RE: [PATCH v9 3/3] trace,x86: code-sharing between non-trace andtrace irq handlers

[Seiji Aguchi]

> > How important is it that the tracepoint is *inside* the enter/exit
> > handling?  If not, it would be simpler to just do:
> >
> > smp_trace_irq_handler()
> > {
> > 	trace_irq_entry();
> > 	smp_irq_handler();
> > 	trace_irq_exit();
> > }
> >
> > ... which seems a bit cleaner.  If this isn't possible, then this
> > patch is fine, but please add to the patch description why the simple
> > wrapper isn't doable.
> 
> The problem is with irq_enter/exit() being called. They must be called before trace_irq_enter/exit(), because of the rcu_irq_enter()
> must be called before any tracepoints are used, as tracepoints use rcu to synchronize.
>

I tried to place tracepoints outside the enter/exit handling. But it didn't work because of the rcu_irq_enter().

> Now perhaps we could do this and have trace_irq_entry().
> 
> Not only that, the tracepoint callbacks expect irq_enter() to already be called.
> 
> Hmm, if irq_enter() can nest, which I think it can, perhaps we can call
> irq_enter() first. I'm not sure if that will screw up the second
> irq_entry() inside smp_irq_handler().
> 
> smp_trace_irq_hander()
> {
> 	irq_entry();
> 	trace_irq_entry();
> 	smp_irq_handler();
> 	trace_irq_exit();
> 	irq_exit();
> }

If irq_enter() is nested, it may have a time penalty because it has to check if it was already called or not.  
It doesn't satisfy a goal of this patch.
Therefore, I think current coding is reasonable.

I will update the patch description.

Seiji
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