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

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Message-ID: <alpine.DEB.2.21.1907291235580.1791@nanos.tec.linutronix.de>
Date:   Mon, 29 Jul 2019 12:38:30 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Guenter Roeck <linux@...ck-us.net>, 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, Peter Zijlstra wrote:
> On Mon, Jul 29, 2019 at 11:58:24AM +0200, Thomas Gleixner wrote:
> > On Mon, 29 Jul 2019, Peter Zijlstra wrote:
> > > On Sat, Jul 27, 2019 at 09:44:50AM -0700, Guenter Roeck wrote:
> > > > [   61.348866] Call Trace:
> > > > [   61.349392]  kick_ilb+0x90/0xa0
> > > > [   61.349629]  trigger_load_balance+0xf0/0x5c0
> > > > [   61.349859]  ? check_preempt_wakeup+0x1b0/0x1b0
> > > > [   61.350057]  scheduler_tick+0xa7/0xd0
> > > 
> > > kick_ilb() iterates nohz.idle_cpus_mask to find itself an idle_cpu().
> > > 
> > > idle_cpus_mask() is set from nohz_balance_enter_idle() and cleared from
> > > nohz_balance_exit_idle(). nohz_balance_enter_idle() is called from
> > > __tick_nohz_idle_stop_tick() when entering nohz idle, this includes the
> > > cpu_is_offline() clause of the idle loop.
> > > 
> > > However, when offline, cpu_active() should also be false, and this
> > > function should no-op.
> > 
> > Ha. That reboot mess is not clearing cpu active as it's not going through
> > the regular cpu hotplug path. It's using reboot IPI which 'stops' the cpus
> > dead in their tracks after clearing cpu online....
> 
> $string-of-cock-compliant-curses
> 
> What a trainwreck...
> 
> So if it doesn't play by the normal rules; how does it expect to work?
> 
> So what do we do? 'Fix' reboot or extend the rules?

Reboot has two modes:

 - Regular reboot initiated from user space

 - Panic reboot

For the regular reboot we can make it go through proper hotplug, for the
panic case not so much.

thanks,

	tglx