Re: cpu stopper threads and load balancing leads to deadlock

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

On Thu, May 03, 2018 at 04:44:50PM +0200, Peter Zijlstra wrote:
> On Thu, May 03, 2018 at 04:16:55PM +0200, Mike Galbraith wrote:
> > On Thu, 2018-05-03 at 15:56 +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... :-(
> > 
> > Could say who cares about oddball offline wakeup stat. <cringe>
> 
> Yeah, nobody.. but I don't want to have to change the wakeup code to
> deal with this if at all possible. That'd just add conditions that are
> 'always' false, except in this exceedingly rare circumstance.
> 
> So ideally we manage to tell RCU that it needs to pay attention while
> we're doing this here thing, which is what I thought RCU_NONIDLE() was
> about.

One straightforward approach would be to provide a arch-specific
Kconfig option that tells notify_cpu_starting() not to bother invoking
rcu_cpu_starting().  Then x86 selects this Kconfig option and invokes
rcu_cpu_starting() itself early enough to avoid splats.

See the (untested, probably does not even build) patch below.

I have no idea where to insert either the "select" or the call to
rcu_cpu_starting(), so I left those out.  I know that putting the
call too early will cause trouble, but I have no idea what constitutes
"too early".  :-/

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 0db8938fbb23..58f7ea1de247 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -948,7 +948,8 @@ void notify_cpu_starting(unsigned int cpu)
 	enum cpuhp_state target = min((int)st->target, CPUHP_AP_ONLINE);
 	int ret;
 
-	rcu_cpu_starting(cpu);	/* Enables RCU usage on this CPU. */
+	if (!IS_ENABLED(CONFIG_RCU_CPU_ONLINE_EARLY))
+		rcu_cpu_starting(cpu);	/* Enables RCU usage on this CPU. */
 	while (st->state < target) {
 		st->state++;
 		ret = cpuhp_invoke_callback(cpu, st->state, true, NULL, NULL);
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 9210379c0353..a874c0d74797 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -238,4 +238,7 @@ config RCU_NOCB_CPU
 	  Say Y here if you want to help to debug reduced OS jitter.
 	  Say N here if you are unsure.
 
+config RCU_CPU_ONLINE_EARLY
+       bool
+
 endmenu # "RCU Subsystem"