[lttng-dev] Enable UST probes only in specific process

[Thibault, Daniel]

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Date: Thu, 31 Jul 2014 14:15:01 -0400
From: Jérémie Galarneau <jeremie.galarneau at efficios.com>
To: Dmitri Shubin <sbn at tbricks.com>

>>> The arguments won't be evaluated if the event is not enabled. 
>>> However, when an event is enabled, the arguments are evaluated 
>>> regardless of the result of the filter's expression evaluation since 
>>> it could be based on the event's content.
>>
>> Yes, exactly.
>> I pass stack trace as an argument to the tracepoint.
>> So it's computed just to be dropped...
>>
>> [...]
>>
>> I.e. here I have separate session per each process I want to trace.
>> It would be easier if I can have channel per process, like e.g.
>> $ lttng enable-channel -p 12345
>>
>> For instance in dtrace I can explicitly specify process I want to 
>> trace by appending its PID to probe name.
>
> You'll be glad to learn that we'll be adding this feature by the end of the year. Probably within the 2.7 release timeframe.

Once this feature is in place, it will mean the tracepoint arguments get evaluated only if the candidate event passes the PID "pre-filter", right?  Will we be able to set the pre-filter to PID ranges or sets of values?  I suppose there will be similar filtering options with regard to user IDs?

Daniel U. Thibault
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