On Sun, Oct 25, 2020 at 8:12 AM Pingfan Liu [off-list ref] wrote:

On Thu, Oct 22, 2020 at 4:37 PM Thomas Gleixner [off-list ref] wrote:

quoted

On Thu, Oct 22 2020 at 13:56, Pingfan Liu wrote:

quoted

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