Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 20 May 2020 08:36:06 -0700 | Subject | Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu() |
| |
On Tue, May 19, 2020 at 7:23 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > On Tue, May 19, 2020 at 05:26:58PM -0700, Andy Lutomirski wrote: > > On Tue, May 19, 2020 at 2:20 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > > > > > Andy Lutomirski <luto@kernel.org> writes: > > > > On Tue, May 19, 2020 at 1:20 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > > >> Thomas Gleixner <tglx@linutronix.de> writes: > > > >> 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? > > > > > > Citing Paul from IRC: > > > > > > "The way things are right now, you can leave out the rcu_irq_enter() > > > if this is not a nohz_full CPU. > > > > > > Or if this is a nohz_full CPU, and the tick is already > > > enabled, in that case you could also leave out the rcu_irq_enter(). > > > > > > Or even if this is a nohz_full CPU and it does not have the tick > > > enabled, if it has been in the kernel less than a few tens of > > > milliseconds, still OK to avoid invoking rcu_irq_enter() > > > > > > But my guess is that it would be a lot simpler to just always call > > > it. > > > > > > Hope that helps. > > > > Maybe? > > > > Unless I've missed something, the effect here is that #PF hitting in > > an RCU-watching context will skip rcu_irq_enter(), whereas all IRQs > > (because you converted them) as well as other faults and traps will > > call rcu_irq_enter(). > > > > Once upon a time, we did this horrible thing where, on entry from user > > mode, we would turn on interrupts while still in CONTEXT_USER, which > > means we could get an IRQ in an extended quiescent state. This means > > that the IRQ code had to end the EQS so that IRQ handlers could use > > RCU. But I killed this a few years ago -- x86 Linux now has a rule > > that, if IF=1, we are *not* in an EQS with the sole exception of the > > idle code. > > > > In my dream world, we would never ever get IRQs while in an EQS -- we > > would do MWAIT with IF=0 and we would exit the EQS before taking the > > interrupt. But I guess we still need to support HLT, which means we > > have this mess. > > > > But I still think we can plausibly get rid of the conditional. > > You mean the conditional in rcu_nmi_enter()? In a NO_HZ_FULL=n system, > this becomes:
So, I meant the conditional in tglx's patch that makes page faults special.
> > > If we > > get an IRQ or (egads!) a fault in idle context, we'll have > > !__rcu_is_watching(), but, AFAICT, we also have preemption off. > > Or we could be early in the kernel-entry code or late in the kernel-exit > code, but as far as I know, preemption is disabled on those code paths. > As are interrupts, right? And interrupts are disabled on the portions > of the CPU-hotplug code where RCU is not watching, if I recall correctly.
Interrupts are off in the parts of the entry/exit that RCU considers to be user mode. We can get various faults, although these should be either NMI-like or events that genuinely or effectively happened in user mode.
> > A nohz_full CPU does not enable the scheduling-clock interrupt upon > entry to the kernel. Normally, this is fine because that CPU will very > quickly exit back to nohz_full userspace execution, so that RCU will > see the quiescent state, either by sampling it directly or by deducing > the CPU's passage through that quiescent state by comparing with state > that was captured earlier. The grace-period kthread notices the lack > of a quiescent state and will eventually set ->rcu_urgent_qs to > trigger this code. > > But if the nohz_full CPU stays in the kernel for an extended time, > perhaps due to OOM handling or due to processing of some huge I/O that > hits in-memory buffers/cache, then RCU needs some way of detecting > quiescent states on that CPU. This requires the scheduling-clock > interrupt to be alive and well. > > Are there other ways to get this done? But of course! RCU could > for example use smp_call_function_single() or use workqueues to force > execution onto that CPU and enable the tick that way. This gets a > little involved in order to avoid deadlock, but if the added check > in rcu_nmi_enter() is causing trouble, something can be arranged. > Though that something would cause more latency excursions than > does the current code. > > Or did you have something else in mind?
I'm trying to understand when we actually need to call the function. Is it just the scheduling interrupt that's supposed to call rcu_irq_enter()? But the scheduling interrupt is off, so I'm confused.
| |