linux-kernel - Re: [PATCH 0/5] ftrace: to kill a daemon

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Date:	Fri, 8 Aug 2008 13:46:07 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Roland McGrath <roland@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 0/5] ftrace: to kill a daemon

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> On Fri, 8 Aug 2008, Mathieu Desnoyers wrote:
> > * Steven Rostedt (rostedt@...dmis.org) wrote:
> > > 
> > > I originally used jumps instead of nops, but unfortunately, they actually 
> > > hurt performance more than adding nops. Ingo told me it was probably due
> > > to using up the jump predictions of the CPU.
> > > 
> > 
> > Hrm, are you sure you use a single 5-bytes nop instruction then, or do
> > you use a mix of various nop sizes (add_nops) on some architectures ?
> 
> I use (for x86) what is in include/asm-x86/nops.h depending on what the
> cpuid gives us.
> 

That's bad :

#define GENERIC_NOP5 GENERIC_NOP1 GENERIC_NOP4

#define K8_NOP5 K8_NOP3 K8_NOP2

#define K7_NOP5 K7_NOP4 ASM_NOP1

So, when you try, later, to replace these instructions with a single
5-bytes instruction, a preempted thread could iret in the middle of your
5-bytes insn and cause an illegal instruction ?


> > 
> > You can consume the branch prediction buffers for conditional branches,
> > but I doubt static jumps have this impact ? I don't see what "jump
> > predictions" you are referring to here exactly.
> 
> I don't know the details, but we definitely saw a drop in preformance 
> between using nops and static jumps.
> 

Generated by replacing all the call by 5-bytes jumps e9 00 00 00 00
instead of the 5-bytes add_nops ? On which architectures ?

Mathieu


> -- Steve
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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