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Subject: Re: [PATCH] nvme: default to 0 poll queues
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Christoph Hellwig <hch@lst.de>,
        Keith Busch <keith.busch@intel.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20181209004953.GA11638@roeck-us.net>
 <fa7aabad-774b-4e5b-686f-3696762291c1@kernel.dk>
 <4ad5653b-1cd4-a770-2290-ca032eeb7072@roeck-us.net>
 <C7D155CA-8DA8-4DF2-BBC2-14E6AF436F24@kernel.dk>
From:   Guenter Roeck <linux@roeck-us.net>
Message-ID: <0a6c9479-58b9-5af7-7fb8-880730554e69@roeck-us.net>
Date:   Sat, 8 Dec 2018 23:32:07 -0800
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On 12/8/18 10:31 PM, Jens Axboe wrote:
> On Dec 8, 2018, at 11:22 PM, Guenter Roeck <linux@roeck-us.net> wrote:
>>
>>> On 12/8/18 9:38 PM, Jens Axboe wrote:
>>>> On 12/8/18 5:49 PM, Guenter Roeck wrote:
>>>> Hi,
>>>>
>>>>> On Mon, Nov 19, 2018 at 08:18:24AM -0700, Jens Axboe wrote:
>>>>> We need a better way of configuring this, and given that polling is
>>>>> (still) a bit niche, let's default to using 0 poll queues. That way
>>>>> we'll have the same read/write/poll behavior as 4.20, and users that
>>>>> want to test/use polling are required to do manual configuration of the
>>>>> number of poll queues.
>>>>>
>>>>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>> ---
>>>>
>>>> This patch results in a boot stall when booting parisc (hppa) images
>>>> from nvme in qemu.
>>>>
>>>> ...
>>>> Fusion MPT SAS Host driver 3.04.20
>>>> rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
>>>> rcu:    (detected by 0, t=5252 jiffies, g=141, q=22)
>>>> rcu: All QSes seen, last rcu_sched kthread activity 5252 (-66742--71994), jiffies_till_next_fqs=1, root ->qsmask 0x0
>>>> kworker/u8:3    R  running task        0    85      2 0x00000004
>>>> Workqueue: nvme-reset-wq nvme_reset_work
>>>> Backtrace:
>>>>   [<10190d20>] show_stack+0x28/0x38
>>>>   [<101dd1e0>] sched_show_task.part.3+0xc4/0x144
>>>>   [<101dd290>] sched_show_task+0x30/0x38
>>>>   [<10221e18>] rcu_check_callbacks+0x760/0x7a4
>>>>
>>>> rcu: rcu_sched kthread starved for 5252 jiffies! g141 f0x2 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0
>>>> rcu: RCU grace-period kthread stack dump:
>>>> rcu_sched       R  running task        0    10      2 0x00000000
>>>> Backtrace:
>>>>   [<10995b1c>] __schedule+0x214/0x648
>>>>   [<10995f94>] schedule+0x44/0xa8
>>>>   [<1099a7c4>] schedule_timeout+0x114/0x1a0
>>>>   [<10220e70>] rcu_gp_kthread+0x744/0x968
>>>>   [<101d5438>] kthread+0x154/0x15c
>>>>   [<1019501c>] ret_from_kernel_thread+0x1c/0x24
>>>>
>>>> [ continued ]
>>>>
>>>> This is only seen in SMP configurations; non-SMP configurations are ok.
>>>> Reverting the patch fixes the problem. v4.20-rcX and earlier kernels
>>>> also boot without problems.
>>>>
>>>> For reference, here is the qemu command line. This is with qemu 3.0.
>>>>
>>>> qemu-system-hppa -kernel vmlinux -no-reboot \
>>>>     -snapshot \
>>>>     -device nvme,serial=foo,drive=d0 \
>>>>     -drive file=rootfs.ext2,if=none,format=raw,id=d0 \
>>>>     -append 'root=/dev/nvme0n1 rw rootwait panic=-1 console=ttyS0,115200 ' \
>>>>     -nographic -monitor null
>>>>
>>>> Please let me know if you need additional information.
>>> Hmm, I think the queue reduction case has a logic error. Actually there
>>> are two bugs:
>>> 1) Ensure we don't keep overwriting the queue count we ask for
>>> 2) Don't include poll_queues in the vectors we need
>>> Untested... And not super pretty. But does this work for you?
>>
>> It solves the boot problem on parisc/hppa. I didn't test with any other architectures.
>> Should I run a complete test sequence ?
> 
> That’d be great, thanks.
> 

Ok, started.

Guenter