Subject: Re: [PATCH] powerpc: on crash, kexec'ed kernel needs all CPUs are online

From: Laurent Vivier <[email-masked]>
Date: Wed, 4 Nov 2015 14:54:51 +0100

On 04/11/2015 13:34, Hari Bathini wrote:
> On 10/16/2015 12:30 AM, Laurent Vivier wrote:
>> On kexec, all secondary offline CPUs are onlined before
>> starting the new kernel, this is not done in the case of kdump.
>>
>> If kdump is configured and a kernel crash occurs whereas
>> some secondaries CPUs are offline (SMT=off),
>> the new kernel is not able to start them and displays some
>> "Processor X is stuck.".
>>
>> Starting with POWER8, subcore logic relies on all threads of
>> core being booted. So, on startup kernel tries to start all
>> threads, and asks OPAL (or RTAS) to start all CPUs (including
>> threads). If a CPU has been offlined by the previous kernel,
>> it has not been returned to OPAL, and thus OPAL cannot restart
>> it: this CPU has been lost...
>>
>> Signed-off-by: Laurent Vivier<[email-masked]>
> 
> 
> Hi Laurent,

Hi Hari,

> Sorry for jumping too late into this.

better late than never :)

> Are you seeing this issue even with the below patches:
> 
> pseries:
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=c1caae3de46a072d0855729aed6e793e536a4a55
> 
> 
> opal/powernv:
> https://github.com/open-power/skiboot/commit/9ee56b5

Very interesting. Is there a way to have a firmware with the fix ?

Thanks,
Laurent

> Thanks
> Hari
> 
>> ---
>>   arch/powerpc/kernel/crash.c | 20 ++++++++++++++++++++
>>   1 file changed, 20 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/crash.c b/arch/powerpc/kernel/crash.c
>> index 51dbace..3ca9452 100644
>> --- a/arch/powerpc/kernel/crash.c
>> +++ b/arch/powerpc/kernel/crash.c
>> @@ -19,6 +19,7 @@
>>   #include <linux/delay.h>
>>   #include <linux/irq.h>
>>   #include <linux/types.h>
>> +#include <linux/cpu.h>
>>     #include <asm/processor.h>
>>   #include <asm/machdep.h>
>> @@ -299,11 +300,30 @@ int crash_shutdown_unregister(crash_shutdown_t
>> handler)
>>   }
>>   EXPORT_SYMBOL(crash_shutdown_unregister);
>>   +/*
>> + * The next kernel will try to start all secondary CPUs and if
>> + * there are not online it will fail to start them.
>> + *
>> + */
>> +static void wake_offline_cpus(void)
>> +{
>> +    int cpu = 0;
>> +
>> +    for_each_present_cpu(cpu) {
>> +        if (!cpu_online(cpu)) {
>> +            pr_info("kexec: Waking offline cpu %d.\n", cpu);
>> +            cpu_up(cpu);
>> +        }
>> +    }
>> +}
>> +
>>   void default_machine_crash_shutdown(struct pt_regs *regs)
>>   {
>>       unsigned int i;
>>       int (*old_handler)(struct pt_regs *regs);
>>   +    wake_offline_cpus();
>> +
>>       /*
>>        * This function is only called after the system
>>        * has panicked or is otherwise in a critical state.
>