[PATCH 1/2] arm64: smp: Add function to determine if cpus are stuck in the kernel

[will.deacon@arm.com (Will Deacon)]

HI James,

On Fri, Jun 17, 2016 at 05:48:35PM +0100, James Morse wrote:
> On 17/06/16 12:16, Suzuki K Poulose wrote:
> > On 17/06/16 11:27, Mark Rutland wrote:
> >> On Fri, Jun 17, 2016 at 10:34:56AM +0100, James Morse wrote:
> >>> kernel/smp.c has a fancy counter that keeps track of the number of CPUs
> >>> it marked as not-present and left in cpu_park_loop(). If there are any
> >>> CPUs spinning in here, features like kexec or hibernate may release them
> >>> by overwriting this memory.
> >>>
> >>> This problem also occurs on machines using spin-tables to release
> >>> secondary cores.
> >>> After commit 44dbcc93ab67 ("arm64: Fix behavior of maxcpus=N")
> >>> we bring all known cpus into the secondary holding pen, but may not bring
> >>> them up depending on 'maxcpus'. This memory can't be re-used by kexec
> >>> or hibernate.
> >>>
> >>> Add a function cpus_are_stuck_in_kernel() to determine if either of these
> >>> cases have occurred.
> 
> >> It might also be stuck in __no_granule_support, if it never made it to C
> >> code. In that case, the CPU in charge of bringing up that new CPU will
> >> increment the counter in __cpu_up.
> > 
> > Just to clarify, *in all the cases*, the CPU in charge of bringing up updates
> > the cpus_stuck_in_kernel.
> 
> Ah, my mistake. I will switch it for Mark's suggestion.
> 
> >>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> >>> index 678e0842cb3b..e197502f94fd 100644
> >>> --- a/arch/arm64/kernel/smp.c
> >>> +++ b/arch/arm64/kernel/smp.c
> >>> @@ -909,3 +909,16 @@ int setup_profiling_timer(unsigned int multiplier)
> >>>   {
> >>>       return -EINVAL;
> >>>   }
> >>> +
> >>> +bool cpus_are_stuck_in_kernel(void)
> >>> +{
> >>> +    bool ret = !!cpus_stuck_in_kernel;
> >>> +#ifdef CONFIG_HOTPLUG_CPU
> >>> +    int any_cpu = raw_smp_processor_id();
> >>> +
> >>> +    if (num_possible_cpus() > 1 && !cpu_ops[any_cpu]->cpu_die)
> >>> +        ret = true;
> >>> +#endif
> > 
> > Minor nit: Moving the cpu_die check to a static inline function with
> > an obvious name might make the code look better.
> > 
> >     return !!cpus_stuck_in_kernel || !have_cpu_die() ?
> > 
> 
> That would be better!

Can you post a new version of this, please?

Will