Regression: Failed boots bisected to 4cd13c21b207 "softirq: Let ksoftirqd do its job"

[Brian Starkey]

Hi,

I'm running an ARM FVP (virtual platform - simluated hardware), which
is failing to reach a login prompt due to extremely slow progress
during boot. systemd gives up waiting for the ttyAMA0 device to
appear, and never starts the getty.

I've bisected this to commit 4cd13c21b207 "softirq: Let ksoftirqd do
its job".

Without this commit, the system boots to a login prompt in 2 minutes.
With this commit, the system eventually manages to bring up sshd after
22 minutes, but as mentioned, the dev-ttyAMA0.device unit has timed
out and so I don't get a prompt on my console.

I only hit the issue when my rootfs is mounted over NFS, and with only
a single core enabled. The (simulated) network device is an SMC91C111.
With multiple cores enabled or a non-NFS filesystem, everything seems
to work OK.

I don't have an identical real hardware platform to try, but I
could not reproduce it on a real ARM Juno board, which is similar.

It looks from the logs that udev's workers are unable to make
progress, so the device nodes don't get created. Don't pay too much
attention to the timestamps in the logs below, they are "inside" the
virtual platform, and don't reflect wall-clock time.
Log before 4cd13c21b207:
	https://drive.google.com/open?id=0B8siaK6ZjvEwMktoa0NUS2hJd1U
Log after 4cd13c21b207:
	https://drive.google.com/open?id=0B8siaK6ZjvEwZXlfeFFSQl9xZTQ
Kernel config: arch/arm64/configs/defconfig

I'm not sure how to debug this further, so if you have any suggestions
I'd be glad to hear them.

Many thanks,
Brian