mm: compaction: Abort async compaction if locks are contended or taking too long
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-by: Jim Schutt <jaschut@sandia.gov>
Tested-by: Jim Schutt <jaschut@sandia.gov>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/compaction.c b/mm/compaction.c
index bcce789..7fcd3a5 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -51,6 +51,47 @@
}
/*
+ * 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);
+}
+
+/*
* Isolate free pages onto a private freelist. Caller must hold zone->lock.
* If @strict is true, will abort returning 0 on any invalid PFNs or non-free
* pages inside of the pageblock (even though it may still end up isolating
@@ -173,7 +214,7 @@
}
/* 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, };
@@ -181,8 +222,14 @@
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 */
@@ -228,6 +275,8 @@
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
@@ -247,25 +296,22 @@
/* 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
@@ -349,9 +395,10 @@
}
}
- 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);
@@ -461,7 +508,16 @@
* 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);
isolated = isolate_freepages_block(pfn, end_pfn,
@@ -773,7 +829,7 @@
static unsigned long compact_zone_order(struct zone *zone,
int order, gfp_t gfp_mask,
- bool sync)
+ bool sync, bool *contended)
{
struct compact_control cc = {
.nr_freepages = 0,
@@ -782,6 +838,7 @@
.migratetype = allocflags_to_migratetype(gfp_mask),
.zone = zone,
.sync = sync,
+ .contended = contended,
};
INIT_LIST_HEAD(&cc.freepages);
INIT_LIST_HEAD(&cc.migratepages);
@@ -803,7 +860,7 @@
*/
unsigned long try_to_compact_pages(struct zonelist *zonelist,
int order, gfp_t gfp_mask, nodemask_t *nodemask,
- bool sync)
+ bool sync, bool *contended)
{
enum zone_type high_zoneidx = gfp_zone(gfp_mask);
int may_enter_fs = gfp_mask & __GFP_FS;
@@ -827,7 +884,8 @@
nodemask) {
int status;
- status = compact_zone_order(zone, order, gfp_mask, sync);
+ status = compact_zone_order(zone, order, gfp_mask, sync,
+ contended);
rc = max(status, rc);
/* If a normal allocation would succeed, stop compacting */