Re: [PATCH] nvme: default to 0 poll queues

[Guenter Roeck]

On 12/8/18 10:31 PM, Jens Axboe wrote:
> On Dec 8, 2018, at 11:22 PM, Guenter Roeck <linux@xxxxxxxxxxxx> 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@xxxxxx>
> > > > > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> > > > > ---
> > > >
> > > > 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 ?
>
> Thatâd be great, thanks.

Ok, started.

Guenter