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

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Message-ID: <20101020152745.GA7348@redhat.com>
Date:	Wed, 20 Oct 2010 11:27:45 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	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, 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 07:05:15PM -0400, Steven Rostedt wrote:
> On Tue, 2010-10-19 at 15:49 -0700, H. Peter Anvin wrote:
> > On 10/19/2010 03:41 PM, Mathieu Desnoyers wrote:
> > >>
> > >> OK, first of all, there are some serious WTFs here:
> > >>
> > >> # define JUMP_LABEL_INITIAL_NOP ".byte 0xe9 \n\t .long 0\n\t"
> > >>
> > >> A jump instruction is one of the worst possible NOPs.  Why are we doing
> > >> this?
> > > 
> > > This code is dynamically patched at boot time (and module load time) with a
> > > better nop, just like the function tracer does.
> > > 
> > 
> > That's just ridiculous... start out with something sane and you at least
> > have the chance of not having to patch it.
> 
> Yep we can fix this. Jason?
> 

sure. The idea of the 'jmp 0' was simply to be an lcd for x86, if
there's a better lcd for x86, I'll update it. But note, that since the
'jmp 0' is patched to a better nop at boot, we wouldn't see much gain.
And in the boot path we are using 'text_poke_early()', so avoiding that
isn't going to improve things much.

I've got a few fixup patches in the queue that I'm going to post first,
and then I'll take a look at this change.

thanks,

-Jason

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