[lttng-dev] efficient use of LTTng in legacy codebase

[Máté Ferenczy]

It sounds great, I like the idea. Just to make sure I understand your suggestion, are we talking about converting the variable argument list to string in include/lttng/ust-tracepoint-event-write.h?

For example:
#define ctf_format_string(_item, …)                               \
        do {                                                      \
            char _buf[BUF_MAXLEN];                               \
            snprintf(buf, sizeof(_buf), __VA_ARGS__);            \
            _ctf_string(_item, _buf, 0);                         \
       } while(0)

Or do you have some better place to put this logic?

Thanks,
Mate

From: Mathieu Desnoyers [mailto:mathieu.desnoyers at efficios.com]
Sent: Wednesday, December 11, 2013 4:48 PM
To: Máté Ferenczy
Cc: lttng-dev at lists.lttng.org
Subject: Re: efficient use of LTTng in legacy codebase

________________________________
From: "Máté Ferenczy" <mate.ferenczy at ericsson.com<mailto:mate.ferenczy at ericsson.com>>
To: "mathieu desnoyers" <mathieu.desnoyers at efficios.com<mailto:mathieu.desnoyers at efficios.com>>
Cc: lttng-dev at lists.lttng.org<mailto:lttng-dev at lists.lttng.org>
Sent: Wednesday, December 11, 2013 6:52:22 PM
Subject: RE: efficient use of LTTng in legacy codebase

I found this email thread: https://www.mail-archive.com/lttng-dev@lists.lttng.org/msg02278.html
According to this I could just check caa_unlikely(__tracepoint_##provider##___##name.state). However, Dave claims that this state will stay true even if the tracepoint is disabled. Is there anything else I should check for?

I would recommend doing this in the UST probe provider instead. Within the TRACEPOINT_EVENT "TP_FIELD" section, when you declare your string field, you could parse the format string there in the last argument of a ctf_string() (a statement expression). The format string would be received as parameter by the tracepoint, along with a variable arg list (...). In the probe, it would be turned into a va_list (see stdarg(3)), and the parameters could be decoded into a string there. This has the benefit of moving the serialization call out-of-line into the probe provider rather than clobbering the tracepoint call site.

Thoughts ?

Thanks,

Mathieu

Thanks,
Mate

From: Máté Ferenczy
Sent: Wednesday, December 11, 2013 12:17 PM
To: 'mathieu.desnoyers at efficios.com'
Cc: 'lttng-dev at lists.lttng.org'
Subject: efficient use of LTTng in legacy codebase
Hello Mathieu,
Our team was asked to support LTTng UST solution in a legacy codebase. The existing codebase is fairly large, and the requirement is that we provide all the existing logs and debugs in the system as LTTng traces.
The planned solution so far was just to pass everything we have as text (strings) to LTTng. For that we obviously have to convert the printf-like format string and parameter list to a string by calling snprintf before giving the string to the LTTng API. That would however mean, that these snprintfs are getting called every time even if there is no listener/active trace session for the given trace. In this case, all those unnecessary snprintf calls may end up significantly impacting the performance of our applications.
In order to work around this, we were thinking that if there was a way to query the LTTng library from our application code whether there is an active tracing session for the given trace level, we could avoid calling those snprintfs in case they are not needed. Do you think it would be feasible to easily get this data from LTTng lib? If yes, can you provide us some pointers where to look at in order to do those changes ourselves?
Any suggestions are welcome.
Thank you,
Mate Ferenczy


--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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