linux-kernel - Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints

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Date:	Tue, 19 Oct 2010 18:38:43 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Koki Sanagi <sanagi.koki@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	nhorman@...driver.com, scott.a.mcmillan@...el.com,
	laijs@...fujitsu.com, "H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>, eric.dumazet@...il.com,
	kaneshige.kenji@...fujitsu.com, David Miller <davem@...emloft.net>,
	izumi.taku@...fujitsu.com, kosaki.motohiro@...fujitsu.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"Luck, Tony" <tony.luck@...el.com>
Subject: Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints

On Tue, Oct 19, 2010 at 11:55:19PM +0200, Thomas Gleixner wrote:
> On Tue, 19 Oct 2010, Jason Baron wrote:
> > On Tue, Oct 19, 2010 at 09:49:45PM +0200, Thomas Gleixner wrote:
> > > > > On Tue, 19 Oct 2010, Steven Rostedt wrote:
> > > 
> > > So it trades a conditional vs. two jumps ? WTF ??
> > > 
> > 
> > right, so the 'jmpq' on boot on x86 gets patched with 5 byte no-op
> > sequence. So in the disabled case we have no-op followed by a jump
> > around the disabled code.
> 
> And that's supposed to be useful ? We do _NOT_ want to jump around
> disabled stuff. The noped out case should fall through into the non
> traced code. Otherwise that whole jumplabel thing is completely
> useless.
> 
> > > I thought that jumplabel magic was supposed to get rid of the jump
> > > over the tracing code ? In fact it adds another jump. Whatfor ?
> > > 
> > 
> > yes, that is the plan. gcc does not yet support hot/cold labels...once
> > it does the second jump will go away and the entire tracepoint code will
> > be moved to a 'cold' section. It's not quite completely optimal yet, but
> > we are getting there.
> 
> Then do not advertise it as the brilliant solution for all tracing
> matters.
> 

I'm not sure I did, the documentation says that we have nop followed by
a jmp:

+The new code is a 'nopl' followed by a 'jmp'. Thus:
+
+nopl - 0f 1f 44 00 00 - 5 bytes
+jmp  - eb 3e          - 2 bytes


http://marc.info/?l=linux-kernel&m=128717355231182&w=2`

> > > Now even worse, when you NOP out the jmpq then your tracepoint is
> > > still not enabled. Brilliant !
> > > 
> > 
> > The 'jmpq' in the enabled case is patched with a jmpq to the body of the
> > tracepoint itself.
> 
> Brilliant.
>  
> > > Did you guys ever look at the assembly output of that insane shite you
> > > are advertising with lengthy explanations ? 
> > > 
> > > Obviously _NOT_
> > > 
> > > Come back when you can show me a clean imlementation of all this crap
> > > which reproduces with my jumplabel enabled stock compiler. And please
> > > just send me a patch w/o the blurb.
> > > 
> > > And sane looks like:
> > > 
> > >     jmpq   2f  <---- This gets noped out 
> > > 1:
> > >     mov    %r12,%rdi
> > >     callq  *(%r12)
> > >     [whatever cleanup it takes ]
> > >     leaveq 
> > >     retq   
> > > 
> > > 2f:
> > >     [tracing gunk]
> > >     jmp    1b
> > > 
> > 
> > yes, this is what the code should look like when we get support for
> > hot/cold labels. I've discussed this support with gcc folk, and its the
> > next step here. So yes, this is exacatly where we are headed.
> 
> So and at the same time the whole tracing crowd tells me, that this is
> already a done deal. See previous advertisments from DrTracing. I'm
> seriously grumpy about this especially in the context of a patch which
> fixes one of the worst interfaces I've seen in years.
> 
> Thanks,
> 
> 	tglx

sorry if I mislead anybody about the current state of of 'jump labels'.
But we have the same goal in mind, and a clear path to get there. If you
don't agree with the approach - I'm all ears. And you are right - the code is
not where it should be yet.

thanks,

-Jason
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