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From:   Pingfan Liu <kernelfans@gmail.com>
Date:   Mon, 8 Jul 2019 16:36:23 +0800
Message-ID: <CAFgQCTvwS+yEkAmCJnsCfnr0JS01OFtBnDg4cr41_GqU79A4Gg@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/numa: instance all parsed numa node
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     x86@kernel.org, Michal Hocko <mhocko@suse.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Tony Luck <tony.luck@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@suse.de>,
        Pavel Tatashin <pavel.tatashin@microsoft.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Stephen Rothwell <sfr@canb.auug.org.au>, Qian Cai <cai@lca.pw>,
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        Bjorn Helgaas <bhelgaas@google.com>,
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        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jul 8, 2019 at 3:44 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Fri, 5 Jul 2019, Pingfan Liu wrote:
>
> > I hit a bug on an AMD machine, with kexec -l nr_cpus=4 option. nr_cpus option
> > is used to speed up kdump process, so it is not a rare case.
>
> But fundamentally wrong, really.
>
> The rest of the CPUs are in a half baken state and any broadcast event,
> e.g. MCE or a stray IPI, will result in a undiagnosable crash.
Very appreciate if you can pay more word on it? I tried to figure out
your point, but fail.

For "a half baked state", I think you concern about LAPIC state, and I
expand this point like the following:

For IPI: when capture kernel BSP is up, the rest cpus are still loop
inside crash_nmi_callback(), so there is no way to eject new IPI from
these cpu. Also we disable_local_APIC(), which effectively prevent the
LAPIC from responding to IPI, except NMI/INIT/SIPI, which will not
occur in crash case.

For MCE, I am not sure whether it can broadcast or not between cpus,
but as my understanding, it can not. Then is it a problem?

From another view point, is there any difference between nr_cpus=1 and
nr_cpus> 1 in crashing case? If stray IPI raises issue to nr_cpus>1,
it does for nr_cpus=1.

Thanks,
  Pingfan