Re: [PATCH v2 18/20] scsi: ufs: Optimize the command queueing code

Asutosh Das (asd)
2021-11-22T17:46:07Z

On 11/19/2021 11:57 AM, Bart Van Assche wrote:
> Remove the clock scaling lock from ufshcd_queuecommand() since it is a
> performance bottleneck. Freeze request queues instead of polling the
> doorbell registers to wait until pending commands have completed.
>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
> drivers/scsi/ufs/ufshcd.c | 124 +++++++++++++-------------------------
> drivers/scsi/ufs/ufshcd.h | 1 +
> 2 files changed, 44 insertions(+), 81 deletions(-)
>
> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> index a6d3f71c6b00..9cf4a22f1950 100644
> --- a/drivers/scsi/ufs/ufshcd.c
> +++ b/drivers/scsi/ufs/ufshcd.c
> @@ -1070,65 +1070,6 @@ static bool ufshcd_is_devfreq_scaling_required(struct ufs_hba *hba,
> return false;
> }
>
[...]
> /**
> * ufshcd_scale_gear - scale up/down UFS gear
> * @hba: per adapter instance
> @@ -1176,37 +1117,63 @@ static int ufshcd_scale_gear(struct ufs_hba *hba, bool scale_up)
>
> static int ufshcd_clock_scaling_prepare(struct ufs_hba *hba)
> {
> - #define DOORBELL_CLR_TOUT_US (1000 * 1000) /* 1 sec */
> - int ret = 0;
> + struct scsi_device *sdev;
> +
> /*
> - * make sure that there are no outstanding requests when
> - * clock scaling is in progress
> + * Make sure that no commands are in progress while the clock frequency
> + * is being modified.
> + *
> + * Since ufshcd_exec_dev_cmd() and ufshcd_issue_devman_upiu_cmd() lock
> + * the clk_scaling_lock before calling blk_get_request(), lock
> + * clk_scaling_lock before freezing the request queues to prevent lock
> + * inversion.
> */
> - ufshcd_scsi_block_requests(hba);
> down_write(&hba->clk_scaling_lock);
> -
> - if (!hba->clk_scaling.is_allowed ||
> - ufshcd_wait_for_doorbell_clr(hba, DOORBELL_CLR_TOUT_US)) {
> - ret = -EBUSY;
> - up_write(&hba->clk_scaling_lock);
> - ufshcd_scsi_unblock_requests(hba);
> - goto out;
> - }
> -
> + if (!hba->clk_scaling.is_allowed)
> + goto busy;
> + blk_freeze_queue_start(hba->tmf_queue);
> + blk_freeze_queue_start(hba->cmd_queue);
> + shost_for_each_device(sdev, hba->host)
> + blk_freeze_queue_start(sdev->request_queue);
This would still issue the requests present in the queue before freezing
and that's a concern.
> + /*
> + * Calling synchronize_rcu_expedited() reduces the time required to
> + * freeze request queues from milliseconds to microseconds.
> + */
> + synchronize_rcu_expedited();
> + shost_for_each_device(sdev, hba->host)
> + if (blk_mq_freeze_queue_wait_timeout(sdev->request_queue, HZ)
> + <= 0)
> + goto unfreeze;
> + if (blk_mq_freeze_queue_wait_timeout(hba->cmd_queue, HZ) <= 0 ||
> + blk_mq_freeze_queue_wait_timeout(hba->tmf_queue, HZ / 10) <= 0)
> + goto unfreeze;
> /* let's not get into low power until clock scaling is completed */
> ufshcd_hold(hba, false);
> + return 0;
>
> -out:
> - return ret;
> +unfreeze:
> + shost_for_each_device(sdev, hba->host)
> + blk_mq_unfreeze_queue(sdev->request_queue);
> + blk_mq_unfreeze_queue(hba->cmd_queue);
> + blk_mq_unfreeze_queue(hba->tmf_queue);
> +
> +busy:
> + up_write(&hba->clk_scaling_lock);
> + return -EBUSY;
> }
>
> static void ufshcd_clock_scaling_unprepare(struct ufs_hba *hba, bool writelock)
> {
> + struct scsi_device *sdev;
> +
> + shost_for_each_device(sdev, hba->host)
> + blk_mq_unfreeze_queue(sdev->request_queue);
> + blk_mq_unfreeze_queue(hba->cmd_queue);
> + blk_mq_unfreeze_queue(hba->tmf_queue);
> if (writelock)
> up_write(&hba->clk_scaling_lock);
> else
> up_read(&hba->clk_scaling_lock);
> - ufshcd_scsi_unblock_requests(hba);
> ufshcd_release(hba);
> }
>
> @@ -2699,9 +2666,6 @@ static int ufshcd_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *cmd)
>
> WARN_ONCE(tag < 0, "Invalid tag %d\n", tag);
>
> - if (!down_read_trylock(&hba->clk_scaling_lock))
> - return SCSI_MLQUEUE_HOST_BUSY;
> -
> /*
> * Allows the UFS error handler to wait for prior ufshcd_queuecommand()
> * calls.
> @@ -2790,8 +2754,6 @@ static int ufshcd_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *cmd)
> out:
> rcu_read_unlock();
>
> - up_read(&hba->clk_scaling_lock);
> -
> if (ufs_trigger_eh()) {
> unsigned long flags;
>
> diff --git a/drivers/scsi/ufs/ufshcd.h b/drivers/scsi/ufs/ufshcd.h
> index e9bc07c69a80..7ec463c97d64 100644
> --- a/drivers/scsi/ufs/ufshcd.h
> +++ b/drivers/scsi/ufs/ufshcd.h
> @@ -778,6 +778,7 @@ struct ufs_hba_monitor {
> * @clk_list_head: UFS host controller clocks list node head
> * @pwr_info: holds current power mode
> * @max_pwr_info: keeps the device max valid pwm
> + * @clk_scaling_lock: used to serialize device commands and clock scaling
> * @desc_size: descriptor sizes reported by device
> * @urgent_bkops_lvl: keeps track of urgent bkops level for device
> * @is_urgent_bkops_lvl_checked: keeps track if the urgent bkops level for
>

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