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

From: [email-masked] (Stephen Warren)
Date: Fri, 26 Jul 2013 11:08:07 -0600

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 at gmail.com wrote:
>> From: Vijaya Kumar K <[email-masked]>
>>
>> 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?