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

From: Vivek Goyal <[email-masked]>
Date: Wed, 4 Jun 2014 09:41:08 -0400

On Wed, Jun 04, 2014 at 01:58:40AM +0530, Srivatsa S. Bhat wrote:
> 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: [email-masked]
> >> Signed-off-by: Srivatsa S. Bhat <[email-masked]>
> >> ---
> >>
> >>  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?

You are right. I did not notice that only arm is doing that. Looks like
it is calling into some platform code, I am not sure what exactly arm
does for disabling cpu.

x86 code calls stop_other_cpus() in machine_shutdown() which sends
REBOOT_VECTOR to other cpus and calls stop_this_cpu() which in turn
does.

        for (;;)
                halt();

IIUC, upon receipt of certain interrupts cpu will come out of halt state.
Not sure how safe it is from kexec point of view as we will be replacing
original kernel that means if cpu comes out of halt state it might be
running some random code.

Eric/hpa might know better the context here and what safeguards us on x86.

So one should not make cpu spin on some code as kexec will change that
code. It should be some other platform specific mechanism which brings
cpu in to hlt like state. So that way arm seems to be doing right thing.

I am not sure what powerpc does to stop cpus.

Thanks
Vivek