Messages in this thread |  | | | Date | Wed, 27 Jan 2010 11:25:15 +0200 | | From | Avi Kivity <> | | Subject | Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP) |
| |
On 01/27/2010 11:08 AM, Ingo Molnar wrote: > >> I see it exactly the opposite. Only a very small minority of cases will >> have such severe memory corruption that tracing will fall apart because of >> random writes to memory; especially on 64-bit where the address space is >> sparse. On the other hand, knowing that the cost is a few dozen cycles >> rather than a thousand or so means that you can trace production servers >> running full loads without worrying about whether tracing will affect >> whatever it is you're trying to observe. >> >> I'm not against slow reliable tracing, but we shouldn't ignore the need for >> speed. >> > I havent seen a conscise summary of your points in this thread, so let me > summarize it as i've understood them (hopefully not putting words into your > mouth): AFAICS you are arguing for some crazy fragile architecture-specific > solution that traps INT3 into ring3 just to shave off a few cycles, and then > use user-space state to trace into. >
That's a good summary, except for the words "crazy fragile", "trap INT3 into ring3" and "a few cycles".
Instead of using int 3, put a jump instruction in the program. This shaves a lot more than a few cycles.
> If so then you ignore the obvious solution to _that_ problem: dont use INT3 at > all, but rebuild (or re-JIT) your program with explicit callbacks. It's _MUCH_ > faster than _any_ breakpoint based solution - literally just the cost of a > function call (or not even that - i've written very fast inlined tracers - > they do rock when it comes to performance). Problem solved and none of the > INT3 details matters at all. >
However did I not think of that? Yes, and let's rip off kprobes tracing from the kernel, we can always rebuild it.
Well, I'm observing an issue in a production system now. I may not want to take it down, or if I take it down I may not be able to observe it again as the problem takes a couple of days to show up, or I may not have the full source, or it takes 10 minutes to build and so an iterative edit/build/run cycle can stretch for hours.
Adding a vma to a running program is very unlikely to affect it. If the program makes random accesses to memory, it will likely segfault very quickly before we ever get to trace it.
> INT3 only matters to _transparent_ probing, and for that, the cost of INT3 is > almost _by definition_ less important than the fact that we can do transparent > tracing. If performance were the overriding issue they'd use dedicated > callbacks - and the INT3 technique wouldnt matter at all. >
INT3 isn't transparent. The only thing that comes close to full transparency is hardware breakpoints. So we have a tradeoff between transparency and speed, and except for the wierdest bugs, this level of transparency won't be needed.
> ( Also, just like we were able to extend the kprobes code with more and more > optimizations, the same can be done with any user-space probing as well, to > make it faster. But at the core of it has to be a sane design that is > transparent and controlled by the kernel, so that it has the option to apply > more and more otimizations - yours isnt such and its limitations are > designed-in.
No design is fully transparent, and I don't see why my design can't be controlled by the kernel?
> Which is neither smart nor useful. ) >
This style of arguing is neither smart or useful as well.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
|  |