[lttng-dev] lttng-track on userspace channel

[Deneau, Tom]

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.

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?

-- 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