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

[Bart Van Assche <bvanassche@acm.org>]

On 11/22/21 3:02 PM, Asutosh Das (asd) wrote:
> Current code waits for the already issued requests to complete. It 
> doesn't issue the yet-to-be issued requests. Wouldn't freezing the queue 
> issue the requests in the context of scaling_{up/down}?
> If yes, I don't think the current code is doing that.

Hi Asutosh,

How about the patch below that preserves most of the existing code for
preparing for clock scaling?

Thanks,

Bart.


Subject: [PATCH] scsi: ufs: Optimize the command queueing code

Remove the clock scaling lock from ufshcd_queuecommand() since it is a
performance bottleneck. Instead, use synchronize_rcu_expedited() to wait
for ongoing ufshcd_queuecommand() calls.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
  drivers/scsi/ufs/ufshcd.c | 12 +++++++-----
  drivers/scsi/ufs/ufshcd.h |  1 +
  2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index 5d214456bf82..1d929c28efaf 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c
@@ -1196,6 +1196,13 @@ static int ufshcd_clock_scaling_prepare(struct ufs_hba *hba)
  	/* let's not get into low power until clock scaling is completed */
  	ufshcd_hold(hba, false);

+	/*
+	 * Wait for ongoing ufshcd_queuecommand() calls. Calling
+	 * synchronize_rcu_expedited() instead of synchronize_rcu() reduces the
+	 * waiting time from milliseconds to microseconds.
+	 */
+	synchronize_rcu_expedited();
+
  out:
  	return ret;
  }
@@ -2699,9 +2706,6 @@ 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 +2794,6 @@ 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 c13ae56fbff8..695bede14dac 100644
--- a/drivers/scsi/ufs/ufshcd.h
+++ b/drivers/scsi/ufs/ufshcd.h
@@ -777,6 +777,7 @@ 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