Re: cpu stopper threads and load balancing leads to deadlock

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, May 03, 2018 at 03:56:17PM +0200, Peter Zijlstra wrote:
> On Thu, May 03, 2018 at 03:32:39PM +0200, Mike Galbraith wrote:
> 
> > Dang.  With $subject fix applied as well..
> 
> That's a NO then... :-(
> 
> > [  151.103732] smpboot: Booting Node 0 Processor 2 APIC 0x4
> > [  151.104908] =============================
> > [  151.104909] WARNING: suspicious RCU usage
> > [  151.104910] 4.17.0.g66d489e-tip-default #84 Tainted: G            E    
> > [  151.104911] -----------------------------
> > [  151.104912] kernel/sched/core.c:1625 suspicious rcu_dereference_check() usage!
> > [  151.104913] 
> >                other info that might help us debug this:
> > 
> > [  151.104914] 
> >                RCU used illegally from offline CPU!
> >                rcu_scheduler_active = 2, debug_locks = 0
> > [  151.104916] 3 locks held by swapper/2/0:
> > [  151.104916]  #0: 00000000560adb60 (stop_cpus_mutex){+.+.}, at: stop_machine_from_inactive_cpu+0x86/0x140
> > [  151.104923]  #1: 00000000e4fb0238 (&p->pi_lock){-.-.}, at: try_to_wake_up+0x2d/0x5f0
> > [  151.104929]  #2: 000000003341403b (rcu_read_lock){....}, at: rcu_read_lock+0x0/0x80
> > [  151.104934] 
> >                stack backtrace:
> > [  151.104937] CPU: 2 PID: 0 Comm: swapper/2 Kdump: loaded Tainted: G            E     4.17.0.g66d489e-tip-default #84
> > [  151.104938] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
> > [  151.104938] Call Trace:
> > [  151.104942]  dump_stack+0x78/0xb3
> > [  151.104945]  ttwu_stat+0x121/0x130
> > [  151.104949]  try_to_wake_up+0x2c2/0x5f0
> > [  151.104953]  ? cpu_stop_park+0x30/0x30
> > [  151.104956]  wake_up_q+0x4a/0x70
> > [  151.104959]  cpu_stop_queue_work+0x6b/0xa0
> > [  151.104963]  queue_stop_cpus_work+0x61/0xb0
> > [  151.104968]  stop_machine_from_inactive_cpu+0xd8/0x140
> 
> > > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> > > index f89014a2c238..a32518c2ba4a 100644
> > > --- a/kernel/stop_machine.c
> > > +++ b/kernel/stop_machine.c
> > > @@ -650,8 +650,10 @@ int stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data,
> > >  	/* Schedule work on other CPUs and execute directly for local CPU */
> > >  	set_state(&msdata, MULTI_STOP_PREPARE);
> > >  	cpu_stop_init_done(&done, num_active_cpus());
> > > -	queue_stop_cpus_work(cpu_active_mask, multi_cpu_stop, &msdata,
> > > -			     &done);
> > > +
> > > +	RCU_NONIDLE(queue_stop_cpus_work(cpu_active_mask, multi_cpu_stop,
> > > +				         &msdata, &done));
> > > +
> > >  	ret = multi_cpu_stop(&msdata);
> 
> Paul, any clue on what else to try here? The whole MTRR setup is
> radically crazy but it's something we're stuck with (yay hardware) :/
> 
> So the issue is that we're doing wakeups from an offline CPU (very early
> during bringup) and RCU (rightfully) complains about that. I thought
> RCU_NONIDLE() was the magic incantation that makes RCU 'watch', but
> clearly it's not enough here.

Huh.

No, RCU_NONIDLE() only works for idle, not for offline.

Maybe...  Let me take a look.  There must be some way to mark a
specific lock acquisition and release as being lockdep-invisible...

Another approach would be to have an architecture-specific thing that
caused RCU to be enabled way earlier on x86.

							Thanx, Paul