On 06/04/2014 07:16 PM, Vivek Goyal wrote: > 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. > I didn't do that because that would mean that the _disable() would be performed inside kernel/kexec.c and the corresponding _enable() would be performed in arch/powerpc/kernel/machine_kexec_64.c -- with no apparent connection between them, which would have made them hard to relate. > 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. > Yes, I agree. I'm trying out a solution at the moment (see the 4 preliminary patches I sent in my reply to Ben). If that works, we won't need the enable call on powerpc. Regards, Srivatsa S. Bhat
