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Re: user-space probes -- plan B from outer space


jkenisto wrote:

> [...]  I was thinking a user-mode (instrumentation) program + a
> kernel module that defines handlers that could be invoked from the
> instrumentation program.  The latter (which requires kernel
> enhancements) is necessary only for convenient & efficient
> coordination of user-space and kernel-space instrumentation.  [...]

This scheme is more advanced than the hypothetical "plan B", which
would require no kernel changes.

> [...]  Yes, pretty slow.  In my prototype, my user-mode handler just
> increments a counter.  On my Pentium M, overhead per probepoint hit
> is ~14.2 usec, compared with 1.03 usec for the uprobes version last
> posted to LKML.  [...]

It would be good to see your code.  I'm surprised the slowdown is
*only* a factor of 14.

> My proposed approach to user/kernel data sharing is a new system
> call or ptrace request that just passes a pid, a handler ID, and a
> pointer to an area in user space that the handler (installed via a
> kernel module) can read and/or write. [...]

What would this user space area be used for?  If all the handlers run
in kernel space, why not keep all the data there too?

- FChE


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