Messages in this thread | | | Date | Wed, 16 Apr 2008 10:00:09 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 1/2] Marker probes in futex.c |
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* Arjan van de Ven (arjan@infradead.org) wrote: > > > > 4631: b0 00 mov $0x0,%al > > > 4633: 84 c0 test %al,%al > > > 4635: 0f 85 c6 00 00 00 jne 4701 > > the use of partial registers here is unfortunate and probably quite expensive ;( > >
Yes, but it saves instruction cache. That's a tradeoff.
> > > If we want to support NMI context and have the ability to > > > instrument preemptable code without too much headache, we must > > > insure that every modification will leave the code in a "correct" > > > state and that we do not grow the size of any reachable > > > instruction. Also, we must insure gcc did not put code between > > > these instructions. Modifying non-relocatable instructions would > > > also be a pain, since we would have to deal with instruction > > > pointer relocation in the breakpoint code when the code > > > modification is being done. > > you also need to make sure no cpu is executing that code ever.. > but you already deal with that right? >
By "insure that every modification will leave the code in a "correct" state", I mean that at any given time before, during or after the code modification, if an NMI comes on any CPU and try to run the modified code, it should have a valid version of the code to execute. Does it make more sense ?
> > > > > > Luckily, gcc almost never place any code between the mov, test and > > > jne instructions. But since we cannot we sure, we could dynamically > > > check for this code pattern after the mov instruction. If we find > > > it, then we play with it as if it was a single asm block, but if we > > > don't find what we expect, then we use standard immediate values > > > for that. I expect the heavily optimised version will be usable > > > almost all the time. > > I expect gcc to start using the macro-fusion capable ones more and more over time at least, > and for that the compare and jmp need to be consecutive. >
Early reasults of the work I've done last night : I can detect about 96% of the ~120 markers I've put in my instrumented kernel.
Not only does the compare and jmp need to be consecutive, but the movb $0x0,%al also does. I *could* try to detect specific code inserted in between, but I really have to make sure I don't get burned by the compiler inserting a jmp there.
I'll post my work shortly.
Mathieu
> > -- > If you want to reach me at my work email, use arjan@linux.intel.com > For development, discussion and tips for power savings, > visit http://www.lesswatts.org
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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