Re: + panic-avoid-the-extra-noise-dmesg.patch added to -mm tree

Sergey Senozhatsky
2018-12-05T05:29:12Z

On (12/05/18 11:57), Sergey Senozhatsky wrote:
> On (12/05/18 10:47), Feng Tang wrote:
> >
> > Btw, just FYI, I just tried the sysrq (using minicom CTL + A + F + 'magic key'),
> > it works with system is running, but failed after I trigger a panic, I will
> > check more though I'm not very familiar with sysrq yet.

OK... So, apparently, what's happening is panic() calls smp_send_stop().
And smp_send_stop()->native_stop_other_cpus() on x86 disables local APIC.
So no fun anymore.

If I keep APIC enabled on panic CPU, then I have my keyboard working,
including PageUp-PageDown scrolling, sysrq handling, and so on.

I think I'm not the only one who'd want scrollback to work after panic
(yes, fremebuffer for debugging).

Andi Kleen [1] wrote (Cc-ed):
: Oops/warnings are getting longer and longer, often scrolling away
: from the screen, and if the kernel crashes backscroll does not work
: anymore, so precious information is lost.

PeterZ,
for those folks who sometimes have to use framebuffer for debugging
(just a trivial "let me scrollback and see the panic backtrace") and
not always have access to serial console, can we have local APIC
enabled on the panic_cpu? Or is it a terrible thing to ask for?

[1] https://lore.kernel.org/lkml/878tcvt592.fsf@linux.intel.com/T/#u

-ss