Messages in this thread | | | Date | Tue, 11 Feb 2020 07:06:15 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook |
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On Tue, Feb 11, 2020 at 09:05:03AM -0500, Steven Rostedt wrote: > On Tue, 11 Feb 2020 12:49:54 +0100 > Peter Zijlstra <peterz@infradead.org> wrote: > > > On Mon, Feb 10, 2020 at 05:06:43PM -0500, Steven Rostedt wrote: > > > + if (!rcu_watching) { \ > > > + /* Can not use RCU if rcu is not watching and in NMI */ \ > > > + if (in_nmi()) \ > > > + return; \ > > > + rcu_irq_enter_irqson(); \ > > > + } \ > > > > I saw the same weirdness in __trace_stack(), and I'm confused by it. > > > > How can we ever get to: in_nmi() && !rcu_watching() ? That should be a > > BUG. In particular, nmi_enter() has rcu_nmi_enter(). > > > > Paul, can that really happen? > > The stack tracer connects to the function tracer and is called at all > the places that function tracing can be called from. As I like being > able to trace RCU internal functions (especially as they are complex), > I don't want to set them all to notrace. But, for callbacks that > require RCU to be watching, we need this check, because there's some > internal state that we can be in an NMI and RCU is not watching (as > there's some places in nmi_enter that can be traced!). > > And if we are tracing preempt_enable and preempt_disable (as Joel added > trace events there), it may be the case for trace events too.
Ah, thank you for the reminder!
Should Documentation/RCU/Design/Requirements/Requirements.rst be updated to include this?
And I have to ask... What happens if we are very early in from-idle NMI entry (or very late in NMI exit), such that both in_nmi() and rcu_is_watching() are returning false? Or did I miss a turn somewhere?
Thanx, Paul
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