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

[Vivek Goyal]

On Wed, Jun 04, 2014 at 08:09:25AM +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2014-06-04 at 01:58 +0530, Srivatsa S. Bhat wrote:
> > 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?
> 
> The problem is with our "soft offline" which we do on some platforms. When we
> offline we don't actually send the CPUs back to firmware or anything like that.
> 
> We put them into a very low low power loop inside Linux.
> 
> The new kernel has no way to extract them from that loop. So we must re-"online"
> them before we kexec so they can be passed to the new kernel normally (or returned
> to firmware like we do on powernv).

Srivatsa,

Looks like your patch has been merged.

I don't like the following change in arch independent code.

/*
 * 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();

As it is very powerpc specific requirement, can you enable hotplug in powerpc
arch dependent code as a short term solution.

Ideally one needs to fix the requirement of online all cpus in powerpc
as a long term solution and then get rid of hotplug enable call.

Thanks
Vivek