When I run a parallel reading performan testing on a md raid1 device with
two NVMe SSDs, I observe very bad throughput in supprise: by fio with 64KB
block size, 40 seq read I/O jobs, 128 iodepth, overall throughput is
only 2.7GB/s, this is around 50% of the idea performance number.
The perf reports locking contention happens at allow_barrier() and
wait_barrier() code,
- 41.41% fio [kernel.kallsyms] [k] _raw_spin_lock_irqsave
- _raw_spin_lock_irqsave
+ 89.92% allow_barrier
+ 9.34% __wake_up
- 37.30% fio [kernel.kallsyms] [k] _raw_spin_lock_irq
- _raw_spin_lock_irq
- 100.00% wait_barrier
The reason is, in these I/O barrier related functions,
- raise_barrier()
- lower_barrier()
- wait_barrier()
- allow_barrier()
They always hold conf->resync_lock firstly, even there are only regular
reading I/Os and no resync I/O at all. This is a huge performance penalty.
The solution is a lockless-like algorithm in I/O barrier code, and only
holding conf->resync_lock when it has to.
The original idea is from Hannes Reinecke, and Neil Brown provides
comments to improve it. I continue to work on it, and make the patch into
current form.
In the new simpler raid1 I/O barrier implementation, there are two
wait barrier functions,
- wait_barrier()
Which calls _wait_barrier(), is used for regular write I/O. If there is
resync I/O happening on the same I/O barrier bucket, or the whole
array is frozen, task will wait until no barrier on same barrier bucket,
or the whold array is unfreezed.
- wait_read_barrier()
Since regular read I/O won't interfere with resync I/O (read_balance()
will make sure only uptodate data will be read out), it is unnecessary
to wait for barrier in regular read I/Os, waiting in only necessary
when the whole array is frozen.
The operations on conf->nr_pending[idx], conf->nr_waiting[idx], conf->
barrier[idx] are very carefully designed in raise_barrier(),
lower_barrier(), _wait_barrier() and wait_read_barrier(), in order to
avoid unnecessary spin locks in these functions. Once conf->
nr_pengding[idx] is increased, a resync I/O with same barrier bucket index
has to wait in raise_barrier(). Then in _wait_barrier() if no barrier
raised in same barrier bucket index and array is not frozen, the regular
I/O doesn't need to hold conf->resync_lock, it can just increase
conf->nr_pending[idx], and return to its caller. wait_read_barrier() is
very similar to _wait_barrier(), the only difference is it only waits when
array is frozen. For heavy parallel reading I/Os, the lockless I/O barrier
code almostly gets rid of all spin lock cost.
This patch significantly improves raid1 reading peroformance. From my
testing, a raid1 device built by two NVMe SSD, runs fio with 64KB
blocksize, 40 seq read I/O jobs, 128 iodepth, overall throughput
increases from 2.7GB/s to 4.6GB/s (+70%).
Changelog
V4:
- Change conf->nr_queued[] to atomic_t.
- Define BARRIER_BUCKETS_NR_BITS by (PAGE_SHIFT - ilog2(sizeof(atomic_t)))
V3:
- Add smp_mb__after_atomic() as Shaohua and Neil suggested.
- Change conf->nr_queued[] from atomic_t to int.
- Change conf->array_frozen from atomic_t back to int, and use
READ_ONCE(conf->array_frozen) to check value of conf->array_frozen
in _wait_barrier() and wait_read_barrier().
- In _wait_barrier() and wait_read_barrier(), add a call to
wake_up(&conf->wait_barrier) after atomic_dec(&conf->nr_pending[idx]),
to fix a deadlock between _wait_barrier()/wait_read_barrier and
freeze_array().
V2:
- Remove a spin_lock/unlock pair in raid1d().
- Add more code comments to explain why there is no racy when checking two
atomic_t variables at same time.
V1:
- Original RFC patch for comments.
Signed-off-by: Coly Li <colyli@xxxxxxx>
Cc: Shaohua Li <shli@xxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: Neil Brown <neilb@xxxxxxx>
Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
Cc: Guoqing Jiang <gqjiang@xxxxxxxx>
---
drivers/md/raid1.c | 165 ++++++++++++++++++++++++++++++++++++-----------------
drivers/md/raid1.h | 31 +++++-----
2 files changed, 130 insertions(+), 66 deletions(-)
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 621b3ae..7376b3d 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -226,7 +226,7 @@ static void reschedule_retry(struct r1bio *r1_bio)
idx = sector_to_idx(r1_bio->sector);
spin_lock_irqsave(&conf->device_lock, flags);
list_add(&r1_bio->retry_list, &conf->retry_list);
- conf->nr_queued[idx]++;
+ atomic_inc(&conf->nr_queued[idx]);
spin_unlock_irqrestore(&conf->device_lock, flags);
wake_up(&conf->wait_barrier);
@@ -836,11 +836,21 @@ static void raise_barrier(struct r1conf *conf, sector_t sector_nr)
spin_lock_irq(&conf->resync_lock);
/* Wait until no block IO is waiting */
- wait_event_lock_irq(conf->wait_barrier, !conf->nr_waiting[idx],
+ wait_event_lock_irq(conf->wait_barrier,
+ !atomic_read(&conf->nr_waiting[idx]),
conf->resync_lock);
/* block any new IO from starting */
- conf->barrier[idx]++;
+ atomic_inc(&conf->barrier[idx]);
+ /*
+ * In raise_barrier() we firstly increase conf->barrier[idx] then
+ * check conf->nr_pending[idx]. In _wait_barrier() we firstly
+ * increase conf->nr_pending[idx] then check conf->barrier[idx].
+ * A memory barrier here to make sure conf->nr_pending[idx] won't
+ * be fetched before conf->barrier[idx] is increased. Otherwise
+ * there will be a race between raise_barrier() and _wait_barrier().
+ */
+ smp_mb__after_atomic();
/* For these conditions we must wait:
* A: while the array is in frozen state
@@ -851,42 +861,81 @@ static void raise_barrier(struct r1conf *conf, sector_t sector_nr)
*/
wait_event_lock_irq(conf->wait_barrier,
!conf->array_frozen &&
- !conf->nr_pending[idx] &&
- conf->barrier[idx] < RESYNC_DEPTH,
+ !atomic_read(&conf->nr_pending[idx]) &&
+ atomic_read(&conf->barrier[idx]) < RESYNC_DEPTH,
conf->resync_lock);
- conf->nr_pending[idx]++;
+ atomic_inc(&conf->nr_pending[idx]);
spin_unlock_irq(&conf->resync_lock);
}
static void lower_barrier(struct r1conf *conf, sector_t sector_nr)
{
- unsigned long flags;
int idx = sector_to_idx(sector_nr);
- BUG_ON(conf->barrier[idx] <= 0);
+ BUG_ON(atomic_read(&conf->barrier[idx]) <= 0);
- spin_lock_irqsave(&conf->resync_lock, flags);
- conf->barrier[idx]--;
- conf->nr_pending[idx]--;
- spin_unlock_irqrestore(&conf->resync_lock, flags);
+ atomic_dec(&conf->barrier[idx]);
+ atomic_dec(&conf->nr_pending[idx]);
wake_up(&conf->wait_barrier);
}
static void _wait_barrier(struct r1conf *conf, int idx)
{
- spin_lock_irq(&conf->resync_lock);
- if (conf->array_frozen || conf->barrier[idx]) {
- conf->nr_waiting[idx]++;
- /* Wait for the barrier to drop. */
- wait_event_lock_irq(
- conf->wait_barrier,
- !conf->array_frozen && !conf->barrier[idx],
- conf->resync_lock);
- conf->nr_waiting[idx]--;
- }
+ /*
+ * We need to increase conf->nr_pending[idx] very early here,
+ * then raise_barrier() can be blocked when it waits for
+ * conf->nr_pending[idx] to be 0. Then we can avoid holding
+ * conf->resync_lock when there is no barrier raised in same
+ * barrier unit bucket. Also if the array is frozen, I/O
+ * should be blocked until array is unfrozen.
+ */
+ atomic_inc(&conf->nr_pending[idx]);
+ /*
+ * In _wait_barrier() we firstly increase conf->nr_pending[idx], then
+ * check conf->barrier[idx]. In raise_barrier() we firstly increase
+ * conf->barrier[idx], then check conf->nr_pending[idx]. A memory
+ * barrier is necessary here to make sure conf->barrier[idx] won't be
+ * fetched before conf->nr_pending[idx] is increased. Otherwise there
+ * will be a race between _wait_barrier() and raise_barrier().
+ */
+ smp_mb__after_atomic();
+
+ /*
+ * Don't worry about checking two atomic_t variables at same time
+ * here. If during we check conf->barrier[idx], the array is
+ * frozen (conf->array_frozen is 1), and chonf->barrier[idx] is
+ * 0, it is safe to return and make the I/O continue. Because the
+ * array is frozen, all I/O returned here will eventually complete
+ * or be queued, no race will happen. See code comment in
+ * frozen_array().
+ */
+ if (!READ_ONCE(conf->array_frozen) &&
+ !atomic_read(&conf->barrier[idx]))
+ return;
- conf->nr_pending[idx]++;
+ /*
+ * After holding conf->resync_lock, conf->nr_pending[idx]
+ * should be decreased before waiting for barrier to drop.
+ * Otherwise, we may encounter a race condition because
+ * raise_barrer() might be waiting for conf->nr_pending[idx]
+ * to be 0 at same time.
+ */
+ spin_lock_irq(&conf->resync_lock);
+ atomic_inc(&conf->nr_waiting[idx]);
+ atomic_dec(&conf->nr_pending[idx]);
+ /*
+ * In case freeze_array() is waiting for
+ * get_unqueued_pending() == extra
+ */
+ wake_up(&conf->wait_barrier);
+ /* Wait for the barrier in same barrier unit bucket to drop. */
+ wait_event_lock_irq(conf->wait_barrier,
+ !conf->array_frozen &&
+ !atomic_read(&conf->barrier[idx]),
+ conf->resync_lock);
+ atomic_inc(&conf->nr_pending[idx]);
+ atomic_dec(&conf->nr_waiting[idx]);
spin_unlock_irq(&conf->resync_lock);
}
@@ -894,18 +943,32 @@ static void wait_read_barrier(struct r1conf *conf, sector_t sector_nr)
{
int idx = sector_to_idx(sector_nr);
- spin_lock_irq(&conf->resync_lock);
- if (conf->array_frozen) {
- conf->nr_waiting[idx]++;
- /* Wait for array to unfreeze */
- wait_event_lock_irq(
- conf->wait_barrier,
- !conf->array_frozen,
- conf->resync_lock);
- conf->nr_waiting[idx]--;
- }
+ /*
+ * Very similar to _wait_barrier(). The difference is, for read
+ * I/O we don't need wait for sync I/O, but if the whole array
+ * is frozen, the read I/O still has to wait until the array is
+ * unfrozen. Since there is no ordering requirement with
+ * conf->barrier[idx] here, memory barrier is unnecessary as well.
+ */
+ atomic_inc(&conf->nr_pending[idx]);
- conf->nr_pending[idx]++;
+ if (!READ_ONCE(conf->array_frozen))
+ return;
+
+ spin_lock_irq(&conf->resync_lock);
+ atomic_inc(&conf->nr_waiting[idx]);
+ atomic_dec(&conf->nr_pending[idx]);
+ /*
+ * In case freeze_array() is waiting for
+ * get_unqueued_pending() == extra
+ */
+ wake_up(&conf->wait_barrier);
+ /* Wait for array to be unfrozen */
+ wait_event_lock_irq(conf->wait_barrier,
+ !conf->array_frozen,
+ conf->resync_lock);
+ atomic_inc(&conf->nr_pending[idx]);
+ atomic_dec(&conf->nr_waiting[idx]);
spin_unlock_irq(&conf->resync_lock);
}
@@ -926,11 +989,7 @@ static void wait_all_barriers(struct r1conf *conf)
static void _allow_barrier(struct r1conf *conf, int idx)
{
- unsigned long flags;
-
- spin_lock_irqsave(&conf->resync_lock, flags);
- conf->nr_pending[idx]--;
- spin_unlock_irqrestore(&conf->resync_lock, flags);
+ atomic_dec(&conf->nr_pending[idx]);
wake_up(&conf->wait_barrier);
}
@@ -955,7 +1014,8 @@ static int get_unqueued_pending(struct r1conf *conf)
int idx, ret;
for (ret = 0, idx = 0; idx < BARRIER_BUCKETS_NR; idx++)
- ret += conf->nr_pending[idx] - conf->nr_queued[idx];
+ ret += atomic_read(&conf->nr_pending[idx]) -
+ atomic_read(&conf->nr_queued[idx]);
return ret;
}
@@ -1000,8 +1060,8 @@ static void unfreeze_array(struct r1conf *conf)
/* reverse the effect of the freeze */
spin_lock_irq(&conf->resync_lock);
conf->array_frozen = 0;
- wake_up(&conf->wait_barrier);
spin_unlock_irq(&conf->resync_lock);
+ wake_up(&conf->wait_barrier);
}
/* duplicate the data pages for behind I/O
@@ -2387,8 +2447,13 @@ static void handle_write_finished(struct r1conf *conf, struct r1bio *r1_bio)
spin_lock_irq(&conf->device_lock);
list_add(&r1_bio->retry_list, &conf->bio_end_io_list);
idx = sector_to_idx(r1_bio->sector);
- conf->nr_queued[idx]++;
+ atomic_inc(&conf->nr_queued[idx]);
spin_unlock_irq(&conf->device_lock);
+ /*
+ * In case freeze_array() is waiting for condition
+ * get_unqueued_pending() == extra to be true.
+ */
+ wake_up(&conf->wait_barrier);
md_wakeup_thread(conf->mddev->thread);
} else {
if (test_bit(R1BIO_WriteError, &r1_bio->state))
@@ -2525,9 +2590,7 @@ static void raid1d(struct md_thread *thread)
retry_list);
list_del(&r1_bio->retry_list);
idx = sector_to_idx(r1_bio->sector);
- spin_lock_irqsave(&conf->device_lock, flags);
- conf->nr_queued[idx]--;
- spin_unlock_irqrestore(&conf->device_lock, flags);
+ atomic_dec(&conf->nr_queued[idx]);
if (mddev->degraded)
set_bit(R1BIO_Degraded, &r1_bio->state);
if (test_bit(R1BIO_WriteError, &r1_bio->state))
@@ -2549,7 +2612,7 @@ static void raid1d(struct md_thread *thread)
r1_bio = list_entry(head->prev, struct r1bio, retry_list);
list_del(head->prev);
idx = sector_to_idx(r1_bio->sector);
- conf->nr_queued[idx]--;
+ atomic_dec(&conf->nr_queued[idx]);
spin_unlock_irqrestore(&conf->device_lock, flags);
mddev = r1_bio->mddev;
@@ -2666,7 +2729,7 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
* If there is non-resync activity waiting for a turn, then let it
* though before starting on this new sync request.
*/
- if (conf->nr_waiting[idx])
+ if (atomic_read(&conf->nr_waiting[idx]))
schedule_timeout_uninterruptible(1);
/* we are incrementing sector_nr below. To be safe, we check against
@@ -2926,22 +2989,22 @@ static struct r1conf *setup_conf(struct mddev *mddev)
goto abort;
conf->nr_pending = kcalloc(BARRIER_BUCKETS_NR,
- sizeof(int), GFP_KERNEL);
+ sizeof(atomic_t), GFP_KERNEL);
if (!conf->nr_pending)
goto abort;
conf->nr_waiting = kcalloc(BARRIER_BUCKETS_NR,
- sizeof(int), GFP_KERNEL);
+ sizeof(atomic_t), GFP_KERNEL);
if (!conf->nr_waiting)
goto abort;
conf->nr_queued = kcalloc(BARRIER_BUCKETS_NR,
- sizeof(int), GFP_KERNEL);
+ sizeof(atomic_t), GFP_KERNEL);
if (!conf->nr_queued)
goto abort;
conf->barrier = kcalloc(BARRIER_BUCKETS_NR,
- sizeof(int), GFP_KERNEL);
+ sizeof(atomic_t), GFP_KERNEL);
if (!conf->barrier)
goto abort;
diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index 3442e8f..dd22a37 100644
--- a/drivers/md/raid1.h
+++ b/drivers/md/raid1.h
@@ -10,18 +10,19 @@
/*
* In struct r1conf, the following members are related to I/O barrier
* buckets,
- * int *nr_pending;
- * int *nr_waiting;
- * int *nr_queued;
- * int *barrier;
- * Each of them points to array of integers, each array is designed to
- * have BARRIER_BUCKETS_NR elements and occupy a single memory page. The
- * data width of integer variables is 4, equal to 1<<(ilog2(sizeof(int))),
- * BARRIER_BUCKETS_NR_BITS is defined as (PAGE_SHIFT - ilog2(sizeof(int)))
- * to make sure an array of integers with BARRIER_BUCKETS_NR elements just
- * exactly occupies a single memory page.
+ * atomic_t *nr_pending;
+ * atomic_t *nr_waiting;
+ * atomic_t *nr_queued;
+ * atomic_t *barrier;
+ * Each of them points to array of atomic_t variables, each array is
+ * designed to have BARRIER_BUCKETS_NR elements and occupy a single
+ * memory page. The data width of atomic_t variables is 4 bytes, equal
+ * to 1<<(ilog2(sizeof(atomic_t))), BARRIER_BUCKETS_NR_BITS is defined
+ * as (PAGE_SHIFT - ilog2(sizeof(int))) to make sure an array of
+ * atomic_t variables with BARRIER_BUCKETS_NR elements just exactly
+ * occupies a single memory page.
*/
-#define BARRIER_BUCKETS_NR_BITS (PAGE_SHIFT - ilog2(sizeof(int)))
+#define BARRIER_BUCKETS_NR_BITS (PAGE_SHIFT - ilog2(sizeof(atomic_t)))
#define BARRIER_BUCKETS_NR (1<<BARRIER_BUCKETS_NR_BITS)
struct raid1_info {
@@ -83,10 +84,10 @@ struct r1conf {
*/
wait_queue_head_t wait_barrier;
spinlock_t resync_lock;
- int *nr_pending;
- int *nr_waiting;
- int *nr_queued;
- int *barrier;
+ atomic_t *nr_pending;
+ atomic_t *nr_waiting;
+ atomic_t *nr_queued;
+ atomic_t *barrier;
int array_frozen;
/* Set to 1 if a full sync is needed, (fresh device added).
--
2.6.6
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