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

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Message-ID: <20161117164200.GA24653@e106950-lin.cambridge.arm.com>
Date:   Thu, 17 Nov 2016 16:42:01 +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 Thu, Nov 17, 2016 at 07:29:14AM -0800, Eric Dumazet wrote:
>On Wed, Nov 16, 2016 at 1:01 PM, Brian Starkey <brian.starkey@....com> wrote:
>> On Wed, Nov 16, 2016 at 10:49:06AM -0800, Eric Dumazet wrote:
>>>
>>> On Wed, Nov 16, 2016 at 10:01 AM, Brian Starkey <brian.starkey@....com>
>>> wrote:
>>>
>>>>
>>>> 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.
>>>
>>>
>>> Oh this code looks ugly :(
>>>
>>> Do you have CONFIG_SMP=y or not ?
>>
>>
>> Yeah CONFIG_SMP=y (and CONFIG_PREEMPT=y too, fwiw).
>>
>> I did try forcing it into the no-op locking (as though config SMP
>> wasn't set), it didn't help (and it doesn't look like that would be
>> safe with CONFIG_PREEMPT=y either).
>>
>> The bit in smc_hardware_send_pkt looks like skipping softirq
>> invocation when there's already one running wouldn't give the same
>> behaviour as before:
>>
>>         if (!smc_special_trylock(&lp->lock, flags)) {
>>                 netif_stop_queue(dev);
>>                 tasklet_schedule(&lp->tx_task);
>>                 return;
>>         }
>>
>> ... that said, I've no idea if that matters.
>>
>> Of course I also don't know if the network driver is even to blame :-(
>>
>
>I believe the problem is in SMC_WAIT_MMU_BUSY()
>
>Could you try this patch ? (inlined and attached)
>

No joy with this patch :-(

I had to add an ioaddr argument because apparently that macro depends
on local context (yuck...), but it doesn't help my issue.

FWIW I don't see any timeouts, either with or without the patch.
(I don't know for sure, but I would guess that the model of the
network card doesn't model whatever stall that loop is checking for.
It probably just completes all MMU operations immediately)

-Brian