Messages in this thread | | | Date | Fri, 8 Aug 2008 13:46:07 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 0/5] ftrace: to kill a daemon |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > > On Fri, 8 Aug 2008, Mathieu Desnoyers wrote: > > * Steven Rostedt (rostedt@goodmis.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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