Messages in this thread | | | Date | Fri, 8 Aug 2008 14:41:06 -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: > > > > > > > > > > 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. :/
Hmm, good point. Unless...
Can a processor be preempted in a middle of nops? What do nops do for a processor? Can it skip them nicely in one shot?
This means I'll have to do the benchmarks again, and see what the performance difference of a jmp and a nop is significant. I'm thinking that if the processor can safely skip nops without any type of processing, this may be the reason that nops are better than a jmp. A jmp causes the processor to do a little more work.
I might even run a test to see if I can force a processor that uses the three-two nops to preempt between them.
I can add a test in x86 ftrace.c to check to see which nop was used, and use the jmp if the arch does not have a 5 byte nop.
I'm assuming that jmp is more expensive than the nops because otherwise a jmp 0 would have been used as a 5 byte nop.
-- Steve
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