[PATCH v33 00/14] add kdump support

[David Woodhouse]

On Fri, 2017-03-17 at 17:10 +0000, Marc Zyngier wrote:
> 
> > > > 
> > > > FWIW if I trigger a crash on CPU 1 my kdump (still 4.9.8+v32) doesn't work.
> > > > I end up booting the kdump kernel on CPU#1 and then it gets distinctly unhappy...
> > > > 
> > > > [    0.000000] Booting Linux on physical CPU 0x1
> > > > ...
> > > > [    0.017125] Detected PIPT I-cache on CPU1
> > > > [    0.017138] GICv3: CPU1: found redistributor 0 region 0:0x00000000f0280000
> > > > [    0.017147] CPU1: Booted secondary processor [411fd073]
> > > > [    0.017339] Detected PIPT I-cache on CPU2
> > > > [    0.017347] GICv3: CPU2: found redistributor 2 region 0:0x00000000f02c0000
> > > > [    0.017354] CPU2: Booted secondary processor [411fd073]
> > > > [    0.017537] Detected PIPT I-cache on CPU3
> > > > [    0.017545] GICv3: CPU3: found redistributor 3 region 0:0x00000000f02e0000
> > > > [    0.017551] CPU3: Booted secondary processor [411fd073]
> > > > [    0.017576] Brought up 4 CPUs
> > > > [    0.017587] SMP: Total of 4 processors activated.
> > > > ...
> > > > [   31.745809] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > > [   31.751299] 	1-...: (30 GPs behind) idle=c90/0/0 softirq=0/0 fqs=0 
> > > > [   31.757557] 	2-...: (30 GPs behind) idle=608/0/0 softirq=0/0 fqs=0 
> > > > [   31.763814] 	3-...: (30 GPs behind) idle=604/0/0 softirq=0/0 fqs=0 
> > > > [   31.770069] 	(detected by 0, t=5252 jiffies, g=-270, c=-271, q=0)
> > > > [   31.776161] Task dump for CPU 1:
> > > > [   31.779381] swapper/1       R  running task        0     0      1 0x00000080
> > > > [   31.786446] Task dump for CPU 2:
> > > > [   31.789666] swapper/2       R  running task        0     0      1 0x00000080
> > > > [   31.796725] Task dump for CPU 3:
> > > > [   31.799945] swapper/3       R  running task        0     0      1 0x00000080
> > > > 
> > > > Is some of that platform-specific?
> > > That sounds like timer interrupts aren't being taken.
> > > 
> > > Given that the CPUs have come up, my suspicion would be that the GIC's
> > > been left in some odd state, that the kdump kernel hasn't managed to
> > > recover from.
> > > 
> > > Marc may have an idea.
> > I thought kdump was UP only? Anyway, this doesn't look too good.
> > 
> > It would be interesting to find out whether we're still taking
> > interrupts. Also, being able to reproduce this on mainline would be useful.
> > 
> > I wonder if we don't have a bug when booting on something other than
> > CPU#0, possibly on a GICv3 platform... I'll give it a go.
> Went ahead and tried a couple of kexecs with various CPUs disabled in
> order to force kexec not to boot on CPU#0, and the VM did boot just fine.
> 
> So I'd really appreciate a mainline reproducer.

I booted an up-to-date 4.11-rc2 kernel with the v33 patch set. I cannot
reproduce.

But then again, I can't reproduce it on 4.9 *either* any more. And that
is precisely the same kernel image I uploaded earlier. So it appears to
be sporadic, and just *happened* to hit me the first time I tried...
which is probably just as well or I'd never have tried that again :)

I'll keep trying.
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