Messages in this thread | | | Date | Tue, 28 Sep 2021 09:35:46 +0100 | From | Mark Rutland <> | Subject | Re: [PATCHv2 0/5] arm64/irqentry: remove duplicate housekeeping of |
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On Tue, Sep 28, 2021 at 09:32:22AM +0100, Mark Rutland wrote: > On Mon, Sep 27, 2021 at 05:09:22PM -0700, Paul E. McKenney wrote: > > On Mon, Sep 27, 2021 at 10:23:18AM +0100, Mark Rutland wrote: > > > On Fri, Sep 24, 2021 at 03:59:54PM -0700, Paul E. McKenney wrote: > > > > On Fri, Sep 24, 2021 at 06:36:15PM +0100, Mark Rutland wrote: > > > > > [Adding Paul for RCU, s390 folk for entry code RCU semantics] > > > > > > > > > > On Fri, Sep 24, 2021 at 09:28:32PM +0800, Pingfan Liu wrote: > > > > > > After introducing arm64/kernel/entry_common.c which is akin to > > > > > > kernel/entry/common.c , the housekeeping of rcu/trace are done twice as > > > > > > the following: > > > > > > enter_from_kernel_mode()->rcu_irq_enter(). > > > > > > And > > > > > > gic_handle_irq()->...->handle_domain_irq()->irq_enter()->rcu_irq_enter() > > > > > > > > > > > > Besides redundance, based on code analysis, the redundance also raise > > > > > > some mistake, e.g. rcu_data->dynticks_nmi_nesting inc 2, which causes > > > > > > rcu_is_cpu_rrupt_from_idle() unexpected. > > > > > > > > > > Hmmm... > > > > > > > > > > The fundamental questionss are: > > > > > > > > > > 1) Who is supposed to be responsible for doing the rcu entry/exit? > > > > > > > > > > 2) Is it supposed to matter if this happens multiple times? > > > > > > > > > > For (1), I'd generally expect that this is supposed to happen in the > > > > > arch/common entry code, since that itself (or the irqchip driver) could > > > > > depend on RCU, and if that's the case thatn handle_domain_irq() > > > > > shouldn't need to call rcu_irq_enter(). That would be consistent with > > > > > the way we handle all other exceptions. > > > > > > > > > > For (2) I don't know whether the level of nesting is suppoosed to > > > > > matter. I was under the impression it wasn't meant to matter in general, > > > > > so I'm a little surprised that rcu_is_cpu_rrupt_from_idle() depends on a > > > > > specific level of nesting. > > > > > > > > > > >From a glance it looks like this would cause rcu_sched_clock_irq() to > > > > > skip setting TIF_NEED_RESCHED, and to not call invoke_rcu_core(), which > > > > > doesn't sound right, at least... > > > > > > > > > > Thomas, Paul, thoughts? > > > > > > > > It is absolutely required that rcu_irq_enter() and rcu_irq_exit() calls > > > > be balanced. Normally, this is taken care of by the fact that irq_enter() > > > > invokes rcu_irq_enter() and irq_exit() invokes rcu_irq_exit(). Similarly, > > > > nmi_enter() invokes rcu_nmi_enter() and nmi_exit() invokes rcu_nmi_exit(). > > > > > > Sure; I didn't mean to suggest those weren't balanced! The problem here > > > is *nesting*. Due to the structure of our entry code and the core IRQ > > > code, when handling an IRQ we have a sequence: > > > > > > irq_enter() // arch code > > > irq_enter() // irq code > > > > > > < irq handler here > > > > > > > irq_exit() // irq code > > > irq_exit() // arch code > > > > > > ... and if we use something like rcu_is_cpu_rrupt_from_idle() in the > > > middle (e.g. as part of rcu_sched_clock_irq()), this will not give the > > > expected result because of the additional nesting, since > > > rcu_is_cpu_rrupt_from_idle() seems to expect that dynticks_nmi_nesting > > > is only incremented once per exception entry, when it does: > > > > > > /* Are we at first interrupt nesting level? */ > > > nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting); > > > if (nesting > 1) > > > return false; > > > > > > What I'm trying to figure out is whether that expectation is legitimate, > > > and assuming so, where the entry/exit should happen. > > > > Oooh... > > > > The penalty for fooling rcu_is_cpu_rrupt_from_idle() is that RCU will > > be unable to detect a userspace quiescent state for a non-nohz_full > > CPU. That could result in RCU CPU stall warnings if a user task runs > > continuously on a given CPU for more than 21 seconds (60 seconds in > > some distros). And this can easily happen if the user has a CPU-bound > > thread that is the only runnable task on that CPU. > > > > So, yes, this does need some sort of resolution. > > > > The traditional approach is (as you surmise) to have only a single call > > to irq_enter() on exception entry and only a single call to irq_exit() > > on exception exit. If this is feasible, it is highly recommended. > > Cool; that's roughly what I was expecting / hoping to hear! > > > In theory, we could have that "1" in "nesting > 1" be a constant supplied > > by the architecture (you would want "3" if I remember correctly) but > > in practice could we please avoid this? For one thing, if there is > > some other path into the kernel for your architecture that does only a > > single irq_enter(), then rcu_is_cpu_rrupt_from_idle() just doesn't stand > > a chance. It would need to compare against a different value depending > > on what exception showed up. Even if that cannot happen, it would be > > better if your architecture could remain in blissful ignorance of the > > colorful details of ->dynticks_nmi_nesting manipulations. > > I completely agree. I think it's much harder to keep that in check than > to enforce a "once per architectural exception" policy in the arch code. > > > Another approach would be for the arch code to supply RCU a function that > > it calls. If there is such a function (or perhaps better, if some new > > Kconfig option is enabled), RCU invokes it. Otherwise, it compares to > > "1" as it does now. But you break it, you buy it! ;-) > > I guess we could look at the exception regs and inspect the original > context, but it sounds overkill... > > I think the cleanest thing is to leave this to arch code, and have the > common IRQ code stay well clear. Unfortunately most architectures > (including arch/arm) still need the common IRQ code to handle this, so > we'll have to make that conditional on Kconfig, something like the below > (build+boot tested only). > > If there are no objections, I'll go check who else needs the same > treatment (IIUC at least s390 will), and spin that as a real > patch/series.
Ah, looking again this is basically Pinfan's patch 2, so ignore the below, and I'll review Pingfan's patch instead.
Thanks, Mark.
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