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If there are too many pending per work I/O, too many
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high priority work thread can be generated so that
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system performance can be effected.
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This patch limits the max pending per work I/O as 16,
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and will fackback to single queue mode when the max
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number is reached.
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This patch fixes Fedora 22 live booting performance
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regression when it is booted from squashfs over dm
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based on loop, and looks the following reasons are
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related with the problem:
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- not like other filesyststems(such as ext4), squashfs
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is a bit special, and I observed that increasing I/O jobs
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to access file in squashfs only improve I/O performance a
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little, but it can make big difference for ext4
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- nested loop: both squashfs.img and ext3fs.img are mounted
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as loop block, and ext3fs.img is inside the squashfs
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- during booting, lots of tasks may run concurrently
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Fixes: b5dd2f6047ca108001328aac0e8588edd15f1778
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Cc: stable@vger.kernel.org (v4.0)
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Reported-by: Justin M. Forbes <jforbes@fedoraproject.org>
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Tested-by: Justin M. Forbes <jforbes@fedoraproject.org>
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Signed-off-by: Ming Lei <ming.lei@canonical.com>
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---
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drivers/block/loop.c | 19 +++++++++++++++++--
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drivers/block/loop.h | 2 ++
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2 files changed, 19 insertions(+), 2 deletions(-)
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diff --git a/drivers/block/loop.c b/drivers/block/loop.c
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index ae3fcb4..5a728c6 100644
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--- a/drivers/block/loop.c
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+++ b/drivers/block/loop.c
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@@ -1425,13 +1425,24 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
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const struct blk_mq_queue_data *bd)
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{
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struct loop_cmd *cmd = blk_mq_rq_to_pdu(bd->rq);
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+ struct loop_device *lo = cmd->rq->q->queuedata;
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+ bool single_queue = !!(cmd->rq->cmd_flags & REQ_WRITE);
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+
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+ /*
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+ * Fallback to single queue mode if the pending per work
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+ * I/O number reaches 16, otherwise too many high priority
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+ * worker thread may effect system performance as reported
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+ * in fedora live booting from squashfs over loop.
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+ */
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+ if (atomic_read(&lo->pending_per_work_io) >= 16)
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+ single_queue = true;
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blk_mq_start_request(bd->rq);
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- if (cmd->rq->cmd_flags & REQ_WRITE) {
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- struct loop_device *lo = cmd->rq->q->queuedata;
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+ if (single_queue) {
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bool need_sched = true;
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+ cmd->per_work_io = false;
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spin_lock_irq(&lo->lo_lock);
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if (lo->write_started)
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need_sched = false;
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@@ -1443,6 +1454,8 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
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if (need_sched)
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queue_work(loop_wq, &lo->write_work);
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} else {
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+ atomic_inc(&lo->pending_per_work_io);
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+ cmd->per_work_io = true;
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queue_work(loop_wq, &cmd->read_work);
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}
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@@ -1467,6 +1480,8 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
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if (ret)
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cmd->rq->errors = -EIO;
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blk_mq_complete_request(cmd->rq);
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+ if (cmd->per_work_io)
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+ atomic_dec(&lo->pending_per_work_io);
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}
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static void loop_queue_write_work(struct work_struct *work)
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diff --git a/drivers/block/loop.h b/drivers/block/loop.h
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index 301c27f..eb855f5 100644
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--- a/drivers/block/loop.h
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+++ b/drivers/block/loop.h
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@@ -57,6 +57,7 @@ struct loop_device {
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struct list_head write_cmd_head;
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struct work_struct write_work;
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bool write_started;
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+ atomic_t pending_per_work_io;
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int lo_state;
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struct mutex lo_ctl_mutex;
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@@ -68,6 +69,7 @@ struct loop_device {
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struct loop_cmd {
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struct work_struct read_work;
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struct request *rq;
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+ bool per_work_io;
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struct list_head list;
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};
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--
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1.9.1
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