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

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Date:	Fri, 15 Feb 2013 21:39:56 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Seiji Aguchi <seiji.aguchi@....com>,
	Steven Rostedt <rostedt@...dmis.org>
CC:	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Thomas Gleixner (tglx@...utronix.de)" <tglx@...utronix.de>,
	"'mingo@...e.hu' (mingo@...e.hu)" <mingo@...e.hu>,
	"Borislav Petkov (bp@...en8.de)" <bp@...en8.de>,
	Satoru Moriya <satoru.moriya@....com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	"Luck, Tony (tony.luck@...el.com)" <tony.luck@...el.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: RE: [PATCH v9 3/3] trace,x86: code-sharing between non-trace and trace irq handlers

Fair enough.  Sounds good.

Seiji Aguchi <seiji.aguchi@....com> wrote:

>> > 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

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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