Messages in this thread | | | Date | Fri, 6 Nov 2020 10:37:56 +0000 | From | Will Deacon <> | Subject | Re: [PATCH] arm64/smp: Move rcu_cpu_starting() earlier |
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On Thu, Nov 05, 2020 at 09:15:24PM -0500, Qian Cai wrote: > On Thu, 2020-11-05 at 15:28 -0800, Paul E. McKenney wrote: > > On Thu, Nov 05, 2020 at 06:02:49PM -0500, Qian Cai wrote: > > > On Thu, 2020-11-05 at 22:22 +0000, Will Deacon wrote: > > > > Hmm, this patch has caused a regression in the case that we fail to > > > > online a CPU because it has incompatible CPU features and so we park it > > > > in cpu_die_early(). We now get an endless spew of RCU stalls because the > > > > core will never come online, but is being tracked by RCU. So I'm tempted > > > > to revert this and live with the lockdep warning while we figure out a > > > > proper fix. > > > > > > > > What's the correct say to undo rcu_cpu_starting(), given that we cannot > > > > invoke the full hotplug machinery here? Is it correct to call > > > > rcutree_dying_cpu() on the bad CPU and then rcutree_dead_cpu() from the > > > > CPU doing cpu_up(), or should we do something else? > > > It looks to me that rcu_report_dead() does the opposite of > > > rcu_cpu_starting(), > > > so lift rcu_report_dead() out of CONFIG_HOTPLUG_CPU and use it there to > > > rewind, > > > Paul? > > > > Yes, rcu_report_dead() should do the trick. Presumably the earlier > > online-time CPU-hotplug notifiers are also unwound? > I don't think that is an issue here. cpu_die_early() set CPU_STUCK_IN_KERNEL, > and then __cpu_up() will see a timeout waiting for the AP online and then deal > with CPU_STUCK_IN_KERNEL according. Thus, something like this? I don't see > anything in rcu_report_dead() depends on CONFIG_HOTPLUG_CPU=y.
Cheers both for suggesting rcu_report_dead().
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c > index 09c96f57818c..10729d2d6084 100644 > --- a/arch/arm64/kernel/smp.c > +++ b/arch/arm64/kernel/smp.c > @@ -421,6 +421,8 @@ void cpu_die_early(void) > > update_cpu_boot_status(CPU_STUCK_IN_KERNEL); > > + rcu_report_dead(cpu);
I think this is in the wrong place, see:
https://lore.kernel.org/r/20201106103602.9849-1-will@kernel.org
which seems to fix the problem for me.
Will
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