linux-kernel - Re: a question about how to debug this case in ftrace

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Date: Wed, 26 Jun 2024 10:17:00 +0800
From: Song Chen <chensong_2000@....cn>
To: Steven Rostedt <rostedt@...dmis.org>, Andrew Halaney <ahalaney@...hat.com>
Cc: Derek Barbosa <debarbos@...hat.com>, pmladek@...e.com,
 john.ogness@...utronix.de, senozhatsky@...omium.org,
 linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 williams@...hat.com, jlelli@...hat.com
Subject: Re: a question about how to debug this case in ftrace

Hi Steven,

Some mobile manufactures used to reserve a space in emmc to dump message 
before oops, your work is smarter than this for it's getting rid of 
dependency of hardware.

I look forward to trying it when it comes out, many thanks.

BR

Song

在 2024/6/25 21:44, Steven Rostedt 写道:
> On Tue, 25 Jun 2024 08:20:15 -0500
> Andrew Halaney <ahalaney@...hat.com> wrote:
> 
>> So I ended up doing a sequence like:
>>
>>      tracing_off();
>>      ftrace_dump(DUMP_ALL);
> 
> I've done that several times too.
> 
>>
>> in the softlockup code when it was detected. Ideally I wanted to look at
>> the vmcore and look at the ftrace data in there (since debugging printk
>> by using printk is a little confusing), but there was a makedumpfile bug
>> I hit... so I went with the hacky route to prove to myself what was
>> going on. I think since then that's been resolved. Hope that helps!
> 
> You may be interested in some work I'm doing that allows you to read
> the ring buffer from a previous kernel after a crash.
> 
> https://lore.kernel.org/linux-trace-kernel/20240612231934.608252486@goodmis.org/
> 
> I also have a way to ask for any memory, that should be able to get the
> same location most times, via a "reserve_mem=" kernel command line
> parameter.
> 
> https://lore.kernel.org/linux-trace-kernel/20240613233415.734483785@goodmis.org/
> 
> They are both destined for the next merge window. After that, I have
> one more patch that ties the two together, so that you can have a
> kernel command line of:
> 
>   reserve_mem=12M:4096:trace trace_instance=bootmap@...ce
> 
> and then when you boot up, you would have a trace instance that would
> be mapped to that memory. If your machine doesn't clear memory after a
> crash, you can read the data from the crash on the next boot.
> 
> -- Steve
> 
>