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Date:   Sat, 8 Dec 2018 16:49:53 -0800
From:   Guenter Roeck <linux@roeck-us.net>
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
Subject: Re: [PATCH] nvme: default to 0 poll queues
Message-ID: <20181209004953.GA11638@roeck-us.net>
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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.

Thanks,
Guenter