[lttng-dev] lttng-track on userspace channel

[Mathieu Desnoyers]

----- On Jun 1, 2017, at 2:24 PM, Mathieu Desnoyers mathieu.desnoyers at efficios.com wrote:

> ----- 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.

Here is the upstream lttng-modules commit that fixes the issue, backported
to 2.10, 2.9, 2.8:

commit 12d4702757b86de457904f6e344b4f412cef611a
Author: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
Date:   Thu Jun 1 14:24:11 2017 -0400

    Fix: pid tracker should track "pgid"
    
    The "pid" notion exposed by LTTng translates to the "pgid" notion in the
    Linux kernel. Therefore using "current->pid" as argument to the PID
    tracker actually ends up behaving as a "tid" tracker, which does not
    match the intent nor the user-space tracer behavior.
    
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>

Thanks for letting us know about this problem.

Mathieu

> 
>> 
>> 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