[lttng-dev] Lttng: display active threads in multiple cores.

[Erica Bugden]

Hello Zvika,

On 2024-04-02 23:52, Zvi Vered wrote:
> Hi Erica,
> 
> Thank you very much for your answer.
> Can you please tell what is the added value of ftrace (compared to using 
> only lttng) ?

I don't think I understand the intention behind your question. I'll make 
some guesses below and you're welcome to clarify if you wish.

ftrace and LTTng are different tools that have some overlap in the 
tracing use cases they can address. ftrace is a Linux kernel tracer that 
is included in the kernel; it isn't an LTTng add-on.

Both ftrace and LTTng can trace the Linux kernel if the tracepoints have 
been included. LTTng doesn't use ftrace, but most kernels that are 
configured to include the tracepoints typically also include ftrace.

That being said, if you only want to trace userspace applications with 
LTTng and don't also want kernel traces, then you don't need an 
ftrace-enabled kernel.

Best,
Erica

> 
> Best regards,
> Zvika
> 
> 
> On Tue, Apr 2, 2024 at 5:11 PM Erica Bugden <ebugden at efficios.com 
> <mailto:ebugden at efficios.com>> wrote:
> 
>     Hello Zvika,
> 
>     On 2024-03-29 01:09, Zvi Vered via lttng-dev wrote:
>      > Hi Christopher,
>      >
>      > Thank you very much for your reply.
>      > Can you please explain what do you mean by ftrace-enabled kernel ?
> 
>     I believe what Christopher means by "ftrace-enabled" kernel is that the
>     Linux kernel has been configured to include ftrace. Both the ftrace
>     tracer and the LTTng tracer use the same kernel tracepoints to extract
>     execution information and these tracepoints are included in the kernel
>     if ftrace is included.
> 
>     Most Linux distributions will include ftrace by default. However, you
>     can check whether this is the case by searching for `tracefs` in
>     `/proc/filesystems` (assuming it's already mounted) or by trying to
>     mount `tracefs`. `tracefs` is the filesystem ftrace uses to communicate
>     with users.
> 
>     More details about how to check if ftrace is enabled and how to enable
>     it if not:
>     https://wiki.linuxfoundation.org/realtime/documentation/howto/tools/ftrace <https://wiki.linuxfoundation.org/realtime/documentation/howto/tools/ftrace>
> 
>     The "More Information" section points to the primary sources (Linux
>     kernel documentation), but I find this page to be a good starting point.
> 
>     Best,
>     Erica
> 
>      >
>      > Best regards,
>      > Zvika
>      >
>      > On Wed, Mar 27, 2024 at 7:32 PM Christopher Harvey via lttng-dev
>      > <lttng-dev at lists.lttng.org <mailto:lttng-dev at lists.lttng.org>
>     <mailto:lttng-dev at lists.lttng.org
>     <mailto:lttng-dev at lists.lttng.org>>> wrote:
>      >
>      >     you can use an ftrace-enabled kernel with lttng (maybe even just
>      >     tracecompass) or perfetto to get that kind of trace
>      >
>      >
>     https://archive.eclipse.org/tracecompass.incubator/doc/org.eclipse.tracecompass.incubator.ftrace.doc.user/User-Guide.html <https://archive.eclipse.org/tracecompass.incubator/doc/org.eclipse.tracecompass.incubator.ftrace.doc.user/User-Guide.html> <https://archive.eclipse.org/tracecompass.incubator/doc/org.eclipse.tracecompass.incubator.ftrace.doc.user/User-Guide.html <https://archive.eclipse.org/tracecompass.incubator/doc/org.eclipse.tracecompass.incubator.ftrace.doc.user/User-Guide.html>>
>      >
>      >     or
>      >
>      > https://ui.perfetto.dev/ <https://ui.perfetto.dev/>
>     <https://ui.perfetto.dev/ <https://ui.perfetto.dev/>>
>      >
>      >     On Wed, Mar 27, 2024, at 5:26 AM, Zvi Vered via lttng-dev wrote:
>      >      > Hello,
>      >      >
>      >      > I have an application with 4 threads.
>      >      > I'm required to display on the graph when thread starts
>     working
>      >     till it
>      >      > blocks for the next semaphore.
>      >      >
>      >      > But without using the lttng userspace library.
>      >      >
>      >      > Is it possible ?
>      >      >
>      >      > Thank you,
>      >      > Zvika
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