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To: Jim Keniston <jkenisto@us.ibm.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, Mark Wielaard <mjw@redhat.com>,
        utrace-devel <utrace-devel@redhat.com>
Subject: Re: [RFC] [PATCH 4/7] Uprobes Implementation
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From: fche@redhat.com (Frank Ch. Eigler)
Date: Sat, 16 Jan 2010 10:50:48 -0500
In-Reply-To: <1263597085.5007.82.camel@localhost.localdomain> (Jim Keniston's message of "Fri, 15 Jan 2010 15:11:25 -0800")
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Jim Keniston <jkenisto@us.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