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

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Message-ID: <20161116180156.GA21156@e106950-lin.cambridge.arm.com>
Date:   Wed, 16 Nov 2016 18:01:56 +0000
From:   Brian Starkey <brian.starkey@....com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Regression: Failed boots bisected to 4cd13c21b207 "softirq: Let
 ksoftirqd do its job"

Hi Eric,

On Wed, Nov 16, 2016 at 07:52:42AM -0800, Eric Dumazet wrote:
>On Wed, Nov 16, 2016 at 5:55 AM, Brian Starkey <brian.starkey@....com> 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.
>

The smc91x driver does seem to have some trickiness around softirqs.
I'm not familiar with net drivers, but I'll see if I can figure
anything out there.

>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/

You're right in saying that this didn't help.

Thanks,
Brian