Messages in this thread | | | Date | Fri, 8 Aug 2008 13:22:59 -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 Thu, 7 Aug 2008, Mathieu Desnoyers wrote: > > > > > Hi Steven, > > > > If you really want to stop using stop_machine, I think you should have a > > look at my immediate values infrastructure : > > > > see: > > http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=arch/x86/kernel/immediate.c;h=0958c02b49eed3bbc00bdc5aceeee17a6d0c7ab4;hb=HEAD > > > > particularly replace_instruction_safe(), which uses a temporary > > breakpoint to safely replace an instruction on a live SMP system without > > using stop_machine. Feel free to try it in ftrace. :) > > > > You may need to tweak the imv_notifier(), which is responsible for > > executing the breakpoint. The only special thing this handler has to > > know is the non-relocatable instructions you might want to insert (it > > only cares about the new instructions, not the old ones being replaced). > > The current code deals with 5-bytes jumps. Note that the instruction is > > executed in the breakpoint handler with preemption disabled, which might > > not be well suited for a call instruction. > > > > I would recommend to patch in a 5-bytes jmp with 0 offset > > (e9 00 00 00 00) when disabled (this makes a single 5-bytes instruction > > and thus makes sure no instruction pointer can iret in the middle). > > > > When enabling the site, you could patch-in the original call, but you > > should tweak the imv_notifier() so that it uses the jmp offset 0 in the > > bypass instead of the function call, because preemption would otherwise > > be disabled around the call when executed in the breakpoint bypass. > > > > Therefore, simply statically forcing the bypass code to e9 00 00 00 00 > > in all the cases (a nop) would do the job for your needs. > > 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 ?
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.
Mathieu
> -- Steve >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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