[PATCH v9 6/6] block: loop: support DIO & AIO

From: Ming Lei
Date: Thu Aug 06 2015 - 04:43:19 EST

There are about 3 advantages to use direct I/O and AIO on
read/write loop's backing file:

1) double cache can be avoided, then memory usage gets
decreased a lot

2) not like user space direct I/O, there isn't cost of
pinning pages

3) avoid context switch for obtaining good throughput
- in buffered file read, random I/O top throughput is often obtained
only if they are submitted concurrently from lots of tasks; but for
sequential I/O, most of times they can be hit from page cache, so
concurrent submissions often introduce unnecessary context switch
and can't improve throughput much. There was such discussion[1]
to use non-blocking I/O to improve the problem for application.
- with direct I/O and AIO, concurrent submissions can be
avoided and random read throughput can't be affected meantime

Follows my fio test result:

1. 16 jobs fio test inside ext4 file system over loop block
1) How to run
- linux kernel: 4.1.0-rc2-next-20150506 with the patchset
- the loop block is over one image on HDD.
- linux psync, 16 jobs, size 400M, ext4 over loop block
- test result: IOPS from fio output

2) Throughput result:
-------------------------------------------------------------
test cases |randread |read |randwrite |write |
-------------------------------------------------------------
base |240 |8705 |3763 |20914
-------------------------------------------------------------
base+loop aio |242 |9258 |4577 |21451
-------------------------------------------------------------

3) context switch
- context switch decreased by ~16% with loop aio for randread,
and decreased by ~33% for read

4) memory usage
- After these four tests with loop aio: ~10% memory becomes used
- After these four tests without loop aio: more than 55% memory
becomes used

2. single job fio test inside ext4 file system over loop block(for Maxim Patlasov)
1) How to run
- linux kernel: 4.1.0-rc2-next-20150506 with the patchset
- the loop block is over one image on HDD.
- linux psync, 1 job, size 4000M, ext4 over loop block
- test result: IOPS from fio output

2) Throughput result:
-------------------------------------------------------------
test cases |randread |read |randwrite |write |
-------------------------------------------------------------
base |109 |21180 |4192 |22782
-------------------------------------------------------------
base+loop aio |114 |21018 |5404 |22670
-------------------------------------------------------------

3) context switch
- context switch decreased by ~10% with loop aio for randread,
and decreased by ~50% for read

4) memory usage
- After these four tests with loop aio: ~10% memory becomes used
- After these four tests without loop aio: more than 55% memory
becomes used

Both 'context switch' and 'memory usage' data are got from sar.

[1] https://lwn.net/Articles/612483/
[2] sar graph when running fio over loop without the patchset
http://kernel.ubuntu.com/~ming/block/loop-aio/v3/lo-nonaio.pdf

[3] sar graph when running fio over loop with the patchset
http://kernel.ubuntu.com/~ming/block/loop-aio/v3/lo-aio.pdf

[4] sar graph when running fio over loop without the patchset
http://kernel.ubuntu.com/~ming/block/loop-aio/v3/lo-nonaio-1job.pdf

[5] sar graph when running fio over loop with the patchset
http://kernel.ubuntu.com/~ming/block/loop-aio/v3/lo-aio-1job.pdf

Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
---
drivers/block/loop.c | 147 +++++++++++++++++++++++++++++++++++++++++++++++++--
drivers/block/loop.h | 5 ++
2 files changed, 149 insertions(+), 3 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 06a737b..edaa18a 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -200,6 +200,8 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
blk_mq_freeze_queue(lo->lo_queue);
lo->use_dio = use_dio;
lo->dio_align = dio_align;
+ lo->last_use_dio = use_dio;
+ atomic_set(&lo->pending_dio, 0);
if (use_dio)
lo->lo_flags |= LO_FLAGS_DIRECT_IO;
else
@@ -436,6 +438,128 @@ static int lo_req_flush(struct loop_device *lo, struct request *rq)
return ret;
}

+static inline void handle_partial_read(struct loop_cmd *cmd, long bytes)
+{
+ if (bytes < 0 || (cmd->rq->cmd_flags & REQ_WRITE))
+ return;
+
+ if (unlikely(bytes < blk_rq_bytes(cmd->rq))) {
+ struct bio *bio = cmd->rq->bio;
+
+ bio_advance(bio, bytes);
+ zero_fill_bio(bio);
+ }
+}
+
+static void lo_rw_aio_complete(struct kiocb *iocb, long ret, long ret2)
+{
+ struct loop_cmd *cmd = container_of(iocb, struct loop_cmd, iocb);
+ struct request *rq = cmd->rq;
+ struct loop_device *lo = cmd->rq->q->queuedata;
+
+ handle_partial_read(cmd, ret);
+
+ if (ret > 0)
+ ret = 0;
+ else if (ret < 0)
+ ret = -EIO;
+
+ rq->errors = ret;
+ blk_mq_complete_request(rq);
+
+ if (atomic_dec_and_test(&lo->pending_dio)) {
+ if (lo->waiter)
+ wake_up_process(lo->waiter);
+ }
+}
+
+static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
+ loff_t pos, bool rw)
+{
+ struct iov_iter iter;
+ struct bio_vec *bvec;
+ struct bio *bio = cmd->rq->bio;
+ struct file *file = lo->lo_backing_file;
+ int ret;
+
+ /* nomerge for loop request queue */
+ WARN_ON(cmd->rq->bio != cmd->rq->biotail);
+
+ bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
+ iov_iter_bvec(&iter, ITER_BVEC | rw, bvec,
+ bio_segments(bio), blk_rq_bytes(cmd->rq));
+
+ cmd->iocb.ki_pos = pos;
+ cmd->iocb.ki_filp = file;
+ cmd->iocb.ki_complete = lo_rw_aio_complete;
+ cmd->iocb.ki_flags = IOCB_DIRECT;
+
+ atomic_inc(&lo->pending_dio);
+
+ if (rw == WRITE)
+ ret = file->f_op->write_iter(&cmd->iocb, &iter);
+ else
+ ret = file->f_op->read_iter(&cmd->iocb, &iter);
+
+ if (ret != -EIOCBQUEUED)
+ cmd->iocb.ki_complete(&cmd->iocb, ret, 0);
+ return 0;
+}
+
+static inline void lo_drain_pending_dio(struct loop_device *lo)
+{
+ lo->waiter = current;
+
+ /* order between writing waiter and reading pending_dio */
+ smp_mb__before_atomic();
+
+ for (;;) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ if (!atomic_read(&lo->pending_dio))
+ break;
+ schedule();
+ }
+ set_current_state(TASK_RUNNING);
+ lo->waiter = NULL;
+}
+
+static inline int lo_rw_simple(struct loop_device *lo,
+ struct request *rq, loff_t pos, bool rw)
+{
+ struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
+
+ /*
+ * When working at direct I/O, under very unusual cases,
+ * such as unaligned direct I/O from application and
+ * access to loop block device with 'unaligned' offset & size,
+ * wait for completion of the pending dio/aio before swiching
+ * to buffered I/O for avoiding probable race between dio and
+ * buffered I/O.
+ *
+ * Page cache flush is handled inside VFS direct I/O path if
+ * it is needed.
+ */
+ if (unlikely(lo->use_dio && lo->last_use_dio && !cmd->use_aio))
+ lo_drain_pending_dio(lo);
+
+ if (cmd->use_aio)
+ return lo_rw_aio(lo, cmd, pos, rw);
+
+ /*
+ * lo_write_simple and lo_read_simple should have been covered
+ * by io submit style function like lo_rw_aio(), one blocker
+ * is that lo_read_simple() need to call flush_dcache_page after
+ * the page is written from kernel, and it isn't easy to handle
+ * this in io submit style function which submits all segments
+ * of the req at one time. And direct read IO doesn't need to
+ * run flush_dcache_page().
+ */
+ if (rw == WRITE)
+ return lo_write_simple(lo, rq, pos);
+ else
+ return lo_read_simple(lo, rq, pos);
+}
+
static int do_req_filebacked(struct loop_device *lo, struct request *rq)
{
loff_t pos;
@@ -451,13 +575,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
else if (lo->transfer)
ret = lo_write_transfer(lo, rq, pos);
else
- ret = lo_write_simple(lo, rq, pos);
+ ret = lo_rw_simple(lo, rq, pos, WRITE);

} else {
if (lo->transfer)
ret = lo_read_transfer(lo, rq, pos);
else
- ret = lo_read_simple(lo, rq, pos);
+ ret = lo_rw_simple(lo, rq, pos, READ);
}

return ret;
@@ -1545,6 +1669,13 @@ int loop_unregister_transfer(int number)
EXPORT_SYMBOL(loop_register_transfer);
EXPORT_SYMBOL(loop_unregister_transfer);

+static inline bool req_dio_aligned(struct loop_device *lo,
+ const struct request *rq)
+{
+ return !((blk_rq_pos(rq) << 9) & lo->dio_align) &&
+ !(blk_rq_bytes(rq) & lo->dio_align);
+}
+
static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
const struct blk_mq_queue_data *bd)
{
@@ -1556,6 +1687,13 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
if (lo->lo_state != Lo_bound)
return -EIO;

+ if (lo->use_dio && !lo->transfer &&
+ req_dio_aligned(lo, bd->rq) &&
+ !(cmd->rq->cmd_flags & (REQ_FLUSH | REQ_DISCARD)))
+ cmd->use_aio = true;
+ else
+ cmd->use_aio = false;
+
queue_kthread_work(&lo->worker, &cmd->work);

return BLK_MQ_RQ_QUEUE_OK;
@@ -1571,11 +1709,14 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
goto failed;

ret = do_req_filebacked(lo, cmd->rq);
+ lo->last_use_dio = cmd->use_aio;

failed:
if (ret)
cmd->rq->errors = -EIO;
- blk_mq_complete_request(cmd->rq);
+ /* complete non-aio request */
+ if (!cmd->use_aio || ret)
+ blk_mq_complete_request(cmd->rq);
}

static void loop_queue_work(struct kthread_work *work)
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index 63f8e14..439054b 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -60,6 +60,9 @@ struct loop_device {
struct task_struct *worker_task;
unsigned dio_align;
bool use_dio;
+ bool last_use_dio;
+ atomic_t pending_dio;
+ struct task_struct *waiter;

struct request_queue *lo_queue;
struct blk_mq_tag_set tag_set;
@@ -70,6 +73,8 @@ struct loop_cmd {
struct kthread_work work;
struct request *rq;
struct list_head list;
+ bool use_aio; /* use AIO interface to handle I/O */
+ struct kiocb iocb;
};

/* Support for loadable transfer modules */
--
1.9.1

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