Messages in this thread | | | Date | Tue, 1 May 2018 21:19:51 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] tracing/irqtrace: only call trace_hardirqs_on/off when state changes |
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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.
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