[lttng-dev] Problem with adding vpid and vtid context to UST

[David Goulet]

Hi Amit,

Amit Margalit:
> Hi,
> 
> Every time I try this, I get the following error:
> 
> Error: Error starting tracing for app pid: 5663 (ret: -1027)
> 
> Here's my sequence:
> 
> lttng create foo -o /store/
> lttng enable-channel -u
> lttng add-context -u -t vpid -t vtid -t procname
> lttng enable-event -u -a
> lttng start foo
> 
> At this point I start my application and get that error. When the application
> exits, I stop the tracing, and run lttng view.
> 
> The trace session seems to have worked just fine, and "procname" was indeed
> added to the context, but vtid and vpid were not.

What's going on here is that you enable a channel then add contexts. This is
perfectly OK (although your example is missing a channel name). So, the
following "lttng enable-event" creates a default channel "channel0" since no
"-c" was specified.

So basically, the contexts are available on the first channel and not the second
one.

This sequence should work:

lttng create foo -o /store/
lttng enable-event -u -a
lttng add-context -u -t vpid -t vtid -t procname
lttng start foo

We should detail that a bit better in the documentation that an add-context
needs to be done *after* a channel is enabled. This allows you to create
multiple channel with different contexts.

Now, an other issue here in terms of usability is that the enable-event *after*
an enable-channel should be apply to this channel. I think there is been a
discussion at some point on lttng-dev on in the bug tracker.

> 
> And BTW, the documentation doesn't clearly state that pid and tid are
> kernel-only contexts, and that one should use vtid and vpid.

Yes! The list of contexts accepted per domain would be quite useful and I do
agree with you that it's missing in the doc.

We would be *happy* to welcome a contribution for that :).

Cheers!
David

> 
> Any help is appreciated.
> 
> Thanks,
> 
> Amit Margalit
> IBM XIV - /Storage Reinvented/
> XIV-NAS Development Team
> Tel. 03-689-7774
> Fax. 03-689-7230
> 
> 
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