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

["Asutosh Das (asd)" <asutoshd@codeaurora.org>]

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) <asutoshd@codeaurora.org>
> 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?

> 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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