Re: [RFC] [PATCH 4/7] Uprobes Implementation

From: Frank Ch. Eigler
Date: Sat Jan 16 2010 - 10:51:18 EST

Jim Keniston <jkenisto@xxxxxxxxxx> writes:

> [...]
> Years ago, we had pre-utrace versions of uprobes where the uprobes
> breakpoint-handler code was dispatched from the die_notifier, before the
> int3 turned into a SIGTRAP. I believe that's what Peter is
> recommending. On my old Pentium M...
> - a pre-utrace uprobe hit cost about 1 usec;
> - a utrace-based uprobe hit cost about 3 usec;
> [...]
> So yeah, learning about the int3 via utrace after the SIGTRAP gets
> created adds some overhead to uprobes. [...]

Was this test comparing likewise fruit? For example, did it account
for factors where other processes were gdb-int3-instrumented or with
lots of kprobes active? Differently multithreaded? Demultiplexing
probes amongst multiple processes?

(It's counterintuitive that the utrace/kernel int3->sigtrap
dispatching code alone should cause thousands of extra instructions.)

- FChE
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