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From:   Guilherme Piccoli <gpiccoli@canonical.com>
Date:   Mon, 26 Oct 2020 12:06:17 -0300
Message-ID: <CAHD1Q_x99XW1zDr5HpVR27F_ksHLkaxc2W83e-N6F_xLYKyGbQ@mail.gmail.com>
Subject: Re: [PATCH 0/3] warn and suppress irqflood
To:     Pingfan Liu <kernelfans@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Jisheng Zhang <Jisheng.Zhang@synaptics.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Petr Mladek <pmladek@suse.com>, Marc Zyngier <maz@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        afzal mohammed <afzal.mohd.ma@gmail.com>,
        Lina Iyer <ilina@codeaurora.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Maulik Shah <mkshah@codeaurora.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Jonathan Corbet <corbet@lwn.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Oliver Neukum <oneukum@suse.com>, linux-doc@vger.kernel.org,
        Kexec Mailing List <kexec@lists.infradead.org>
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On Sun, Oct 25, 2020 at 8:12 AM Pingfan Liu <kernelfans@gmail.com> wrote:
>
> On Thu, Oct 22, 2020 at 4:37 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > On Thu, Oct 22 2020 at 13:56, Pingfan Liu wrote:
> > > I hit a irqflood bug on powerpc platform, and two years ago, on a x86 platform.
> > > When the bug happens, the kernel is totally occupies by irq.  Currently, there
> > > may be nothing or just soft lockup warning showed in console. It is better
> > > to warn users with irq flood info.
> > >
> > > In the kdump case, the kernel can move on by suppressing the irq flood.
> >
> > You're curing the symptom not the cause and the cure is just magic and
> > can't work reliably.
> Yeah, it is magic. But at least, it is better to printk something and
> alarm users about what happens. With current code, it may show nothing
> when system hangs.

Thanks Pingfan and Thomas for the points - I'd like to have a
mechanism in the kernel to warn users when an IRQ flood is potentially
happening.
Some time ago (2 years) we faced a similar issue in x86-64, a hard to
debug problem in kdump, that eventually was narrowed to a buggy NIC FW
flooding IRQs in kdump kernel, and no messages showed (although kernel
changed a lot since that time, today we might have better IRQ
handling/warning). We tried an early-boot fix, by disabling MSIs (as
per PCI spec) early in x86 boot, but it wasn't accepted - Bjorn asked
pertinent questions that I couldn't respond (I lost the reproducer)
[0].

Cheers,


Guilherme

[0] lore.kernel.org/linux-pci/20181018183721.27467-1-gpiccoli@canonical.com