Re: [PATCH v3 16/17] scsi: ufs: Optimize the command queueing code
From: Asutosh Das (asd) <hidden>
Date: 2021-12-01 23:33:20
On 11/30/2021 3:33 PM, Bart Van Assche wrote:
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. Cc: Asutosh Das (asd) <redacted> 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(-)
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?
quoted hunk ↗ jump to hunk
diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c index c4cf5c4b4893..3e4c62c6f9d2 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; }@@ -2681,9 +2688,6 @@ 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 +2776,6 @@ 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
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