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Subject[PATCH 5/5] mm: compaction: Abort async compaction if locks are contended or taking too long
Date
Jim Schutt reported a problem that pointed at compaction contending
heavily on locks. The workload is straight-forward and in his own words;

The systems in question have 24 SAS drives spread across 3 HBAs,
running 24 Ceph OSD instances, one per drive. FWIW these servers
are dual-socket Intel 5675 Xeons w/48 GB memory. I've got ~160
Ceph Linux clients doing dd simultaneously to a Ceph file system
backed by 12 of these servers.

Early in the test everything looks fine

procs -------------------memory------------------ ---swap-- -----io---- --system-- -----cpu-------
r b swpd free buff cache si so bi bo in cs us sy id wa st
31 15 0 287216 576 38606628 0 0 2 1158 2 14 1 3 95 0 0
27 15 0 225288 576 38583384 0 0 18 2222016 203357 134876 11 56 17 15 0
28 17 0 219256 576 38544736 0 0 11 2305932 203141 146296 11 49 23 17 0
6 18 0 215596 576 38552872 0 0 7 2363207 215264 166502 12 45 22 20 0
22 18 0 226984 576 38596404 0 0 3 2445741 223114 179527 12 43 23 22 0
and then it goes to pot

procs -------------------memory------------------ ---swap-- -----io---- --system-- -----cpu-------
r b swpd free buff cache si so bi bo in cs us sy id wa st
163 8 0 464308 576 36791368 0 0 11 22210 866 536 3 13 79 4 0
207 14 0 917752 576 36181928 0 0 712 1345376 134598 47367 7 90 1 2 0
123 12 0 685516 576 36296148 0 0 429 1386615 158494 60077 8 84 5 3 0
123 12 0 598572 576 36333728 0 0 1107 1233281 147542 62351 7 84 5 4 0
622 7 0 660768 576 36118264 0 0 557 1345548 151394 59353 7 85 4 3 0
223 11 0 283960 576 36463868 0 0 46 1107160 121846 33006 6 93 1 1 0
Note that system CPU usage is very high blocks being written out has
dropped by 42%. He analysed this with perf and found

perf record -g -a sleep 10
perf report --sort symbol --call-graph fractal,5
34.63% [k] _raw_spin_lock_irqsave
|
|--97.30%-- isolate_freepages
| compaction_alloc
| unmap_and_move
| migrate_pages
| compact_zone
| compact_zone_order
| try_to_compact_pages
| __alloc_pages_direct_compact
| __alloc_pages_slowpath
| __alloc_pages_nodemask
| alloc_pages_vma
| do_huge_pmd_anonymous_page
| handle_mm_fault
| do_page_fault
| page_fault
| |
| |--87.39%-- skb_copy_datagram_iovec
| | tcp_recvmsg
| | inet_recvmsg
| | sock_recvmsg
| | sys_recvfrom
| | system_call
| | __recv
| | |
| | --100.00%-- (nil)
| |
| --12.61%-- memcpy
--2.70%-- [...]
There was other data but primarily it is all showing that compaction is
contended heavily on the zone->lock and zone->lru_lock.

commit [b2eef8c0: mm: compaction: minimise the time IRQs are disabled
while isolating pages for migration] noted that it was possible for
migration to hold the lru_lock for an excessive amount of time. Very
broadly speaking this patch expands the concept.

This patch introduces compact_checklock_irqsave() to check if a lock
is contended or the process needs to be scheduled. If either condition
is true then async compaction is aborted and the caller is informed.
The page allocator will fail a THP allocation if compaction failed due
to contention. This patch also introduces compact_trylock_irqsave()
which will acquire the lock only if it is not contended and the process
does not need to schedule.

Reported-and-tested-by: Jim Schutt <jaschut@sandia.gov>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
include/linux/compaction.h | 4 +-
mm/compaction.c | 105 ++++++++++++++++++++++++++++++++++----------
mm/internal.h | 1 +
mm/page_alloc.c | 17 ++++---
4 files changed, 96 insertions(+), 31 deletions(-)
diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index fd20c15..0e38a1d 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -22,7 +22,7 @@ extern int sysctl_extfrag_handler(struct ctl_table *table, int write,
extern int fragmentation_index(struct zone *zone, unsigned int order);
extern unsigned long try_to_compact_pages(struct zonelist *zonelist,
int order, gfp_t gfp_mask, nodemask_t *mask,
- bool sync, struct page **page);
+ bool sync, bool *contended, struct page **page);
extern int compact_pgdat(pg_data_t *pgdat, int order);
extern unsigned long compaction_suitable(struct zone *zone, int order);

@@ -64,7 +64,7 @@ static inline bool compaction_deferred(struct zone *zone, int order)
#else
static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,
int order, gfp_t gfp_mask, nodemask_t *nodemask,
- bool sync, struct page **page)
+ bool sync, bool *contended, struct page **page)
{
return COMPACT_CONTINUE;
}
diff --git a/mm/compaction.c b/mm/compaction.c
index c2d0958..b95e263 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -50,6 +50,47 @@ static inline bool migrate_async_suitable(int migratetype)
return is_migrate_cma(migratetype) || migratetype == MIGRATE_MOVABLE;
}

+/*
+ * Compaction requires the taking of some coarse locks that are potentially
+ * very heavily contended. Check if the process needs to be scheduled or
+ * if the lock is contended. For async compaction, back out in the event
+ * if contention is severe. For sync compaction, schedule.
+ *
+ * Returns true if the lock is held.
+ * Returns false if the lock is released and compaction should abort
+ */
+static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
+ bool locked, struct compact_control *cc)
+{
+ if (need_resched() || spin_is_contended(lock)) {
+ if (locked) {
+ spin_unlock_irqrestore(lock, *flags);
+ locked = false;
+ }
+
+ /* async aborts if taking too long or contended */
+ if (!cc->sync) {
+ if (cc->contended)
+ *cc->contended = true;
+ return false;
+ }
+
+ cond_resched();
+ if (fatal_signal_pending(current))
+ return false;
+ }
+
+ if (!locked)
+ spin_lock_irqsave(lock, *flags);
+ return true;
+}
+
+static inline bool compact_trylock_irqsave(spinlock_t *lock,
+ unsigned long *flags, struct compact_control *cc)
+{
+ return compact_checklock_irqsave(lock, flags, false, cc);
+}
+
static void compact_capture_page(struct compact_control *cc)
{
unsigned long flags;
@@ -87,7 +128,8 @@ static void compact_capture_page(struct compact_control *cc)
continue;

/* Take the lock and attempt capture of the page */
- spin_lock_irqsave(&cc->zone->lock, flags);
+ if (!compact_trylock_irqsave(&cc->zone->lock, &flags, cc))
+ return;
if (!list_empty(&area->free_list[mtype])) {
page = list_entry(area->free_list[mtype].next,
struct page, lru);
@@ -226,7 +268,7 @@ isolate_freepages_range(unsigned long start_pfn, unsigned long end_pfn)
}

/* Update the number of anon and file isolated pages in the zone */
-static void acct_isolated(struct zone *zone, struct compact_control *cc)
+static void acct_isolated(struct zone *zone, bool locked, struct compact_control *cc)
{
struct page *page;
unsigned int count[2] = { 0, };
@@ -234,8 +276,14 @@ static void acct_isolated(struct zone *zone, struct compact_control *cc)
list_for_each_entry(page, &cc->migratepages, lru)
count[!!page_is_file_cache(page)]++;

- __mod_zone_page_state(zone, NR_ISOLATED_ANON, count[0]);
- __mod_zone_page_state(zone, NR_ISOLATED_FILE, count[1]);
+ /* If locked we can use the interrupt unsafe versions */
+ if (locked) {
+ __mod_zone_page_state(zone, NR_ISOLATED_ANON, count[0]);
+ __mod_zone_page_state(zone, NR_ISOLATED_FILE, count[1]);
+ } else {
+ mod_zone_page_state(zone, NR_ISOLATED_ANON, count[0]);
+ mod_zone_page_state(zone, NR_ISOLATED_FILE, count[1]);
+ }
}

/* Similar to reclaim, but different enough that they don't share logic */
@@ -281,6 +329,8 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
struct list_head *migratelist = &cc->migratepages;
isolate_mode_t mode = 0;
struct lruvec *lruvec;
+ unsigned long flags;
+ bool locked;

/*
* Ensure that there are not too many pages isolated from the LRU
@@ -300,25 +350,22 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,

/* Time to isolate some pages for migration */
cond_resched();
- spin_lock_irq(&zone->lru_lock);
+ spin_lock_irqsave(&zone->lru_lock, flags);
+ locked = true;
for (; low_pfn < end_pfn; low_pfn++) {
struct page *page;
- bool locked = true;

/* give a chance to irqs before checking need_resched() */
if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) {
- spin_unlock_irq(&zone->lru_lock);
+ spin_unlock_irqrestore(&zone->lru_lock, flags);
locked = false;
}
- if (need_resched() || spin_is_contended(&zone->lru_lock)) {
- if (locked)
- spin_unlock_irq(&zone->lru_lock);
- cond_resched();
- spin_lock_irq(&zone->lru_lock);
- if (fatal_signal_pending(current))
- break;
- } else if (!locked)
- spin_lock_irq(&zone->lru_lock);
+
+ /* Check if it is ok to still hold the lock */
+ locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
+ locked, cc);
+ if (!locked)
+ break;

/*
* migrate_pfn does not necessarily start aligned to a
@@ -402,9 +449,10 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
}
}

- acct_isolated(zone, cc);
+ acct_isolated(zone, locked, cc);

- spin_unlock_irq(&zone->lru_lock);
+ if (locked)
+ spin_unlock_irqrestore(&zone->lru_lock, flags);

trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);

@@ -514,7 +562,16 @@ static void isolate_freepages(struct zone *zone,
* are disabled
*/
isolated = 0;
- spin_lock_irqsave(&zone->lock, flags);
+
+ /*
+ * The zone lock must be held to isolate freepages. This
+ * unfortunately this is a very coarse lock and can be
+ * heavily contended if there are parallel allocations
+ * or parallel compactions. For async compaction do not
+ * spin on the lock
+ */
+ if (!compact_trylock_irqsave(&zone->lock, &flags, cc))
+ break;
if (suitable_migration_target(page)) {
end_pfn = min(pfn + pageblock_nr_pages, zone_end_pfn);
trace_mm_compaction_freepage_scanpfn(pfn);
@@ -837,8 +894,8 @@ out:
}

static unsigned long compact_zone_order(struct zone *zone,
- int order, gfp_t gfp_mask,
- bool sync, struct page **page)
+ int order, gfp_t gfp_mask, bool sync,
+ bool *contended, struct page **page)
{
struct compact_control cc = {
.nr_freepages = 0,
@@ -848,6 +905,7 @@ static unsigned long compact_zone_order(struct zone *zone,
.zone = zone,
.sync = sync,
.page = page,
+ .contended = contended,
};
INIT_LIST_HEAD(&cc.freepages);
INIT_LIST_HEAD(&cc.migratepages);
@@ -869,7 +927,7 @@ int sysctl_extfrag_threshold = 500;
*/
unsigned long try_to_compact_pages(struct zonelist *zonelist,
int order, gfp_t gfp_mask, nodemask_t *nodemask,
- bool sync, struct page **page)
+ bool sync, bool *contended, struct page **page)
{
enum zone_type high_zoneidx = gfp_zone(gfp_mask);
int may_enter_fs = gfp_mask & __GFP_FS;
@@ -889,7 +947,8 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
nodemask) {
int status;

- status = compact_zone_order(zone, order, gfp_mask, sync, page);
+ status = compact_zone_order(zone, order, gfp_mask, sync,
+ contended, page);
rc = max(status, rc);

/* If a normal allocation would succeed, stop compacting */
diff --git a/mm/internal.h b/mm/internal.h
index b03f05e..e549a7f 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -130,6 +130,7 @@ struct compact_control {
int order; /* order a direct compactor needs */
int migratetype; /* MOVABLE, RECLAIMABLE etc */
struct zone *zone;
+ bool *contended; /* True if a lock was contended */
struct page **page; /* Page captured of requested size */
};

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d1759f5..373d05f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2114,7 +2114,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
struct zonelist *zonelist, enum zone_type high_zoneidx,
nodemask_t *nodemask, int alloc_flags, struct zone *preferred_zone,
int migratetype, bool sync_migration,
- bool *deferred_compaction,
+ bool *contended_compaction, bool *deferred_compaction,
unsigned long *did_some_progress)
{
struct page *page = NULL;
@@ -2129,7 +2129,8 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,

current->flags |= PF_MEMALLOC;
*did_some_progress = try_to_compact_pages(zonelist, order, gfp_mask,
- nodemask, sync_migration, &page);
+ nodemask, sync_migration,
+ contended_compaction, &page);
current->flags &= ~PF_MEMALLOC;

/* If compaction captured a page, prep and use it */
@@ -2182,7 +2183,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
struct zonelist *zonelist, enum zone_type high_zoneidx,
nodemask_t *nodemask, int alloc_flags, struct zone *preferred_zone,
int migratetype, bool sync_migration,
- bool *deferred_compaction,
+ bool *contended_compaction, bool *deferred_compaction,
unsigned long *did_some_progress)
{
return NULL;
@@ -2355,6 +2356,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
unsigned long did_some_progress;
bool sync_migration = false;
bool deferred_compaction = false;
+ bool contended_compaction = false;

/*
* In the slowpath, we sanity check order to avoid ever trying to
@@ -2451,6 +2453,7 @@ rebalance:
nodemask,
alloc_flags, preferred_zone,
migratetype, sync_migration,
+ &contended_compaction,
&deferred_compaction,
&did_some_progress);
if (page)
@@ -2460,10 +2463,11 @@ rebalance:
/*
* If compaction is deferred for high-order allocations, it is because
* sync compaction recently failed. In this is the case and the caller
- * has requested the system not be heavily disrupted, fail the
- * allocation now instead of entering direct reclaim
+ * requested a movable allocation that does not heavily disrupt the
+ * system then fail the allocation instead of entering direct reclaim.
*/
- if (deferred_compaction)
+ if (contended_compaction &&
+ (gfp_mask & (__GFP_MOVABLE|__GFP_REPEAT)) == __GFP_MOVABLE)
goto nopage;

/* Try direct reclaim and then allocating */
@@ -2534,6 +2538,7 @@ rebalance:
nodemask,
alloc_flags, preferred_zone,
migratetype, sync_migration,
+ &contended_compaction,
&deferred_compaction,
&did_some_progress);
if (page)
--
1.7.9.2


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