On Fri, Jun 22, 2018 at 6:26 AM Paul E. McKenney <paul...@linux.vnet.ibm.com> wrote: > > On Thu, Jun 21, 2018 at 10:56:59PM -0700, Joel Fernandes wrote: > > Hi Paul, > > > > On Wed, Jun 20, 2018 at 09:49:02AM -0700, Paul E. McKenney wrote: > > > On Thu, Jun 21, 2018 at 01:05:22AM +0900, Byungchul Park wrote: > > > > On Wed, Jun 20, 2018 at 11:58 PM, Paul E. McKenney > > > > <paul...@linux.vnet.ibm.com> wrote: > > > > > On Wed, Jun 20, 2018 at 05:47:20PM +0900, Byungchul Park wrote: > > > > >> Hello folks, > > > > >> > > > > >> I'm careful in saying that ->dynticks_nmi_nesting can be removed but > > > > >> I > > > > >> think it's possible since the only thing we are interested in with > > > > >> regard to ->dynticks_nesting or ->dynticks_nmi_nesting is whether > > > > >> rcu is > > > > >> idle or not. > > > > > > > > > > Please keep in mind that NMIs cannot be masked, which means that the > > > > > rcu_nmi_enter() and rcu_nmi_exit() pair can be invoked at any point in > > > > > the process, between any consecutive pair of instructions. The saving > > > > > > And yes, I should have looked at this patch more closely before replying. > > > But please see below. > > > > > > > I believe I understand what NMI is and why you introduced > > > > ->dynticks_nmi_nesting. Or am I missing something? > > > > > > Perhaps the fact that there are architectures that can enter interrupt > > > handlers and never leave them when the CPU is non-idle. One example of > > > this is the usermode upcalls in the comment that you removed. > > > > I spent some time tonight and last night trying to understand this concept > > of > > never leaving an interrupt, I hope you don't mind me asking this dumb > > question... perhaps I will learn something : Could you let me know how is it > > possible that an interrupt never exits? > > > > Typically an interrupt never exiting sounds like a hard-lockup. This is how > > hardlock detector works: Since regular interrupts in linux can't nest, the > > hardlockup detector checks if hrtimer interrupts are being handled and if > > not, then it throws a splat, panics the kernel etc. So I am a bit troubled > > by > > this interrupt never exiting concept.. > > > > Further since an interrupt is an atomic context, it cannot sleep or schedule > > into usermode so how are these upcalls handled from the interrupt? > > It has been some years since I traced the code flow, but what happened > back then is that it switches itself from an interrupt handler to not > without actually returning from the interrupt. This can only happen when > interrupting a non-idle process, thankfully, and RCU's dyntick-idle code > relies on this restriction. If I remember correctly, the code ends up > executing in the context of the interrupted process, but it has been some > years, so please apply appropriate skepticism.
... > > I have never seen NMIs be unpaired or improperly nested. However, > given that rcu_irq_enter() invokes rcu_nmi_enter() and rcu_irq_exit() > invokes rcu_nmi_exit(), it is definitely the case that rcu_nmi_enter() > and rcu_nmi_exit() need to deal with unpaired and improperly nested > invocations. This is very strange. There are certainly cases in x86 where an interrupt-ish code path can become less interrupt-ish without returning (killing a task that overflows a kernel stack is an example), but the RCU calls should still nest correctly. Do you know the history of this requirement?