Messages in this thread | | | Date | Fri, 8 Aug 2008 14:21:04 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 0/5] ftrace: to kill a daemon |
| |
* Steven Rostedt (rostedt@goodmis.org) wrote: > > On Fri, 8 Aug 2008, Mathieu Desnoyers wrote: > > * 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 ? > > That's why I use kstop_machine. >
kstop_machine does not guarantee that you won't have _any_ thread preempted with IP pointing exactly in the middle of your instructions _before_ the modification scheduled back in _after_ the modification and thus causing an illegal instruction.
Still buggy. :/
> > > > > > > > > > > > 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 ? > > > > I ran this on my Dell (intel Xeon), which IIRC did show the performance > degration. I unfortunately don't have the time to redo those tests, but > you are welcome to. > > Just look at arch/x86/kernel/ftrace.c and replace the nop with the jump. > In fact, the comments in that file still say it is a jmp. Remember, my > first go was to use the jmp. >
I'll try to find time to compare :
multi-instructions 5-bytes nops (although this approach is just buggy) 5-bytes jump to the next address 2-bytes jump to offset +3.
Mathieu
> -- Steve >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
| |