Re: [PATCH v3 16/17] scsi: ufs: Optimize the command queueing code

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

On 12/1/21 3:33 PM, Asutosh Das (asd) wrote:
> Hi Bart,
> Say an IO (req1) has crossed the scsi_host_queue_ready() check but hasn't yet reached ufshcd_queuecommand() and DBR is 0.
> ufshcd_clock_scaling_prepare() is invoked and completes and scaling proceeds to change the clocks and gear.
> I wonder if the IO (req1) would be issued while scaling is in progress.
> If so, do you think a check should be added in ufshcd_queuecommand() to see if scaling is in progress or if host is blocked?

How about using the patch below instead of patch 16/17 from this patch
series? The patch below should not trigger the race condition mentioned
above.

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, wait until all budget_maps have cleared
to wait for ongoing ufshcd_queuecommand() calls.

Cc: Asutosh Das (asd) <asutoshd@codeaurora.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
  drivers/scsi/ufs/ufshcd.c | 33 +++++++++++++++++++++++----------
  drivers/scsi/ufs/ufshcd.h |  1 +
  2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index c4cf5c4b4893..be679f2a97da 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c
@@ -1070,13 +1070,31 @@ static bool ufshcd_is_devfreq_scaling_required(struct ufs_hba *hba,
  	return false;
  }

+/*
+ * Determine the number of pending commands by counting the bits in the SCSI
+ * device budget maps. This approach has been selected because a bit is set in
+ * the budget map before scsi_host_queue_ready() checks the host_self_blocked
+ * flag. The host_self_blocked flag can be modified by calling
+ * scsi_block_requests() or scsi_unblock_requests().
+ */
+static u32 ufshcd_pending_cmds(struct ufs_hba *hba)
+{
+	struct scsi_device *sdev;
+	u32 pending;
+
+	shost_for_each_device(sdev, hba->host)
+		pending += sbitmap_weight(&sdev->budget_map);
+
+	return pending;
+}
+
  static int ufshcd_wait_for_doorbell_clr(struct ufs_hba *hba,
  					u64 wait_timeout_us)
  {
  	unsigned long flags;
  	int ret = 0;
  	u32 tm_doorbell;
-	u32 tr_doorbell;
+	u32 tr_pending;
  	bool timeout = false, do_last_check = false;
  	ktime_t start;

@@ -1094,8 +1112,8 @@ static int ufshcd_wait_for_doorbell_clr(struct ufs_hba *hba,
  		}

  		tm_doorbell = ufshcd_readl(hba, REG_UTP_TASK_REQ_DOOR_BELL);
-		tr_doorbell = ufshcd_readl(hba, REG_UTP_TRANSFER_REQ_DOOR_BELL);
-		if (!tm_doorbell && !tr_doorbell) {
+		tr_pending = ufshcd_pending_cmds(hba);
+		if (!tm_doorbell && !tr_pending) {
  			timeout = false;
  			break;
  		} else if (do_last_check) {
@@ -1115,12 +1133,12 @@ static int ufshcd_wait_for_doorbell_clr(struct ufs_hba *hba,
  			do_last_check = true;
  		}
  		spin_lock_irqsave(hba->host->host_lock, flags);
-	} while (tm_doorbell || tr_doorbell);
+	} while (tm_doorbell || tr_pending);

  	if (timeout) {
  		dev_err(hba->dev,
  			"%s: timedout waiting for doorbell to clear (tm=0x%x, tr=0x%x)\n",
-			__func__, tm_doorbell, tr_doorbell);
+			__func__, tm_doorbell, tr_pending);
  		ret = -EBUSY;
  	}
  out:
@@ -2681,9 +2699,6 @@ 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.
@@ -2772,8 +2787,6 @@ 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 c3c2792f309f..411c6015bbfe 100644
--- a/drivers/scsi/ufs/ufshcd.h
+++ b/drivers/scsi/ufs/ufshcd.h
@@ -779,6 +779,7 @@ 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