Messages in this thread | | | Date | Tue, 1 May 2018 15:38:40 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] tracing/irqtrace: only call trace_hardirqs_on/off when state changes |
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On Tue, 1 May 2018 21:19:51 +0200 Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, May 01, 2018 at 02:46:20PM -0400, Steven Rostedt wrote: > > On Fri, 17 Nov 2017 02:15:06 +1000 > > Nicholas Piggin <npiggin@gmail.com> wrote: > > > > > In local_irq_save and local_irq_restore, only call irq tracing when > > > the flag state acutally changes. It is not unexpected for the state > > > to go disable->disable. > > > > > > This allows the irq tracing code to better track superfluous > > > enables and disables, and in future could issue warnings. For the > > > most part they are harmless, but they can indicate that the caller > > > has lost track of its irq state. > > > > I missed this before (that was a busy time, I missed a lot of emails > > then :-/ ). > > > > Anyway, this makes sense. > > > > Peter? > > I'm confused. The patch calls the trace hooks less often, so how can it > then better track superfluous calls? > > > > @@ -110,7 +110,8 @@ do { \ > > > #define local_irq_save(flags) \ > > > do { \ > > > raw_local_irq_save(flags); \ > > > - trace_hardirqs_off(); \ > > > + if (!raw_irqs_disabled_flags(flags)) \ > > > + trace_hardirqs_off(); \ > > > } while (0) > > Here we only call the trace hook when we actually did an ON->OFF change > and loose the call on OFF->OFF. > > > > @@ -118,9 +119,11 @@ do { \ > > > do { \ > > > if (raw_irqs_disabled_flags(flags)) { \ > > > raw_local_irq_restore(flags); \ > > > - trace_hardirqs_off(); \ > > > + if (!irqs_disabled()) \ > > > + trace_hardirqs_off(); \ > > Only call on ON->OFF, ignore OFF->OFF. > > > > } else { \ > > > - trace_hardirqs_on(); \ > > > + if (irqs_disabled()) \ > > > + trace_hardirqs_on(); \ > > > raw_local_irq_restore(flags); \ > > > } \ > > > } while (0) > > Only call on OFF->ON, ignore ON->ON. > > > Now, lockdep only minimally tracks these otherwise redundant operations; > see redundant_hardirqs_{on,off} counters, and loosing that doesn't seen > like a big issue. > > But I'm confused how this helps track superfluous things, it looks like > it explicitly tracks _less_ superfluous transitions.
I think it is about triggering on OFF->OFF a warning, as that would only happen if we have:
local_irq_save(flags); [..] local_irq_disable();
-- Steve
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