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A SystemTap update

A SystemTap update

Posted Jan 30, 2009 10:31 UTC (Fri) by mjw (subscriber, #16740)
In reply to: A SystemTap update by jejb
Parent article: A SystemTap update

> Actuallly, only the user space tracing aspect of systemtap is dependent on these. You can still do kernel space tracing without them.

Correct.

> We've spent quite a lot of effort explaining the problems with the utrace/uprobes dependency (especially the issues of having to pull the process symbol table into the kernel and of having the kernel actually execute the compiled code to do the traps).

Could you post the problems you see?

How a tracing tool like systemtap processes and uses the symbol table is kind of orthogonal from utrace and uprobes. utrace and uprobes might make it easier to access them during runtime. But that isn't what Systemtap currently does. If you want a tracer to do these things dynamically at trace event time, or even push the whole thing towards user space in reaction to trace events and hand it off to a user space helper then that is certainly a design choice you can make (unlike tracers, debuggers do this for example since they don't mind suspending the tracee for a longer period). The article does hint at why "offloading" this to a user space helper might not be practical (see the vfs example and the explanation of what might happen if you try to offload something like that to a perl script). But those are tradeoffs you can make independent of the infrastructure you use in the kernel to handle events and trace point insertion.

> There is hope that we might be able to go with a lighter weight infrastructure that simply vectors traps to the user space stap runtime and does all the interpreting in user space.

Yes, there is nothing inherent in utrace or uprobes about how you handle trace events or how you use and insert vector traps into user space. That is the basic idea behind pushing them upstream, because they are useful apart from systemtap. They should also be useful for other tracers like connecting them to ftrace or lttng. You could even use them for a new debugger interface if you aren't interested in a no-overhead tracer. That is what the froggy project is exploring. It seems time to provide something better than the ptrace interface for debuggers.


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