[lttng-dev] 'add-context --userspace' must be preceded by 'enable-channel' or 'enable-event'

[David Goulet]

Hi David,

Yes you are right. I've opened a bug for that. You might want to
subscribe to it :).

https://bugs.lttng.org/issues/391

Thanks a lot!
David

David OShea:
> Hi David, Mathieu,
> 
>> -----Original Message-----
>> From: David Goulet [mailto:dgoulet at efficios.com]
>> Sent: Wednesday, 31 October 2012 7:05 AM
>> To: Mathieu Desnoyers
>> Cc: David OShea; lttng-dev at lists.lttng.org
>> Subject: Re: [lttng-dev] 'add-context --userspace' must be preceded by
>> 'enable-channel' or 'enable-event'
> [...]
>> Mathieu Desnoyers:
>>> If I understand correctly, the two use-cases where you issue
>>> "add-context" prior to enable-event and prior to enable-channel are
>>> behaving as if they are failing (those contexts don't appear in the
>>> trace), but you don't get any error message. David (David Goulet, in
>>> CC), any clue on why lttng-tools behaves that way, and how can we fix
>>> this ?
> 
> This is not quite correct: I don't have a use case where I 'add-context' prior to 'enable-channel'.  The only case that fails is when I do an 'add-context' immediately after 'create'.  Things work properly if I do an 'enable-channel' or 'enable-event' before the 'add-context'.
> 
>>>> Bad: 'add-context' before 'enable-event':
>>>>
>>>> """
>>>> # lttng create
>>>> Session auto-20121029-090142 created.
>>>> Traces will be written in /root/lttng-traces/auto-20121029-090142
>>>> # lttng add-context --userspace -t vpid -t vtid -t procname
>>>> UST context procname added to all channels
>>>> UST context vtid added to all channels
>>>> UST context vpid added to all channels
>>>> # lttng enable-event --userspace --all
>>
>> The context is added to "channel0" here which is the default one
>> created
>> automatically.
>>
>> The lttng-tools session daemon do add the contexts to the channel on
>> the
>> tracer side (ustctl_add_context) so Mathieu we might want to check if
>> the UST tracer do behave correctly by adding the context to all events
>> of a channel. (Note here that -a -u was used hence the "*" event).
>>
>> I also do confirm that lttng-tools is doing the right ustctl call on
>> channel0 here.
> 
> I took a quick look at the source for lttng-tools 2.0.4 and 2.1.0-rc4, and it looks to me like in the case above, the default channel "channel0" is not actually created by 'add-context'.
> 
> In lttng_enable_event(), if no channel name was specified, it uses DEFAULT_CHANNEL_NAME, then in cmd_enable_event_all() (I'm considering the case of enabling all events, which is what I do), in case LTTNG_DOMAIN_UST, if the specified channel doesn't exist, it will create it.  This seems to be why doing the 'enable-event' first works.
> 
> On the other hand, in lttng_add_context(), if no channel name is specified, no channel name is passed to the server, then in cmd_add_context(), context_ust_add() is called, and in that function, if no channel name is specified, it adds the context to every channel.  However, my understanding is that at this point there are no channels created.
> 
> Apologies if I have missed anything here!
> 
> Regards,
> David
> 
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