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

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Date:   Thu, 19 Mar 2020 03:38:26 -0400
From:   "Valdis Klētnieks" <valdis.kletnieks@...edu>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     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 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]?

(And yes, I know that the concept of "this CPU" can change quickly, which is
why per_cpu data may not be accurate for anything more than statistical
aggregation purposes.  It's only worth pondering how to deal with that if it's
worth developing the patch at all... :)


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