Re: printk.time causes rare kernel boot hangs

["Richard W.M. Jones" <rjones@redhat.com>]

On Wed, Jun 14, 2023 at 01:35:36PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 14, 2023 at 11:39:53AM +0100, Richard W.M. Jones wrote:
> > Got it!
> > 
> > #0  arch_static_branch (branch=false, key=<optimized out>)
> >     at ./arch/x86/include/asm/jump_label.h:27
> > #1  static_key_false (key=<optimized out>) at ./include/linux/jump_label.h:207
> > #2  native_write_msr (msr=1760, low=1876580734, high=106)
> >     at ./arch/x86/include/asm/msr.h:147
> > #3  0xffffffff8107997c in paravirt_write_msr (high=<optimized out>, 
> >     low=1876580734, msr=1760) at ./arch/x86/include/asm/paravirt.h:196
> > #4  wrmsrl (val=<optimized out>, msr=1760)
> >     at ./arch/x86/include/asm/paravirt.h:229
> > #5  lapic_next_deadline (delta=<optimized out>, evt=<optimized out>)
> >     at arch/x86/kernel/apic/apic.c:491
> > #6  0xffffffff811f7b1d in clockevents_program_event (dev=0xffff88804e820dc0, 
> >     expires=<optimized out>, force=<optimized out>)
> >     at kernel/time/clockevents.c:334
> > #7  0xffffffff811f81b0 in tick_handle_periodic (dev=0xffff88804e820dc0)
> >     at kernel/time/tick-common.c:133
> > #8  0xffffffff810796c1 in local_apic_timer_interrupt ()
> >     at arch/x86/kernel/apic/apic.c:1095
> > #9  __sysvec_apic_timer_interrupt (regs=regs@entry=0xffffc90000003ee8)
> >     at arch/x86/kernel/apic/apic.c:1112
> > #10 0xffffffff81f9cf09 in sysvec_apic_timer_interrupt (regs=0xffffc90000003ee8)
> >     at arch/x86/kernel/apic/apic.c:1106
> > #11 0xffffffff820015ca in asm_sysvec_apic_timer_interrupt ()
> >     at ./arch/x86/include/asm/idtentry.h:645
> > #12 0x0000000000000000 in ?? ()
> 
> Uuuhh.. something is really fishy here. The thing in common between the
> fingered patch and this stacktrace is the jump_label/static_branch
> usage, but they're quite different paths.
> 
> There is no printk or local_clock() in sight here.

So I should note that the stack trace is collected by connecting to
qemu after the kernel "hangs" ("hangs" in quotes, because it is
probably still running).

It's difficult to collect a stack trace at the moment of the hang
since I basically have to wait for hundreds of boot iterations until I
see the bug manifest.

I wonder if the KVM tracing features would be a better approach here?

> I've got that plain qemu thing running on:
> 
>   defconfig + kvm_guest.config + CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
> 
> and have added "nokaslr" to the -append string. Lets see if it wants to
> go wobbly for me.

Thanks,

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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