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

[Jeff Moyer <jmoyer@redhat.com>]

Ming Lei <ming.lei@canonical.com> writes:

> 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.

Actually, it limits it to 32.  Also, there is no discussion on what
variables might affect this number.  Will that magic number change
depending on the number of cpus on the system, for example?

-Jeff (who despises magic numbers littered throughout the code)

> 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;
>  };