This is the mail archive of the mailing list for the systemtap project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: djprobes status

Ingo Molnar wrote:
> On Fri, 2006-09-15 at 20:50 +0200, Ingo Molnar wrote:
>> Am i missing some difficulty? The performance of probe
>> insertion/removal
>> does not really matter, the critical path is probe execution.
> a third possibility would be to generate not a jump straight into the
> trampoline, but a jump to a kprobes-controlled function:
> 	pushw $target_IP
> 	ret
> this makes the inserted code 6 bytes, but gives full control to kprobes:
> you can start a probe in a non-patched function, which would start by
> first disabling preemption. It's almost as fast as the direct jump:
> modern CPUs are able to optimize such ret sequences just fine.

> at the end of the DCR trampoline you'd not jump back straight to the
> code, but you'd return-jump to another global kprobes function: which
> would re-enable preemption and which would jump back to the next
> instruction.
> this is all still infinitely faster than INT3 + single-stepping.
> this would make probe installation and removal quite simple, at the
> expense of some minimal runtime overhead. (10 cycles at most, on modern
> CPUs)


Actually, the implementation will be much more complex because
we have to think about multiple probes and multiple processors.
But basically I can see the solution in your advice.

Thank you for your advice.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]