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From:   Pingfan Liu <kernelfans@gmail.com>
Date:   Tue, 9 Jul 2019 12:26:36 +0800
Message-ID: <CAFgQCTvTB_QYANc4TJOUXVzeKhn4TXsgjRLrPuXkRxKjgGC6pA@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/numa: instance all parsed numa node
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Thomas Gleixner <tglx@linutronix.de>, 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>,
        Barret Rhoden <brho@google.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        David Rientjes <rientjes@google.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Jul 9, 2019 at 1:53 AM Andy Lutomirski <luto@amacapital.net> wrote:
>
>
>
> > On Jul 8, 2019, at 3:35 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> >> On Mon, 8 Jul 2019, Pingfan Liu wrote:
> >>> 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:
> >
> > It's not only the APIC state. It's the state of the CPUs in general.
> >
> >> 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.
> >
> > Fair enough for the IPI 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?
> >
> > It can and it does.
> >
> > That's the whole point why we bring up all CPUs in the 'nosmt' case and
> > shut the siblings down again after setting CR4.MCE. Actually that's in fact
> > a 'let's hope no MCE hits before that happened' approach, but that's all we
> > can do.
> >
> > If we don't do that then the MCE broadcast can hit a CPU which has some
> > firmware initialized state. The result can be a full system lockup, triple
> > fault etc.
> >
> > So when the MCE hits a CPU which is still in the crashed kernel lala state,
> > then all hell breaks lose.
> >
> >> 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.
> >
> > Anything less than the actual number of present CPUs is problematic except
> > you use the 'let's hope nothing happens' approach. We could add an option
> > to stop the bringup at the early online state similar to what we do for
> > 'nosmt'.
> >
> >
>
> How about we change nr_cpus to do that instead so we never have to have this conversation again?
Are you interest in implementing this?

Thanks,
  Pingfan