Messages in this thread | | | Date | Tue, 24 Nov 2009 15:59:25 -0500 | From | Masami Hiramatsu <> | Subject | Re: [PATCH -tip v5 03/10] kprobes: Introduce kprobes jump optimization |
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Frederic Weisbecker wrote: > On Tue, Nov 24, 2009 at 10:34:16AM -0500, Masami Hiramatsu wrote: >> Frederic Weisbecker wrote: >>> I _might_ have understood. >>> You have set up the optimized flags, then you wait for >>> any old-style int 3 kprobes to complete and route >>> to detour buffer so that you can patch the jump >>> safely in the dead code? (and finish with first byte >>> by patching the int 3 itself) >>> >> >> Yeah, you might get almost correct answer. >> The reason why we have to wait scheduling on all processors >> is that this code may modify N instructions (not a single >> instruction). This means, there is a chance that 2nd to nth >> instructions are interrupted on other cpus when we start >> code modifying. > > > Aaah ok! > > In this case, you probably just need the synchronize_sched() > thing. The delayed work looks unnecessary.
Yeah, the delayed work is for speeding up batch registration which kprobes are already supported. Sometimes ~100 probes can be set via batch registration I/F.
>> Please imagine that 2nd instruction is interrupted and >> stop_machine() replaces the 2nd instruction with jump >> *address* while running interrupt handler. When the interrupt >> returns to original address, there is no valid instructions >> and it causes unexpected result. > > > Yeah. > > >> >> To avoid this situation, we have to wait a scheduler quiescent >> state on all cpus, because it also ensure that all current >> interruption are done. > > > Ok. > > >> This also excuses why we don't need to wait when unoptimizing >> and why it has not supported preemptive kernel yet. > > > I see...so the non-preemptible kernel requirement looks > hard to workaround :-s
It's the next challenge I think :-) Even though, kprobes itself still work on preemptive kernel, so we don't lose any functionality.
>> In unoptimizing case, since there is just a single instruction >> (jump), there is no nth instruction which can be interrupted. >> Thus we can just use a stop_machine(). :-) > > > Ok. > > >> >> On the preemptive kernel, waiting scheduling is not work as we >> see on non-preemptive kernel. Since processes can be preempted >> in interruption, we can't ensure that the current running >> interruption is done. (I assume that a pair of freeze_processes >> and thaw_processes may possibly ensure that, or maybe we can >> share some stack rewinding code with ksplice.) >> So it depends on !PREEMPT. > > > > Right. > However using freeze_processes() and thaw_processes() would be > probably too costly and it's not a guarantee that every processes > go to the refrigerator() :-), because some tasks are not freezable, > like the kernel threads by default if I remember well, unless they > call set_freezable(). That's a pity, we would just have needed > to set __kprobe in refrigerator().
Ah, right. Even though, we still have an option of ksplice code.
Thank you,
> PS: hmm btw I remember about a patch that > tagged refrigerator() as __cold but it looks like it hasn't been > applied.... > > Thanks. >
-- Masami Hiramatsu
Software Engineer Hitachi Computer Products (America), Inc. Software Solutions Division
e-mail: mhiramat@redhat.com
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