linux-kernel - Re: [PATCH] nvme: default to 0 poll queues

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Date:   Sat, 8 Dec 2018 23:31:48 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Christoph Hellwig <hch@....de>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme: default to 0 poll queues

On Dec 8, 2018, at 11:22 PM, Guenter Roeck <linux@...ck-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@....de>
>>>> Signed-off-by: Jens Axboe <axboe@...nel.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.