linux-kernel - Re: sched: Unexpected reschedule of offline CPU#2!

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Message-ID: <alpine.DEB.2.21.1908161217380.1873@nanos.tec.linutronix.de>
Date:   Fri, 16 Aug 2019 12:22:22 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Guenter Roeck <linux@...ck-us.net>
cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>
Subject: Re: sched: Unexpected reschedule of offline CPU#2!

On Mon, 29 Jul 2019, Guenter Roeck wrote:
> On Mon, Jul 29, 2019 at 12:47:45PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 29, 2019 at 12:38:30PM +0200, Thomas Gleixner wrote:
> > > Reboot has two modes:
> > > 
> > >  - Regular reboot initiated from user space
> > > 
> > >  - Panic reboot
> > > 
> > > For the regular reboot we can make it go through proper hotplug, 
> > 
> > That seems sensible.
> > 
> > > for the panic case not so much.
> > 
> > It's panic, shit has already hit fan, one or two more pieces shouldn't
> > something anybody cares about.
> > 
> 
> Some more digging shows that this happens a lot with Google GCE intances,
> typically after a panic. The problem with that, if I understand correctly,
> is that it may prevent coredumps from being written. So, while of course
> the panic is what needs to be fixed, it is still quite annoying, and it
> would help if this can be fixed for panic handling as well.
> 
> How about the patch suggested by Hillf Danton ? Would that help for the
> panic case ?

I have no idea how that patch looks like, but the quick hack is below.

Thanks,

	tglx

8<---------------
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 75fea0d48c0e..625627b1457c 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -601,6 +601,7 @@ void stop_this_cpu(void *dummy)
 	/*
 	 * Remove this CPU:
 	 */
+	set_cpu_active(smp_processor_id(), false);
 	set_cpu_online(smp_processor_id(), false);
 	disable_local_APIC();
 	mcheck_cpu_clear(this_cpu_ptr(&cpu_info));