Re: [PATCH V4 0/5] blk-mq: improvement on handling IO during CPU hotplug

From: John Garry <john.garry@huawei.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, <linux-block@vger.kernel.org>,
	"Bart Van Assche" <bvanassche@acm.org>,
	Hannes Reinecke <hare@suse.com>, "Christoph Hellwig" <hch@lst.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Keith Busch <keith.busch@intel.com>
Subject: Re: [PATCH V4 0/5] blk-mq: improvement on handling IO during CPU hotplug
Date: Wed, 16 Oct 2019 17:19:01 +0100	[thread overview]
Message-ID: <9dbc14ab-65cd-f7ac-384c-2dbe03575ee7@huawei.com> (raw)
In-Reply-To: <20191016120729.GB5515@ming.t460p>

On 16/10/2019 13:07, Ming Lei wrote:
> On Wed, Oct 16, 2019 at 09:58:27AM +0100, John Garry wrote:
>> On 14/10/2019 02:50, Ming Lei wrote:
>>> Hi,
>>>
>>> Thomas mentioned:
>>>     "
>>>      That was the constraint of managed interrupts from the very beginning:
>>>
>>>       The driver/subsystem has to quiesce the interrupt line and the associated
>>>       queue _before_ it gets shutdown in CPU unplug and not fiddle with it
>>>       until it's restarted by the core when the CPU is plugged in again.
>>>     "
>>>
>>> But no drivers or blk-mq do that before one hctx becomes dead(all
>>> CPUs for one hctx are offline), and even it is worse, blk-mq stills tries
>>> to run hw queue after hctx is dead, see blk_mq_hctx_notify_dead().
>>>
>>> This patchset tries to address the issue by two stages:
>>>
>>> 1) add one new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE
>>>
>>> - mark the hctx as internal stopped, and drain all in-flight requests
>>> if the hctx is going to be dead.
>>>
>>> 2) re-submit IO in the state of CPUHP_BLK_MQ_DEAD after the hctx becomes dead
>>>
>>> - steal bios from the request, and resubmit them via generic_make_request(),
>>> then these IO will be mapped to other live hctx for dispatch
>>>
>>> Please comment & review, thanks!
>>>
>>> John, I don't add your tested-by tag since V3 have some changes,
>>> and I appreciate if you may run your test on V3.
>>
>> Hi Ming,
>>
>> So I got around to doing some testing. The good news is that issue which we
>> were experiencing in v3 series seems to have has gone away - alot more
>> stable.
>>
>> However, unfortunately, I did notice some SCSI timeouts:
>>
>> 15508.615074] CPU2: shutdown
>> [15508.617778] psci: CPU2 killed.
>> [15508.651220] CPU1: shutdown
>> [15508.653924] psci: CPU1 killed.
>> [15518.406229] sas: Enter sas_scsi_recover_host busy: 63 failed: 63
>> Jobs: 1 (f=1): [R] [0.0% done] [0[15518.412239] sas: sas_scsi_find_task:
>> aborting task 0x00000000a7159744
>> KB/0KB/0KB /s] [0/0/0 iops] [eta [15518.421708] sas:
>> sas_eh_handle_sas_errors: task 0x00000000a7159744 is done
>> [15518.431266] sas: sas_scsi_find_task: aborting task 0x00000000d39731eb
>> [15518.442539] sas: sas_eh_handle_sas_errors: task 0x00000000d39731eb is
>> done
>> [15518.449407] sas: sas_scsi_find_task: aborting task 0x000000009f77c9bd
>> [15518.455899] sas: sas_eh_handle_sas_errors: task 0x000000009f77c9bd is
>> done
>>
>> A couple of things to note:
>> - I added some debug prints in blk_mq_hctx_drain_inflight_rqs() for when
>> inflights rqs !=0, and I don't see them for this timeout
>> - 0 datarate reported from fio
>>
>> I'll have a look...
>
> What is the output of the following command?
>
> (cd /sys/kernel/debug/block/$SAS_DISK && find . -type f -exec grep -aH . {} \;)
I assume that you want this run at about the time SCSI EH kicks in for 
the timeout, right? If so, I need to find some simple sysfs entry to 
tell me of this occurrence, to trigger the capture. Or add something. My 
script is pretty dump.

BTW, I did notice that we the dump_stack in __blk_mq_run_hw_queue() 
pretty soon before the problem happens - maybe a clue or maybe coincidence.

Thanks,
John

>
> Thanks,
> Ming
>
> .
>