[PATCH] powerpc, kexec: Fix "Processor X is stuck" issue during kexec from ST mode

[Srivatsa S. Bhat]

On 05/28/2014 07:01 PM, Vivek Goyal wrote:
> On Tue, May 27, 2014 at 04:25:34PM +0530, Srivatsa S. Bhat wrote:
>> If we try to perform a kexec when the machine is in ST (Single-Threaded) mode
>> (ppc64_cpu --smt=off), the kexec operation doesn't succeed properly, and we
>> get the following messages during boot:
>>
>> [    0.089866] POWER8 performance monitor hardware support registered
>> [    0.089985] power8-pmu: PMAO restore workaround active.
>> [    5.095419] Processor 1 is stuck.
>> [   10.097933] Processor 2 is stuck.
>> [   15.100480] Processor 3 is stuck.
>> [   20.102982] Processor 4 is stuck.
>> [   25.105489] Processor 5 is stuck.
>> [   30.108005] Processor 6 is stuck.
>> [   35.110518] Processor 7 is stuck.
>> [   40.113369] Processor 9 is stuck.
>> [   45.115879] Processor 10 is stuck.
>> [   50.118389] Processor 11 is stuck.
>> [   55.120904] Processor 12 is stuck.
>> [   60.123425] Processor 13 is stuck.
>> [   65.125970] Processor 14 is stuck.
>> [   70.128495] Processor 15 is stuck.
>> [   75.131316] Processor 17 is stuck.
>>
>> Note that only the sibling threads are stuck, while the primary threads (0, 8,
>> 16 etc) boot just fine. Looking closer at the previous step of kexec, we observe
>> that kexec tries to wakeup (bring online) the sibling threads of all the cores,
>> before performing kexec:
>>
>> [ 9464.131231] Starting new kernel
>> [ 9464.148507] kexec: Waking offline cpu 1.
>> [ 9464.148552] kexec: Waking offline cpu 2.
>> [ 9464.148600] kexec: Waking offline cpu 3.
>> [ 9464.148636] kexec: Waking offline cpu 4.
>> [ 9464.148671] kexec: Waking offline cpu 5.
>> [ 9464.148708] kexec: Waking offline cpu 6.
>> [ 9464.148743] kexec: Waking offline cpu 7.
>> [ 9464.148779] kexec: Waking offline cpu 9.
>> [ 9464.148815] kexec: Waking offline cpu 10.
>> [ 9464.148851] kexec: Waking offline cpu 11.
>> [ 9464.148887] kexec: Waking offline cpu 12.
>> [ 9464.148922] kexec: Waking offline cpu 13.
>> [ 9464.148958] kexec: Waking offline cpu 14.
>> [ 9464.148994] kexec: Waking offline cpu 15.
>> [ 9464.149030] kexec: Waking offline cpu 17.
>>
>> Instrumenting this piece of code revealed that the cpu_up() operation actually
>> fails with -EBUSY. Thus, only the primary threads of all the cores are online
>> during kexec, and hence this is a sure-shot receipe for disaster, as explained
>> in commit e8e5c2155b (powerpc/kexec: Fix orphaned offline CPUs across kexec),
>> as well as in the comment above wake_offline_cpus().
>>
>> It turns out that cpu_up() was returning -EBUSY because the variable
>> 'cpu_hotplug_disabled' was set to 1; and this disabling of CPU hotplug was done
>> by migrate_to_reboot_cpu() inside kernel_kexec().
>>
>> Now, migrate_to_reboot_cpu() was originally written with the assumption that
>> any further code will not need to perform CPU hotplug, since we are anyway in
>> the reboot path. However, kexec is clearly not such a case, since we depend on
>> onlining CPUs, atleast on powerpc.
>>
>> So re-enable cpu-hotplug after returning from migrate_to_reboot_cpu() in the
>> kexec path, to fix this regression in kexec on powerpc.
>>
>> Also, wrap the cpu_up() in powerpc kexec code within a WARN_ON(), so that we
>> can catch such issues more easily in the future.
>>
>> Fixes: c97102ba963 (kexec: migrate to reboot cpu)
>> Cc: stable at vger.kernel.org
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat at linux.vnet.ibm.com>
>> ---
>>
>>  arch/powerpc/kernel/machine_kexec_64.c |    2 +-
>>  kernel/kexec.c                         |    8 ++++++++
>>  2 files changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kernel/machine_kexec_64.c b/arch/powerpc/kernel/machine_kexec_64.c
>> index 59d229a..879b3aa 100644
>> --- a/arch/powerpc/kernel/machine_kexec_64.c
>> +++ b/arch/powerpc/kernel/machine_kexec_64.c
>> @@ -237,7 +237,7 @@ static void wake_offline_cpus(void)
>>  		if (!cpu_online(cpu)) {
>>  			printk(KERN_INFO "kexec: Waking offline cpu %d.\n",
>>  			       cpu);
>> -			cpu_up(cpu);
>> +			WARN_ON(cpu_up(cpu));
>>  		}
>>  	}
>>  }
>> diff --git a/kernel/kexec.c b/kernel/kexec.c
>> index c8380ad..28c5706 100644
>> --- a/kernel/kexec.c
>> +++ b/kernel/kexec.c
>> @@ -1683,6 +1683,14 @@ int kernel_kexec(void)
>>  		kexec_in_progress = true;
>>  		kernel_restart_prepare(NULL);
>>  		migrate_to_reboot_cpu();
>> +
>> +		/*
>> +		 * migrate_to_reboot_cpu() disables CPU hotplug assuming that
>> +		 * no further code needs to use CPU hotplug (which is true in
>> +		 * the reboot case). However, the kexec path depends on using
>> +		 * CPU hotplug again; so re-enable it here.
>> +		 */
>> +		cpu_hotplug_enable();
>>  		printk(KERN_EMERG "Starting new kernel\n");
>>  		machine_shutdown();
> 
> After migrate_to_reboot_cpu(), we are calling machine_shutdown() which
> calls disable_nonboot_cpus() and which in turn calls _cpu_down().
> 

Hmm? I see only 'arm' calling disable_nonboot_cpus() from machine_shutdown().
None of the other architectures call it. Is that a leftover in arm?

> So it is kind of odd that we first migrate to boot cpu, and then disable
> all non-boot cpus and after that powerpc goes ahead and onlines all
> cpus.
> 
> I think this is not a good idea. For whatever reason if powerpc has to
> online all cpus, then it should happne earlier and not in machine_kexec().
> 
> In fact I think generic code expects that all non-boot cpus are disabled
> so that generic code can use all the RAM as it wants to. Now if powerpc
> breaks that assumption, it will lead to various kind of issues.
> 
> So I think we need to go back and see if we can find a way where we
> don't have to online all cpus in first kernel. And second kernel needs
> to have a way to detect it and online things.
> 

Yep, that makes sense. But unfortunately I don't have enough insight into
why exactly powerpc has to online the CPUs before doing a kexec. I just
know from the commit log and the comment mentioned above (and from my own
experiments) that the CPUs will get stuck if they were offline. Perhaps
somebody more knowledgeable can explain this in detail and suggest a proper
long-term solution.

Matt, Ben, any thoughts on this?

Regards,
Srivatsa S. Bhat