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

Vijay Kilari
2013-07-30T10:05:26Z

On Fri, Jul 26, 2013 at 10:35 PM, Stephen Warren <swarren@wwwdotorg.org> wrote:
> On 07/25/2013 11:41 PM, 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.
>
> I'm not sure that's true, unless perhaps you have CONFIG_KEXEC_JUMP enabled?
>
>> 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);
>
> I'm not familiar with that API, but it looks like it's just setting the
> *current* CPU offline. That sounds problematic for two reasons:
>
> 1) Setting the current CPU offline sounds like a bad idea; after all,
> code is still running on it. Presumably you want to offline all other CPUs.
>
machine_crash_nonpanic_core() is a SMP call (smp_call_function) .
Setting cpu offline is called for all other CPUs except the caller.

> 2) On a dual-CPU system, I guess this will leave a single CPU marked
> online, and hence satisfy the test in machine_kexec(). However, on a
> quad-core system, won't this just reduce the online CPU count from 4 to
> 3 and hence the test in machine_kexec() will still fail?
>
Setting CPU offline is called from SMP call function. So it is
called for all the
CPU's on the system except on caller CPU

> Can't you call disable_nonboot_cpus() from machine_crash_nonpanic_core()
> just like machine_shutdown() does?
I thought of using disable_nonboot_cpus(). However crash can happen on
any CPU. So we have to stop only nonpanic CPUs.
The other mechanisms I thought to offline CPUs is
1) Calling __cpu_disable() to put CPU completely offline. However
platform_cpu_disable() does not allow CPU 0 is disable (crash can happen
on any core).
2) Calling machine_halt(). This does not allow smp_send_stop() on
bootable cpu