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

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Message-ID: <e2bf4639-27fb-6a5f-4a71-098ea5f3fe7f@molgen.mpg.de>
Date:   Tue, 11 Oct 2022 11:13:48 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: How to trace serial console init during boot-time?

Dear Steven,


Thank you very much for the quick reply.

Am 08.10.22 um 22:43 schrieb Steven Rostedt:
> On Fri, 7 Oct 2022 15:30:56 +0200 Paul Menzel wrote:

>> 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=function_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.

Thank you. That worked. As I didn’t use a trigger to stop tracing, the 
resulting trace was over 1 GB in size, but I found what I was looking for.

> Otherwise, I could take a look to see what it takes to get function
> graph tracing working that early.
Having earlier tracing is always an improvement, but right now it’s not 
necessary at least for me anymore.

Thank you again.


Kind regards,

Paul


PS: With the trace, I started the thread *Do not delay boot when 
printing log to serial console during startup?* [1].


[1]: 
https://lore.kernel.org/linux-serial/c87cc376-3b0d-8fda-7fc1-555c930faaf1@molgen.mpg.de/