[lttng-dev] lttng-track on userspace channel

[Mathieu Desnoyers]

----- On Jun 1, 2017, at 2:49 PM, Tom Deneau tom.deneau at amd.com wrote:

> Mathieu --
> 
> So just to confirm, once fixed the lttng-track command in both kernel and user
> will track process ids, not thread ids, and if one does want to track thread
> ids,
> one should use --filter with $ctx.tid.

Yes, ideally with single-quotes so the shell does not expand $ctx ;)

> 
> A related question, I got errors when trying to do the following:
>   lttng add-context -u -c u -t tid -t pid
> so had to change to vtid and vpid.
> 
> So in user channel to filter to a tid level I would do something like:
>   lttng add-context -u -c u -t vtid -t vpid
>   lttng enable-event -u -c u -a -filter '$ctx.vtid == 123'
> 
> Is that correct?

Yes, this is correct. It's because lttng-ust, from an application
perspective, only knows about the vtid and vpid (the containerized
values of its thread and process ID).

Thanks,

Mathieu

> 
> -- Tom
> 
>> -----Original Message-----
>> From: Mathieu Desnoyers [mailto:mathieu.desnoyers at efficios.com]
>> Sent: Thursday, June 01, 2017 1:25 PM
>> To: Deneau, Tom <tom.deneau at amd.com>
>> Cc: lttng-dev <lttng-dev at lists.lttng.org>
>> Subject: Re: [lttng-dev] lttng-track on userspace channel
>> 
>> ----- On May 31, 2017, at 7:28 PM, Deneau, Tom tom.deneau at amd.com wrote:
>> 
>> > I am using lttng 2.9.4.
>> >
>> > I have a server database daemon with many threads and I would like to
>> track
>> > events on a smaller subset of those threads.
>> > I noticed that on the kernel channel I can use lttng track --
>> pid=123,124,125
>> >
>> > and this will trace kernel events for just those TIDS (in spite of the
>> > documentation saying it will track whole pids).
>> > But I am happy because tid tracking is what I want.
>> 
>> Argh.. this is an unfortunate discrepancy! It should have been using
>> the kernel "tgid" at kernel-level to match what we do for userspace.
>> I'll prepare a patch fixing that.
>> 
>> LTTng really try hard to stick to "pid means process ID" and "tid means
>> thread ID". Internally in lttng-modules we do all that is needed to map
>> the pid/tgid notions back to tid/pid.
>> 
>> >
>> > However, is there an equivalent way to track certain tids on the
>> userspace
>> > channel?  I find that if I use
>> >    lttng track -u --pid=xxx
>> > it just tracks all the tids under that pid.
>> 
>> You can use e.g.
>> 
>> lttng enable-event -k -a --filter '$ctx.tid == 123'
>> 
>> or if you want it container-aware:
>> 
>> lttng enable-event -k -a --filter '$ctx.vtid == 123'
>> 
>> Thanks,
>> 
>> Mathieu
>> 
>> >
>> > Here is my script (where 79653 is a single tid).  In this case it traces
>> only
>> > kernel events on that tid.
>> > -----------
>> > lttng create
>> > lttng enable-channel u -u --num-subbuf 64 --subbuf-size 512k
>> > lttng enable-channel k -k --num-subbuf 64 --subbuf-size 512k
>> > lttng add-context -k -c k -t tid -t pid
>> > lttng add-context -u -c u -t vtid -t vpid
>> > lttng enable-event -u -c u -a
>> > lttng enable-event -k -c k --syscall futex
>> > lttng track -k --pid=79653
>> > lttng track -u --pid=79653
>> > lttng start
>> > sleep 1.0
>> > lttng stop
>> > lttng destroy
>> >
>> > -- Tom Deneau
>> > _______________________________________________
>> > lttng-dev mailing list
>> > lttng-dev at lists.lttng.org
>> > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
>> 
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
> > http://www.efficios.com

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