linux-kernel - Re: [PATCH] scsi: ufs: core: quiesce request queues before check pending cmds

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Date: Wed, 26 Jun 2024 09:33:33 -0700
From: Bart Van Assche <bvanassche@....org>
To: Ziqi Chen <quic_ziqichen@...cinc.com>,
 Peter Wang (王信友) <peter.wang@...iatek.com>,
 "quic_rampraka@...cinc.com" <quic_rampraka@...cinc.com>,
 "quic_nguyenb@...cinc.com" <quic_nguyenb@...cinc.com>,
 "quic_nitirawa@...cinc.com" <quic_nitirawa@...cinc.com>,
 "beanhuo@...ron.com" <beanhuo@...ron.com>,
 "avri.altman@....com" <avri.altman@....com>,
 "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
 "junwoo80.lee@...sung.com" <junwoo80.lee@...sung.com>,
 "mani@...nel.org" <mani@...nel.org>,
 "quic_cang@...cinc.com" <quic_cang@...cinc.com>
Cc: "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
 "alim.akhtar@...sung.com" <alim.akhtar@...sung.com>,
 "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
 "quic_asutoshd@...cinc.com" <quic_asutoshd@...cinc.com>,
 "quic_mnaresh@...cinc.com" <quic_mnaresh@...cinc.com>,
 "manivannan.sadhasivam@...aro.org" <manivannan.sadhasivam@...aro.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scsi: ufs: core: quiesce request queues before check
 pending cmds

On 6/26/24 6:18 AM, Ziqi Chen wrote:
> On 6/26/2024 12:13 AM, Bart Van Assche wrote:
>> ufshcd_wait_for_doorbell_clr() should be removed and 
>> ufshcd_clock_scaling_prepare() should use blk_mq_freeze_*().
>> See also my patch "ufs: Simplify the clock scaling mechanism
>> implementation" from 5 years ago 
>> (https://lore.kernel.org/linux-scsi/20191112173743.141503-5-bvanassche@acm.org/).
>
> The defect of blk_mq_freeze_*() is that it would bring in significant 
> latency and performance regression. I don't think it is what many people 
> want to see.

This can be solved by inserting a synchronize_srcu_expedited() call
between the blk_freeze_queue_start() and the
blk_mq_freeze_queue_wait_timeout() calls. With that call added, the
latency of the new code should be lower than that of the
io_schedule_timeout(msecs_to_jiffies(20)) call in the current code.

blk_mq_freeze_*() is only slow if the RCU grace period is not expedited.

Bart.