[lttng-dev] How to disable an event that's been enabled by a wildcard selection or -a? (lttng-dev Digest, Vol 64, Issues 30, 38)

[Thibault, Daniel]

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> Date: Fri, 16 Aug 2013 12:08:56 +0000
>
> A question on the suggested feature below:
>
> If we do the following set of commands:
>
> lttng enable-event "a*" -u
> lttng enable-event "!ab" -u
>
> The intention is to enable all events starting with the letter 'a' except the event 'ab'.
> However, if the event 'ab' is already registered in the session, the first command would enable the event 'ab'.
>
> Should the second command now disable the event 'ab', or should the command only apply to future registrations of events ?
>
> JP Ikaheimonen
> Mentor Graphics
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   The second command should definitely disable event 'ab'.  Otherwise the effect of any given command sequence becomes undefined because the system is 'too stateful'.

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> Date: Tue, 20 Aug 2013 11:27:46 +0200
>
> If that is the intention, then it would be better to specify the commands in reverse order:
>
> # lttng enable-event "!ab" -u
> # lttng enable-event "a*" -u
>
> or even better:
>
> # lttng enable-event "!ab" "a*" -u
>
> > However, if the event 'ab' is already registered in the session, the first command would enable the event 'ab'.
> > Should the second command now disable the event 'ab', or should the command only apply to future registrations of events ?
>
> That is exactly what would happen in an *active* *session* if you issue
> the commands in the order you originally suggested.
> Incoming events matching "a*" are recorded up to the point where 'lttng
> enable-event "!ab"' is executed. From then on,  'ab'-events are excluded
> from recording.
>
> Paul Woegerer, SW Development Engineer
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   It makes little sense to me that the sequence:

# lttng enable-event "!ab" -u
# lttng enable-event "a*" -u

should "enable all events except 'ab'"  (the original intent).  There would be no way of ever turning 'ab' back on, unless the command treated the wild card differently from its equivalent expansion.  To illustrate my point, assume the alphabet consists of just {a, b, c}; then "a*" expands to "aa", "ab", "ac", "aaa", "aab", etc.  Issuing the command with the "a*" argument or its equivalent expansion (which is possible here since event names are length-limited) should yield the same result.

I note that the "!" special character was not used in older versions of enable-event, leading to limited backward compatibility of this new interface convention.  Is the intent to remove the disable-event command entirely?

Daniel U. Thibault
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