Messages in this thread | | | Date | Fri, 8 Aug 2008 14:13:23 -0400 (EDT) | From | Steven Rostedt <> | Subject | Re: [PATCH 0/5] ftrace: to kill a daemon |
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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.
> > > > > > > > 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.
-- Steve
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