linux-kernel - Re: [REGRESSION] CPUIDLE_FLAG_RCU_IDLE, blk_mq_freeze_queue

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Date:   Tue, 14 Mar 2023 16:21:15 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Alexey Klimov <alexey.klimov@...aro.org>, peterz@...radead.org
Cc:     draszik@...gle.com, peter.griffin@...aro.org,
        willmcvicker@...gle.com, mingo@...nel.org, ulf.hansson@...aro.org,
        tony@...mide.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, axboe@...nel.dk,
        alim.akhtar@...sung.com, regressions@...ts.linux.dev,
        avri.altman@....com, klimova@...gle.com
Subject: Re: [REGRESSION] CPUIDLE_FLAG_RCU_IDLE, blk_mq_freeze_queue_wait()
 and slow-stuck reboots

On 3/14/23 16:00, Alexey Klimov wrote:
> The delay is found to be in device's ->shutdown() methods called from kernel_restart():
> void kernel_restart_prepare(char *cmd)
> {
> 	blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, cmd);
> 	system_state = SYSTEM_RESTART;
> 	usermodehelper_disable();
> 	device_shutdown();	<---- here
> }
> 
> The driver in question is ufshcd and its ufshcd_wl_shutdown() shutdown method. It often blocks on scsi_device_quiesce() and upon manual checking it seems that it sleeps on blk_mq_freeze_queue_wait()/wait_event() in blk_freeze_queue():
> 
> scsi_device_quiesce(struct scsi_device *sdev)
> {
> 	...
> 	blk_mq_freeze_queue(q);
> 	...
> }
> 	||
> 	V
> void blk_freeze_queue(struct request_queue *q)
> {
> 	/*
> 	 * In the !blk_mq case we are only calling this to kill the
> 	 * q_usage_counter, otherwise this increases the freeze depth
> 	 * and waits for it to return to zero.  For this reason there is
> 	 * no blk_unfreeze_queue(), and blk_freeze_queue() is not
> 	 * exported to drivers as the only user for unfreeze is blk_mq.
> 	 */
> 	blk_freeze_queue_start(q);
> 	blk_mq_freeze_queue_wait(q);	<--- sleeps on wait_event() here
> }

Please let me know if you want me to resubmit patch "scsi: ufs: Remove 
the LUN quiescing code from ufshcd_wl_shutdown()" 
(https://lore.kernel.org/linux-scsi/20220331223424.1054715-14-bvanassche@acm.org/).

Thanks,

Bart.