Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 19 May 2020 13:24:45 -0700 | Subject | Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu() |
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On Tue, May 19, 2020 at 1:20 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > Thomas Gleixner <tglx@linutronix.de> writes: > > Andy Lutomirski <luto@kernel.org> writes: > >> On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@linutronix.de> wrote: > >>> The pagefault handler cannot use the regular idtentry_enter() because that > >>> invokes rcu_irq_enter() if the pagefault was caused in the kernel. Not a > >>> problem per se, but kernel side page faults can schedule which is not > >>> possible without invoking rcu_irq_exit(). > >>> > >>> Adding rcu_irq_exit() and a matching rcu_irq_enter() into the actual > >>> pagefault handling code would be possible, but not pretty either. > >>> > >>> Provide idtentry_entry/exit_cond_rcu() which calls rcu_irq_enter() only > >>> when RCU is not watching. The conditional RCU enabling is a correctness > >>> issue: A kernel page fault which hits a RCU idle reason can neither > >>> schedule nor is it likely to survive. But avoiding RCU warnings or RCU side > >>> effects is at least increasing the chance for useful debug output. > >>> > >>> The function is also useful for implementing lightweight reschedule IPI and > >>> KVM posted interrupt IPI entry handling later. > >> > >> Why is this conditional? That is, couldn't we do this for all > >> idtentry_enter() calls instead of just for page faults? Evil things > >> like NMI shouldn't go through this path at all. > > > > I thought about that, but then ended up with the conclusion that RCU > > might be unhappy, but my conclusion might be fundamentally wrong. > > It's about this: > > rcu_nmi_enter() > { > if (!rcu_is_watching()) { > make it watch; > } else if (!in_nmi()) { > do_magic_nohz_dyntick_muck(); > } > > So if we do all irq/system vector entries conditional then the > do_magic() gets never executed. After that I got lost...
I'm also baffled by that magic, but I'm also not suggesting doing this to *all* entries -- just the not-super-magic ones that use idtentry_enter().
Paul, what is this code actually trying to do?
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