linux-kernel - Re: [RFC] [PATCH 4/7] Uprobes Implementation

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Date:	Sat, 16 Jan 2010 10:50:48 -0500
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Jim Keniston <jkenisto@...ibm.com>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mark Wielaard <mjw@...hat.com>,
	utrace-devel <utrace-devel@...hat.com>
Subject: Re: [RFC] [PATCH 4/7] Uprobes Implementation


Jim Keniston <jkenisto@...ibm.com> 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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