Re: How to trace serial console init during boot-time?

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 7 Oct 2022 15:30:56 +0200
Paul Menzel <pmenzel@molgen.mpg.de> wrote:

> Dear Linux folks,
> 
> 
> I am trying to do boot-time tracing of `univ8250_console_init()`:
> 
>      [    0.126636] ftrace: allocating 41793 entries in 164 pages
>      [    0.132446] ftrace: allocated 164 pages with 3 groups
>      […]
>      [    0.167334] calling  con_init+0x0/0x239 @ 0
>      [    0.170217] Console: colour VGA+ 80x25
>      [    0.190381] printk: console [tty0] enabled
>      [    0.190484] initcall con_init+0x0/0x239 returned 0 after 0 usecs
>      [    0.190487] calling  hvc_console_init+0x0/0x18 @ 0
>      [    0.190489] initcall hvc_console_init+0x0/0x18 returned 0 after 
> 0 usecs
>      [    0.190491] calling  univ8250_console_init+0x0/0x2b @ 0
>      [    1.488645] printk: console [ttyS0] enabled
>      [    1.492945] initcall univ8250_console_init+0x0/0x2b returned 0 
> after 0 usecs
>      […]
>      [    1.670397] calling  trace_init_perf_perm_irq_work_exit+0x0/0x17 @ 1
>      [    1.670399] initcall trace_init_perf_perm_irq_work_exit+0x0/0x17 
> returned 0 after 0 usecs
>      […]
>      [    1.673339] calling  trace_init_flags_sys_enter+0x0/0x13 @ 1
>      [    1.673342] initcall trace_init_flags_sys_enter+0x0/0x13 
> returned 0 after 0 usecs
>      [    1.673344] calling  trace_init_flags_sys_exit+0x0/0x13 @ 1
>      [    1.673346] initcall trace_init_flags_sys_exit+0x0/0x13 returned 
> 0 after 0 usecs
>      [    1.673348] calling  cpu_stop_init+0x0/0x87 @ 1
>      [    1.673364] initcall cpu_stop_init+0x0/0x87 returned 0 after 0 usecs
>      [    1.673366] calling  init_kprobes+0x0/0x149 @ 1
>      [    1.673495] initcall init_kprobes+0x0/0x149 returned 0 after 0 usecs
>      [    1.673497] calling  init_events+0x0/0x4d @ 1
>      [    1.673502] initcall init_events+0x0/0x4d returned 0 after 0 usecs
>      [    1.673504] calling  init_trace_printk+0x0/0xc @ 1
>      [    1.673505] initcall init_trace_printk+0x0/0xc returned 0 after 
> 0 usecs
>      [    1.673507] calling  event_trace_enable_again+0x0/0x23 @ 1
>      [    1.673508] initcall event_trace_enable_again+0x0/0x23 returned 
> 0 after 0 usecs
> 
> The function `univ8250_console_init()` is not available in 
> `/sys/kernel/debug/tracing/available_filter_functions`, so I’d like to 
> trace `univ8250_console_setup()`:
> 
>      initcall_debug log_buf_len=32M trace_buf_size=262144K 
> trace_clock=global 
> trace_options=nooverwrite,funcgraph-abstime,funcgraph-cpu,funcgraph-duration,funcgraph-proc,funcgraph-tail,nofuncgraph-overhead,context-info,graph-time 
> ftrace=fun
> ction_graph ftrace_graph_max_depth=2 
> ftrace_graph_filter=univ8250_console_setup tp_printk
> 
> Unfortunately, `/sys/kernel/debug/tracing/trace` is empty, so I guess 
> the console happens before ftrace is available?

"function" tracing is enabled by then (I just tested it), but
"function_graph" is not. Function graph requires trace events enabled,
but I could see what happens if I do enabled it ;-)

> 
> Is there another way to trace the serial console init, without having to 
> add print statements?
> 

Use "ftrace=function" but then you will get everything. You could add a
"set_filter=<func>:traceoff" to set a trigger that would disable
tracing, if you can find a function that would be good to stop tracing
with.

Otherwise, I could take a look to see what it takes to get function
graph tracing working that early.

-- Steve