Hi, I was thinking more about a programmer's guide to using this mechanism.
What I'd like to do is to have Babeltrace filter events it is showing me from an existing trace. Consider the following example: There are 3 types of tracepoints being captured (without filtering): A debug log message (occasionally) which includes a "debug_level" value (1..10 with 1 being highest importance). Function entry (via gcc -finstrument-functions or other instrumentation). Function exit (similarly). During a 1-hour period, I have 2-3 GB of trace data captured. The trace includes events from 4 different processes. If I use Babeltrace to read the trace I will get an enormously big result in text (~30-40GB at least). I would like to be able to do something like this: $ babeltrace --filter '$ctx.procname == "smbd" || debug_level < 3' /local/lttng-traces/ust/uid/2140/64-bit This would then let me see any events from the "smbd" process (since i added procname to the context using lttng add-context) or debug messages from all processes where the debug level is 1 or 2. Regarding linking - I am referring to the function lttng_enabler_event_link_bytecode(), called from lttng-ust/liblttng-ust/lttng-events.c around line 628. Thanks, Amit Margalit IBM XIV - Storage Reinvented XIV-NAS Development Team Tel. 03-689-7774 Fax. 03-689-7230 From: Jérémie Galarneau <[email protected]> To: Amit Margalit/Israel/IBM@IBMIL Cc: [email protected] Date: 07/18/2013 07:10 PM Subject: Re: [lttng-dev] Question about filtering bytecode Sent by: [email protected] Hi Amit, You may find information on the Babeltrace API in babeltrace/doc/API.txt The filtering mechanism used by the user space tracer is unrelated to Babeltrace. As for the sequence of events, this is how I understand it. When invoking the "lttng" binary with --enable-event and a filter option, a byte code is generated from the filtering rules (lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:704). This byte code is then handed off to the session-daemon. Then, when an instrumented application is launched and connects to the session-daemon, it receives the byte code while its events are being enabled. To answer your question directly, that byte code will be executed by the tracee when the event is "triggered" (lttng-ust/liblttng-ust/lttng-filter-interpreter.c:175). I'm not sure I understand your question about filter/event linking. A filter has to be linked to an event since its rules (expressions) are applied on that event's values. Could you clarify if that's not what you meant? Regards, Jérémie On Thu, Jul 18, 2013 at 3:12 AM, Amit Margalit <[email protected]> wrote: > Hi, > > I'm considering whether I can write a wrapper for the libbabeltrace ctf > iterator which provides a filtered "next()" method. > > I've started reading the code in the hopes of understanding how to do this, > and yet, I'd really appreciate a link to some ordered documentation, or - > any other kind of explanation. > > For example, one thing I still don't understand is the exact sequence, and > where the bytecode is actually executed? What is the meaning and purpose of > "linking" the filter to an event. > > Thanks, > > Amit Margalit > IBM XIV - Storage Reinvented > XIV-NAS Development Team > Tel. 03-689-7774 > Fax. 03-689-7230 > _______________________________________________ > lttng-dev mailing list > [email protected] > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > -- Jérémie Galarneau EfficiOS Inc. http://www.efficios.com
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