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

[Mathieu Desnoyers]

* Steven Rostedt (rostedt@xxxxxxxxxxx) wrote:
> 
> On Fri, 8 Aug 2008, Mathieu Desnoyers wrote:
> > * Steven Rostedt (rostedt@xxxxxxxxxxx) 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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