linux-kernel - Re: [RFC PATCH 3/3] watchdog: Turn console verbosity on when reporting softlockup

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Date:   Thu, 19 Mar 2020 17:01:56 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Valdis Klētnieks <valdis.kletnieks@...edu>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        linux-kernel@...r.kernel.org,
        John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Jisheng Zhang <Jisheng.Zhang@...aptics.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrew Gabbasov <andrew_gabbasov@...tor.com>,
        Dirk Behme <dirk.behme@...bosch.com>,
        Eugeniu Rosca <roscaeugeniu@...il.com>
Subject: Re: [RFC PATCH 3/3] watchdog: Turn console verbosity on when
 reporting softlockup

On (20/03/19 03:38), Valdis Klētnieks wrote:
> On Thu, 19 Mar 2020 15:48:36 +0900, Sergey Senozhatsky said:
> 
> > So the issue is that when we bump `console_loglevel' or `ignore_loglevel'
> > we lift restrictions globally. For all CPUs, for all contexts which call
> > printk(). This may have negative impact. Fuzzing is one example. It
> > usually generates a lot of printk() noise, so lifting global printk()
> > log_level restrictions does cause problems. I recall that fuzzing people
> > really disliked the
> > 			old_level = console_loglevel
> > 			console_loglevel = new_level
> > 			....
> > 			console_loglevel = old_level
> >
> > approach. Because if lets all CPUs and tasks to pollute serial logs.
> > While what we want is to print messages from a particular CPU/task.
> 
> Well... how does this sound for a RFC idea?  We already have CONFIG_PRINTK_TIME
> and CONFIG_PRINTK_CALLER.  How about adding an option to allow printing the
> calling CPU as well, or just extend CALLER to print [pid/cpu] rather than just
> [pid]?

IIRC, CONFIG_PRINTK_CALLER prints pid when printk() caller is a
running task, and CPU-id otherwise.

It was added by fuzzing people, because it's almost (or "simply")
impossible to get through serial logs sometimes.

	-ss