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

[Eric Dumazet]

On Wed, Nov 16, 2016 at 5:55 AM, Brian Starkey <brian.starkey@xxxxxxx> wrote:
> 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
>

Hi Brian.

Thanks a lot for this report.

If issue triggers when/if using one core, it is possible one driver
has a dependency on
softirqs being serviced during an initialization loop.

If the thread is not yielding cpu (holding something like a spinlock
thus disabling preemption),
then ksoftirqd might not be able to run on the (same) cpu.

I sent a patch for busy polling yesterday, but I am almost certain
this would not fix your issue
(assuming you have CONFIG_PREEMPT)

https://patchwork.ozlabs.org/patch/695185/