[PATCH v2] powerpc/kexec: Fix orphaned offline CPUs across kexec
horms at verge.net.au
Thu Jul 29 23:25:38 EDT 2010
On Fri, Jul 30, 2010 at 01:15:14PM +1000, Michael Neuling wrote:
> (adding kexec list to CC)
> In message <4C521FD2.4050301 at ozlabs.org> you wrote:
> > Michael Neuling wrote:
> > > In message <4C511216.30109 at ozlabs.org> you wrote:
> > >> When CPU hotplug is used, some CPUs may be offline at the time a kexec is
> > >> performed. The subsequent kernel may expect these CPUs to be already runn
> > > ,
> > >> and will declare them stuck. On pseries, there's also a soft-offline (ced
> > >> state that CPUs may be in; this can also cause problems as the kexeced ker
> > >> may ask RTAS if they're online -- and RTAS would say they are. Again, stu
> > >>
> > >> This patch kicks each present offline CPU awake before the kexec, so that
> > >> none are lost to these assumptions in the subsequent kernel.
> > >
> > > There are a lot of cleanups in this patch. The change you are making
> > > would be a lot clearer without all the additional cleanups in there. I
> > > think I'd like to see this as two patches. One for cleanups and one for
> > > the addition of wake_offline_cpus().
> > Okay, I can split this. Typofixy-add-debug in one, wake_offline_cpus
> > in another.
> > > Other than that, I'm not completely convinced this is the functionality
> > > we want. Do we really want to online these cpus? Why where they
> > > offlined in the first place? I understand the stuck problem, but is the
> > > solution to online them, or to change the device tree so that the second
> > > kernel doesn't detect them as stuck?
> > Well... There are two cases. If a CPU is soft-offlined on pseries, it
> > must b e woken from that cede loop (in
> > platforms/pseries/hotplug-cpu.c) as we're repla cing code under its
> > feet. We could either special-case the wakeup from this ce de loop to
> > get that CPU to RTAS "stop-self" itself properly. (Kind of like a "
> > wake to die".)
> Makes sense.
> > So that leaves hard-offline CPUs (perhaps including the above): I
> > don't know why they might have been offlined. If it's something
> > serious, like fire, they'd be removed from the present set too (and
> > thus not be considered in this restarting case). We could add a mask
> > to the CPU node to show which of the threads (if any) are running, and
> > alter the startup code to start everything if this mask doesn't exist
> > (non-kexec) or only online currently-running threads if the mask is
> > present. That feels a little weird.
> > My reasoning for restarting everything was: The first time you boot,
> > all of your present CPUs are started up. When you reboot, any CPUs
> > you offlined for fun are restarted. Kexec is (in this non-crash
> > sense) a user-initiated 'quick reboot', so I reasoned that it should
> > look the same as a 'hard reboot' and your new invocation would have
> > all available CPUs running as is usual.
> OK, I like this justification. Would be good to include it in the
> checkin comment since we're changing functionality somewhat.
FWIW, I do too. Personally I like to think of kexec as soft-reboot.
Where soft means, in software, not soft-touch.
More information about the kexec