On Tue, 1 May 2018 21:19:51 +0200
Peter Zijlstra <pet...@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 <npig...@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
