Messages in this thread | | | Subject | Re: [PATCH] powerpc: on crash, kexec'ed kernel needs all CPUs are online | From | Laurent Vivier <> | Date | Fri, 16 Oct 2015 09:57:24 +0200 |
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On 16/10/2015 04:29, David Gibson wrote: > On Thu, 15 Oct 2015 21:00:58 +0200 Laurent Vivier > <lvivier@redhat.com> wrote: > >> On kexec, all secondary offline CPUs are onlined before starting >> the new kernel, this is not done in the case of kdump. >> >> If kdump is configured and a kernel crash occurs whereas some >> secondaries CPUs are offline (SMT=off), the new kernel is not >> able to start them and displays some "Processor X is stuck.". >> >> Starting with POWER8, subcore logic relies on all threads of core >> being booted. So, on startup kernel tries to start all threads, >> and asks OPAL (or RTAS) to start all CPUs (including threads). If >> a CPU has been offlined by the previous kernel, it has not been >> returned to OPAL, and thus OPAL cannot restart it: this CPU has >> been lost... >> >> Signed-off-by: Laurent Vivier <lvivier@redhat.com> > > Nice analysis of the problem. But, I'm a bit uneasy about this > approach to fixing it: Onlining potentially hundreds of CPU threads > seems like a risky operation in a kernel that's already crashed.
I agree.
> I don't have a terribly clear idea of what is the best way to > address this. Here's a few ideas in the right general direction: > > * I'm already looking into a kdump userspace fixes to stop it > attempting to bring up secondary CPUs > > * A working kernel option to say "only allow this many online cpus > ever" which we could pass to the kdump kernel would be nice > > * Paulus had an idea about offline threads returning themselves > directly to OPAL by kicking a flag at kdump/kexec time.
For me the problem is: as these CPUs are offline, I guess the core has been switched to 1 thread per core, so the CPUs (1 to 7 for core 0) don't exist anymore, how can we return them to OPAL ?
> > BenH, Paulus, > > OPAL <-> kernel cpu transitions don't seem to work quite how I > thought they would. IIUC there's a register we can use to directly > control which threads on a core are active. Given that I would > have thought cpu "ownership" OPAL vs. kernel would be on a > per-core, rather than per-thread basis. > > Is there some way we can change the CPU onlining / offlining code > so that if threads aren't in OPAL, we directly enable them, rather > than just hoping they're in a nap loop somewhere? >
Laurent -----BEGIN PGP SIGNATURE----- Version: GnuPG v2
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