linux-kernel - Re: [PATCH v2] panic: add an option to replay all the printk message in buffer

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Date:   Fri, 19 Apr 2019 10:08:35 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>, ying.huang@...el.com
Subject: Re: [PATCH v2] panic: add an option to replay all the printk message
 in buffer

On Thu, Apr 18, 2019 at 01:01:52PM +0200, Petr Mladek wrote:
> On Thu 2019-04-18 17:00:44, Feng Tang wrote:
> > Hi Petr,
> > 
> > On Thu, Apr 18, 2019 at 09:45:52AM +0200, Petr Mladek wrote:
> > > On Thu 2019-04-18 09:00:14, Sergey Senozhatsky wrote:
> > > > I think that PANIC_PRINT_ALL_PRINTK_MSG is a debugging option; a quite
> > > > specific one. So people who ask the kernel to PANIC_PRINT_ALL_PRINTK_MSG
> > > > they know what they are doing, and we probably will not cofuse anyone.
> > > > After all, we don't print any headers when we ftrace_dump() or imitate
> > > > sysrq via sysrq_timer_list_show(), or for any other panic_print_sys_info()
> > > > printouts. So it's OK to just do the simple thing for
> > > > PANIC_PRINT_ALL_PRINTK_MSG.
> > > 
> > > The following functions are currently called from panic_print_sys_info():
> > > 
> > > + show_state():
> > > 	printk(KERN_INFO
> > > 		"  task                        PC stack   pid father\n");
> > > + show_mem():
> > > 	printk("Mem-Info:\n");
> > > 
> > > + sysrq_timer_list_show()
> > > 	no global header; but each section can be easily distinguished
> > > 	because there are several static strings that explains the
> > > 	content
> > > 
> > > + debug_show_all_locks()
> > > 	pr_warn("\nShowing all locks held in the system:\n");
> > > 
> > > + ftrace_dump():
> > > 	printk(KERN_TRACE "Dumping ftrace buffer:\n");
> > > 
> > > 
> > > The person that enabled the debugging option might know what it did
> > > when it process the log the very same day. It might be less clear
> > > for others reading the log.
> > > 
> > > Also it still might be convenient to find the beginning easily.
> > > Or it might help to orientate when several test runs
> > > (over night test) are squashed in a single file. I see
> > > such logs pretty often.
> > 
> > Thanks for checking all the info. For the note, how about adding
> > something like this inside panic_print_sys_info()?
> > 
> > 	if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG) {
> > 		printk("\nprintk: will replay all the messages in buffer:\n");
> > 		console_flush_on_panic(CONSOLE_FLUSH_ALL);
> > 	}
> 
> This will store the message at the end of the buffer. It will repeat
> the message once again when replaying the log. I personally think
> that it is worse than printing nothing.

Got it :) 

> 
> 
> > If you are all fine with it, I'll go post a v3, thanks,
> 
> I suggest to wait few days. We might get opinion from someone
> else...
> 
> Then sent whatever approach makes most sense to you. In fact, you
> are the 3rd person in this triangle. I believe that we will
> respect it. Last many mails are just a bike shedding.
> 
> Just one thing. Please, use "replay" in the function or parameter
> name. The effect of console_flush_on_panic(CONSOLE_FLUSH_ALL)
> is really hard to guess from the name.

Ok. thanks

- Feng