Messages in this thread Patch in this message |  | | Date | Thu, 3 May 2018 09:12:31 -0700 | From | "Paul E. McKenney" <> | Subject | Re: cpu stopper threads and load balancing leads to deadlock |
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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
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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"
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