On Tue, May 01, 2018 at 02:46:20PM -0400, Steven Rostedt wrote:
> On Fri, 17 Nov 2017 02:15:06 +1000
> Nicholas Piggin <[email protected]> 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.
