[PATCH v10 6/6] block: loop: support DIO & AIO

From: Ming Lei
Date: Sun Aug 16 2015 - 22:32:32 EST


There are at least 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

xfstests(-g auto, ext4) is basically passed when running with
direct I/O(aio), one exception is generic/232, but it failed in
loop buffered I/O(4.2-rc6-next-20150814) too.

Follows the fio test result for performance purpose:
4 jobs fio test inside ext4 file system over loop block

1) How to run
- KVM: 4 VCPUs, 2G RAM
- linux kernel: 4.2-rc6-next-20150814(base) with the patchset
- the loop block is over one image on SSD.
- linux psync, 4 jobs, size 1500M, ext4 over loop block
- test result: IOPS from fio output

2) Throughput(IOPS) becomes a bit better with direct I/O(aio)
-------------------------------------------------------------
test cases |randread |read |randwrite |write |
-------------------------------------------------------------
base |8015 |113811 |67442 |106978
-------------------------------------------------------------
base+loop aio |8136 |125040 |67811 |111376
-------------------------------------------------------------

- somehow, it should be caused by more page cache avaiable for
application or one extra page copy is avoided in case of direct I/O

3) context switch
- context switch decreased by ~50% with loop direct I/O(aio)
compared with loop buffered I/O(4.2-rc6-next-20150814)

4) memory usage from /proc/meminfo
-------------------------------------------------------------
| Buffers | Cached
-------------------------------------------------------------
base | > 760MB | ~950MB
-------------------------------------------------------------
base+loop direct I/O(aio) | < 5MB | ~1.6GB
-------------------------------------------------------------

- so there are much more page caches available for application with
direct I/O

[1] https://lwn.net/Articles/612483/

Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
---
drivers/block/loop.c | 98 ++++++++++++++++++++++++++++++++++++++++++++++++++--
drivers/block/loop.h | 2 ++
2 files changed, 97 insertions(+), 3 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 75db3b9..2337608 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -445,6 +445,90 @@ 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;
+
+ handle_partial_read(cmd, ret);
+
+ if (ret > 0)
+ ret = 0;
+ else if (ret < 0)
+ ret = -EIO;
+
+ rq->errors = ret;
+ blk_mq_complete_request(rq);
+}
+
+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;
+
+ 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 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);
+
+ 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;
@@ -460,13 +544,13 @@ 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;
@@ -1570,6 +1654,12 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
if (lo->lo_state != Lo_bound)
return -EIO;

+ if (lo->use_dio && !(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;
@@ -1589,7 +1679,9 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
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 d1de221..fb2237c 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -69,6 +69,8 @@ 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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/