[lttng-dev] LTTng 32 bits on 64 bits systems - kernel events... but no user-event

Mon Jun 7 11:24:28 EDT 2021

Hi, 

> I am currently trying to add lttng 32bits on a 64bits system with a custom
> kernel 4.19.177+ x86_64 (Debian 10.9)
> My former attempt only with only a 100% arm32 bits was successful (raspberry).
> I am facing now a strange issue : I can record kernel event but not userspace
> event... but I can see these user-event (see below)!

Ok. 

> (tuto : [ https://lttng.org/docs/#doc-instrumenting-32-bit-app-on-64-bit-system
> | https://lttng.org/docs/#doc-instrumenting-32-bit-app-on-64-bit-system ] )

> I followed two times the tutorial : on lttng 2.12 and lttng 2.10.
> I am on lttng (LTTng Trace Control) 2.10.11 - KeKriek now

Any particular reasons?

> All installation seemed to install without any error.
> The only thing I added is at the very end where I did a : ln -s
> /usr/local/bin/lttng /usr/bin/lttng (to directly call commands like "lttng
> start")

> Here some interesting outputs, I am using this projet as an example : [
> https://lttng.org/docs/#doc-tracing-your-own-user-application |
> https://lttng.org/docs/#doc-tracing-your-own-user-application ]
> lttng list -u : OK [ https://paste.ubuntu.com/p/hZptnNzySw/ | Ubuntu Pastebin ]
> LTTNG_UST_DEBUG=1 ./hello : seems OK : [ https://paste.ubuntu.com/p/w6xHrJsWJ9/
> | Ubuntu Pastebin ]
> kernel-event output : [ https://paste.ubuntu.com/p/5KfyS8wVdb/ | Ubuntu Pastebin
> ]
> user-event ouput : none - the output folder stated with lttng create
> my-user-session --output=/tmp/my-user-session doesn't even appear
> apt-file search lttng : [ https://paste.ubuntu.com/p/xMcCc7bqwk/ | Ubuntu
> Pastebin ]
> lsmod | grep lttng : [ https://paste.ubuntu.com/p/7YNkNsh9Fr/ | Ubuntu Pastebin
> ]

Can you provide the config.log from the configure steps 

We are missing the most important logs: lttng-sessiond. 

$ lttng-sessiond -vvv --verbose-consumer 

> Note : file /usr/local/bin/lttng (the exe I am using) gives :
> /usr/local/bin/lttng: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV),
> dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux
> 3.2.0, BuildID[sha1]=542a108c40e386c17027c2c8f8fcb30e278b7748, with debug_info,
> not stripped

This is normal. Concretely to support tracing of 32 bit applications only a 32 bits
lttng-consumerd executable is needed which is launched by the lttng-sessiond executable. 

> I am wondering if the installation didn't pick a 64bits RCU and analyses
> programms only on this.

Hmmm, what indicate this? I seriously doubt that this can happen. 

> However, I have currently absolutely no idea how to debug this.

I decided to do the process on a clean ubuntu 20.04 machine.
For 2.12 there is some problems when compiling only lttng-consumerd (fd-tracker lib and libcommon problem). I plan on providing a patch shortly.

But the major problem is that the path passed at configure time for the lttng-consumerd32 bit daemon are simply not used at runtime.
See this bug I just opened [1]. This is valid for 2.10 all the way to master AFAIK.

[1] https://bugs.lttng.org/issues/1318 

In the meantime you will have to pass the lttng-consumerd 32 bit path as argument or env variables to lttng-sessiond as such: 

$ lttng-sessiond -vvv --verbose-consumer --consumerd32-libdir=/usr/local/lib32 --consumerd32-path=/usr/local/lib32/lttng/libexec/lttng-consumerd

or 

$ LTTNG_CONSUMERD32_BIN=/usr/local/lib32/lttng/libexec/lttng-consumerd LTTNG_CONSUMERD32_LIBDIR=/usr/local/lib32 lttng-sessiond -vvv --verbose-consumer

>From there the 32bits application is traced correctly and data is gathered without any problem (at least on 2.12).

```
[15:22:24.500655502] (+?.?????????) ubuntu2004.localdomain lttng_ust_statedump:start: { cpu_id = 3 }, { }
[15:22:24.500661538] (+0.000006036) ubuntu2004.localdomain lttng_ust_statedump:procname: { cpu_id = 3 }, { procname = "hello" }
[15:22:24.502611224] (+0.001949686) ubuntu2004.localdomain lttng_ust_statedump:bin_info: { cpu_id = 3 }, { baddr = 0xF7EC9000, memsz = 20596, path = "/usr/lib32/libdl-2.31.so", is_pic = 1, has_build_id = 1, has_debug_link = 1 }
[15:22:24.502614501] (+0.000003277) ubuntu2004.localdomain lttng_ust_statedump:build_id: { cpu_id = 3 }, { baddr = 0xF7EC9000, _build_id_length = 20, build_id = [ [0] = 0x75, [1] = 0x1E, [2] = 0xDD, [3] = 0x47, [4] = 0x72, [5] = 0x3, [6] 
```

Cheers