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Re: Backward compatibility for insn probe point
Roland McGrath wrote:
>> I poked around the kernel source some more, but couldn't see what was
>> going wrong.
>
> I figured you'd use some stap probes to follow the code paths!
Oh I did, but so much of it is inlined that it was incredibly difficult
to follow.
>> I've changed the itrace code to stop the task after each step trap (so
>> that it acts more like ptrace). I've tested this on several kernels
>> (2.6.18-141.el5/ppc, 2.6.18-128.1.10.el5/x86_64/i686, and
>> 2.6.25-14.fc9.ppc64) and it seems to work correctly.
>>
>> Does this seem like a reasonable work-around? Could there be problems
>> with this approach?
>
> I presume it kills performance. But what works works, that's a what a
> work-around is. I'd hope that you don't make it use this "not really
> right" mode for kernels with the modern utrace interface that doesn't have
> this bug.
Yes, this is only for old utrace. As far as performance goes, I've
benchmarked single-stepping '/bin/ls' on x86_64 with both approaches.
Here's what I saw ('time' output, averages of 5 runs):
- no stopping on each step trap:
real 0m3.735s
user 0m0.328s
sys 0m3.359s
- stopping on each step trap:
real 0m4.101s
user 0m0.336s
sys 0m3.692s
I could also limit this work-around to ppc-only, to not penalize other
architectures.
One last thing. I thought I'd try block stepping, so I got access to an
ia64 machine. Unfortunately, using systemtap insn probes (either single
or block step) lock up the system with a spinlock lockup. Sigh.
--
David Smith
dsmith@redhat.com
Red Hat
http://www.redhat.com
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