[PATCH] block: loop: avoiding too many pending per work I/O

[Ming Lei <ming.lei@canonical.com>]

If there are too many pending per work I/O, too many
high priority work thread can be generated so that
system performance can be effected.

This patch limits the max pending per work I/O as 16,
and will fackback to single queue mode when the max
number is reached.

This patch fixes Fedora 22 live booting performance
regression when it is booted from squashfs over dm
based on loop, and looks the following reasons are
related with the problem:

- not like other filesyststems(such as ext4), squashfs
is a bit special, and I observed that increasing I/O jobs
to access file in squashfs only improve I/O performance a
little, but it can make big difference for ext4

- nested loop: both squashfs.img and ext3fs.img are mounted
as loop block, and ext3fs.img is inside the squashfs

- during booting, lots of tasks may run concurrently

Fixes: b5dd2f6047ca108001328aac0e8588edd15f1778
Cc: stable@vger.kernel.org (v4.0)
Reported-by: Justin M. Forbes <jforbes@fedoraproject.org>
Tested-by: Justin M. Forbes <jforbes@fedoraproject.org>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
 drivers/block/loop.c | 19 +++++++++++++++++--
 drivers/block/loop.h |  2 ++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index d7173cb..4db0301 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1425,13 +1425,24 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 		const struct blk_mq_queue_data *bd)
 {
 	struct loop_cmd *cmd = blk_mq_rq_to_pdu(bd->rq);
+	struct loop_device *lo = cmd->rq->q->queuedata;
+	bool single_queue = !!(cmd->rq->cmd_flags & REQ_WRITE);
+
+	/*
+	 * Fallback to single queue mode if the pending per work
+	 * I/O number reaches 32, otherwise too many high priority
+	 * worker thread may effect system performance as reported
+	 * in fedora live booting from squashfs over loop.
+	 */
+	if (atomic_read(&lo->pending_per_work_io) >= 32)
+		single_queue = true;
 
 	blk_mq_start_request(bd->rq);
 
-	if (cmd->rq->cmd_flags & REQ_WRITE) {
-		struct loop_device *lo = cmd->rq->q->queuedata;
+	if (single_queue) {
 		bool need_sched = true;
 
+		cmd->per_work_io = false;
 		spin_lock_irq(&lo->lo_lock);
 		if (lo->write_started)
 			need_sched = false;
@@ -1443,6 +1454,8 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 		if (need_sched)
 			queue_work(loop_wq, &lo->write_work);
 	} else {
+		cmd->per_work_io = true;
+		atomic_inc(&lo->pending_per_work_io);
 		queue_work(loop_wq, &cmd->read_work);
 	}
 
@@ -1467,6 +1480,8 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
 	if (ret)
 		cmd->rq->errors = -EIO;
 	blk_mq_complete_request(cmd->rq);
+	if (cmd->per_work_io)
+		atomic_dec(&lo->pending_per_work_io);
 }
 
 static void loop_queue_write_work(struct work_struct *work)
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index 301c27f..eb855f5 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -57,6 +57,7 @@ struct loop_device {
 	struct list_head	write_cmd_head;
 	struct work_struct	write_work;
 	bool			write_started;
+	atomic_t		pending_per_work_io;
 	int			lo_state;
 	struct mutex		lo_ctl_mutex;
 
@@ -68,6 +69,7 @@ struct loop_device {
 struct loop_cmd {
 	struct work_struct read_work;
 	struct request *rq;
+	bool per_work_io;
 	struct list_head list;
 };
 
-- 
1.9.1