6 +3475,7 @@ void free_unref_page(struct page *page, unsigned int order)
unsigned long flags;
unsigned long pfn = page_to_pfn(page);
int migratetype;
+ bool freed_pcp = false;

if (!free_unref_page_prepare(page, pfn, order))
return;
@@ -3455,8 +3497,11 @@ void free_unref_page(struct page *page, unsigned int order)
}

local_lock_irqsave(&pagesets.lock, flags);
- free_unref_page_commit(page, migratetype, order);
+ freed_pcp = free_unref_page_commit(page, migratetype, order, false);
local_unlock_irqrestore(&pagesets.lock, flags);
+
+ if (unlikely(!freed_pcp))
+ free_one_page(page_zone(page), page, pfn, order, migratetype, FPI_NONE);
}

/*
@@ -3465,10 +3510,19 @@ void free_unref_page(struct page *page, unsigned int order)
void free_unref_page_list(struct list_head *list)
{
struct page *page, *next;
+ struct per_cpu_pages *pcp;
+ struct zone *locked_zone;
unsigned long flags;
int batch_count = 0;
int migratetype;

+ /*
+ * An empty list is possible. Check early so that the later
+ * lru_to_page() does not potentially read garbage.
+ */
+ if (list_empty(list))
+ return;
+
/* Prepare pages for freeing */
list_for_each_entry_safe(page, next, list, lru) {
unsigned long pfn = page_to_pfn(page);
@@ -3489,8 +3543,31 @@ void free_unref_page_list(struct list_head *list)
}
}

+ /*
+ * Preparation could have drained the list due to failing to prepare
+ * or all pages are being isolated.
+ */
+ if (list_empty(list))
+ return;
+
local_lock_irqsave(&pagesets.lock, flags);
+
+ page = lru_to_page(list);
+ locked_zone = page_zone(page);
+ pcp = this_cpu_ptr(locked_zone->per_cpu_pageset);
+ spin_lock(&pcp->lock);
+
list_for_each_entry_safe(page, next, list, lru) {
+ struct zone *zone = page_zone(page);
+
+ /* Different zone, different pcp lock. */
+ if (zone != locked_zone) {
+ spin_unlock(&pcp->lock);
+ locked_zone = zone;
+ pcp = this_cpu_ptr(zone->per_cpu_pageset);
+ spin_lock(&pcp->lock);
+ }
+
/*
* Non-isolated types over MIGRATE_PCPTYPES get added
* to the MIGRATE_MOVABLE pcp list.
@@ -3500,18 +3577,33 @@ void free_unref_page_list(struct list_head *list)
migratetype = MIGRATE_MOVABLE;

trace_mm_page_free_batched(page);
- free_unref_page_commit(page, migratetype, 0);
+
+ /*
+ * If there is a parallel drain in progress, free to the buddy
+ * allocator directly. This is expensive as the zone lock will
+ * be acquired multiple times but if a drain is in progress
+ * then an expensive operation is already taking place.
+ *
+ * TODO: Always false at the moment due to local_lock_irqsave
+ * and is preparation for converting to local_lock.
+ */
+ if (unlikely(!free_unref_page_commit(page, migratetype, 0, true)))
+ free_one_page(page_zone(page), page, page_to_pfn(page), 0, migratetype, FPI_NONE);

/*
* Guard against excessive IRQ disabled times when we get
* a large list of pages to free.
*/
if (++batch_count == SWAP_CLUSTER_MAX) {
+ spin_unlock(&pcp->lock);
local_unlock_irqrestore(&pagesets.lock, flags);
batch_count = 0;
local_lock_irqsave(&pagesets.lock, flags);
+ pcp = this_cpu_ptr(locked_zone->per_cpu_pageset);
+ spin_lock(&pcp->lock);
}
}
+ spin_unlock(&pcp->lock);
local_unlock_irqrestore(&pagesets.lock, flags);
}

@@ -3680,9 +3772,28 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
int migratetype,
unsigned int alloc_flags,
struct per_cpu_pages *pcp,
- struct list_head *list)
+ struct list_head *list,
+ bool locked)
{
struct page *page;
+ unsigned long __maybe_unused UP_flags;
+
+ /*
+ * spin_trylock is not necessary right now due to due to
+ * local_lock_irqsave and is a preparation step for
+ * a conversion to local_lock using the trylock to prevent
+ * IRQ re-entrancy. If pcp->lock cannot be acquired, the caller
+ * uses rmqueue_buddy.
+ *
+ * TODO: Convert local_lock_irqsave to local_lock.
+ */
+ if (unlikely(!locked)) {
+ pcp_trylock_prepare(UP_flags);
+ if (!spin_trylock(&pcp->lock)) {
+ pcp_trylock_finish(UP_flags);
+ return NULL;
+ }
+ }

do {
if (list_empty(list)) {
@@ -3703,8 +3814,10 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
migratetype, alloc_flags);

pcp->count += alloced << order;
- if (unlikely(list_empty(list)))
- return NULL;
+ if (unlikely(list_empty(list))) {
+ page = NULL;
+ goto out;
+ }
}

page = list_first_entry(list, struct page, pcp_list);
@@ -3712,6 +3825,12 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
pcp->count -= 1 << order;
} while (check_new_pcp(page, order));

+out:
+ if (!locked) {
+ spin_unlock(&pcp->lock);
+ pcp_trylock_finish(UP_flags);
+ }
+
return page;
}

@@ -3736,7 +3855,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
pcp = this_cpu_ptr(zone->per_cpu_pageset);
pcp->free_factor >>= 1;
list = &pcp->lists[order_to_pindex(migratetype, order)];
- page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list);
+ page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list, false);
local_unlock_irqrestore(&pagesets.lock, flags);
if (page) {
__count_zid_vm_events(PGALLOC, page_zonenum(page), 1);
@@ -3771,7 +3890,8 @@ struct page *rmqueue(struct zone *preferred_zone,
migratetype != MIGRATE_MOVABLE) {
page = rmqueue_pcplist(preferred_zone, zone, order,
gfp_flags, migratetype, alloc_flags);
- goto out;
+ if (likely(page))
+ goto out;
}
}

@@ -5343,6 +5463,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
local_lock_irqsave(&pagesets.lock, flags);
pcp = this_cpu_ptr(zone->per_cpu_pageset);
pcp_list = &pcp->lists[order_to_pindex(ac.migratetype, 0)];
+ spin_lock(&pcp->lock);

while (nr_populated < nr_pages) {

@@ -5353,11 +5474,13 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
}

page = __rmqueue_pcplist(zone, 0, ac.migratetype, alloc_flags,
- pcp, pcp_list);
+ pcp, pcp_list, true);
if (unlikely(!page)) {
/* Try and allocate at least one page */
- if (!nr_account)
+ if (!nr_account) {
+ spin_unlock(&pcp->lock);
goto failed_irq;
+ }
break;
}
nr_account++;
@@ -5370,6 +5493,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
nr_populated++;
}

+ spin_unlock(&pcp->lock);
local_unlock_irqrestore(&pagesets.lock, flags);

__count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
@@ -7019,6 +7143,7 @@ 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));

+ spin_lock_init(&pcp->lock);
for (pindex = 0; pindex < NR_PCP_LISTS; pindex++)
INIT_LIST_HEAD(&pcp->lists[pindex]);

--
2.35.3