Re: [PATCH] ARM: kexec: offline non panic CPUs on Kdump panic

Will Deacon
2013-07-26T17:11:55Z

On Fri, Jul 26, 2013 at 06:08:07PM +0100, Stephen Warren wrote:
> On 07/26/2013 04:49 AM, Will Deacon wrote:
> > [Adding Stephen Warren since he has been working in this area]
> >
> > On Fri, Jul 26, 2013 at 06:41:27AM +0100, vijay.kilari@gmail.com wrote:
> >> From: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>
> >>
> >> In case of normal kexec kernel load, all cpu's are offlined
> >> before calling machine_kexec() under kernel_kexec() function.
> >> But in case crash panic cpus are relaxed in
> >> machine_crash_nonpanic_core() SMP function but not offlined.
> >>
> >> When crash kernel is loaded with kexec and on panic trigger
> >> machine_kexec() checks for number of cpus online.
> >> If more than one cpu is online machine_kexec() fails to load
> >> with below error
> >>
> >> kexec: error: multiple CPUs still online
> >>
> >> In machine_crash_nonpanic_core() SMP function, offline CPU
> >> before cpu_relax
>
> >> diff --git a/arch/arm/kernel/machine_kexec.c b/arch/arm/kernel/machine_kexec.c
>
> >> @@ -73,6 +73,7 @@ void machine_crash_nonpanic_core(void *unused)
> >> crash_save_cpu(&regs, smp_processor_id());
> >> flush_cache_all();
> >>
> >> + set_cpu_online(smp_processor_id(), false);
> >> atomic_dec(&waiting_for_crash_ipi);
> >> while (1)
> >> cpu_relax();
> >
> > Ok, I guess this will work since the new kernel is loaded somewhere higher
> > in memory and the crashed kernel will stick around, so the non-crashing CPUs
> > can sit around spinning.
>
> Does a kernel that's used as the crash kernel guarantee:
>
> * Never to re-use the memory that was used by the previous kernel, so
> that the spin loop code/data won't be corrupted, ever, no matter how
> long the crash recovery kernel runs.
>
> * Not use SMP, so there's never a need to re-activate the non-boot CPUs,
> which might not work if they aren't truly disabled but rather just
> running a pin loop?

I *think* this is true, and x86 seems to have code to a similar effect (the
powerpc stuff lost me though). I've never played with crash kernels on SMP
though...

Will