Re: [printk] fbc14616f4: BUG:kernel_reboot-without-warning_in_test_stage

From	Pavel Machek <pavel@ucw.cz>
Newsgroups	linux.kernel
Subject	Re: [printk] fbc14616f4: BUG:kernel_reboot-without-warning_in_test_stage
Date	2017-04-07 09:20 +0200
Message-ID	<ttwlA-40I-9@gated-at.bofh.it> (permalink)
References	(5 earlier) <tr62e-5Sb-13@gated-at.bofh.it> <tr6OC-6qJ-7@gated-at.bofh.it> <ts6CS-52F-19@gated-at.bofh.it> <ttjy1-3Us-13@gated-at.bofh.it> <ttu0p-2mu-5@gated-at.bofh.it>
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On Fri 2017-04-07 13:44:40, Sergey Senozhatsky wrote:
> Hello,
> 
> On (04/06/17 19:33), Pavel Machek wrote:
> > > This patch set gives up part of the printk() reliability for bounded
> > > latency (at least unless we detect we are really in trouble) which is IMHO
> > > a good trade-off for lots of users (and others can just turn this feature
> > > off).
> > 
> > If they can ever realize they were bitten by this feature.
> > 
> > Can we go for different tradeoff?
> > 
> > In console_unlock(), if you detect too much work, print "Too many
> > messages to print, %d bytes delayed" and wake up kernel thread.
> 
> "too many messages" is undefined. console_unlock() can be called from
> IRQ handler or with preemtion disabled, or under spin_lock, or under
> RCU read lock, etc. etc. By the time we decide to wake up printk_kthread
> from console_unlock() it may be already too late.

So lets define "too many messages" as 240 characters. We know printk
worked rather well for us for more than 20 years. Kernel code is used
to printk taking few miliseconds. 

> besides, this does not really address any of the concerns you have
> pointed out in other emails. we might be unable to wake_up printk_kthread
> (because there is a misbehaving higher prio process, or because the
> scheduler is misbehaving, etc. etc.) so the "emergency mode" is still
> here and still requires special handling.

Yeah? So you know modified printk() does not work, that's why
"emergency mode" exists. Unfortunately, you can't rely on fact that
you can detect half-crashed machines by printk levels. You usually
can't.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Thread

[RFC][PATCHv2 0/8] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-29 11:30 +0200
  [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-29 11:30 +0200
    Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Petr Mladek <pmladek@suse.com> - 2017-04-04 11:10 +0200
      Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-04 11:40 +0200
    Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Pavel Machek <pavel@ucw.cz> - 2017-04-06 19:20 +0200
      Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-07 07:20 +0200
        Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Pavel Machek <pavel@ucw.cz> - 2017-04-07 09:30 +0200
          Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-07 10:20 +0200
            Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Pavel Machek <pavel@ucw.cz> - 2017-04-07 14:10 +0200
  [RFC][PATCHv2 5/8] sysrq: switch to printk.emergency mode in unsafe places Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-29 11:30 +0200
    Re: [RFC][PATCHv2 5/8] sysrq: switch to printk.emergency mode in  unsafe places Petr Mladek <pmladek@suse.com> - 2017-03-31 17:40 +0200
      Re: [RFC][PATCHv2 5/8] sysrq: switch to printk.emergency mode in  unsafe places Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-04-01 02:10 +0200
  [RFC][PATCHv2 1/8] printk: move printk_pending out of per-cpu Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-29 11:40 +0200
    Re: [RFC][PATCHv2 1/8] printk: move printk_pending out of per-cpu Petr Mladek <pmladek@suse.com> - 2017-03-31 15:20 +0200
      Re: [RFC][PATCHv2 1/8] printk: move printk_pending out of per-cpu Peter Zijlstra <peterz@infradead.org> - 2017-03-31 15:40 +0200
        Re: [RFC][PATCHv2 1/8] printk: move printk_pending out of per-cpu Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-03 13:30 +0200
          Re: [RFC][PATCHv2 1/8] printk: move printk_pending out of per-cpu Petr Mladek <pmladek@suse.com> - 2017-04-03 14:50 +0200
  [RFC][PATCHv2 6/8] kexec: switch to printk.emergency mode in unsafe places Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-29 11:40 +0200
    Re: [RFC][PATCHv2 6/8] kexec: switch to printk.emergency mode in  unsafe places Petr Mladek <pmladek@suse.com> - 2017-03-31 17:40 +0200
  [RFC][PATCHv2 8/8] printk: enable printk offloading Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-29 11:40 +0200
    Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-03-31 04:40 +0200
      Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-03-31 06:10 +0200
        Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Ye Xiaolong <xiaolong.ye@intel.com> - 2017-03-31 08:50 +0200
          Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-31 16:50 +0200
            Re: [printk]  fbc14616f4: BUG:kernel_reboot-without-warning_in_test_stage ebiederm@xmission.com (Eric W. Biederman) - 2017-03-31 17:40 +0200
              Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Jan Kara <jack@suse.cz> - 2017-04-03 11:40 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Petr Mladek <pmladek@suse.com> - 2017-04-03 12:10 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Pavel Machek <pavel@ucw.cz> - 2017-04-06 19:40 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-07 06:50 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Pavel Machek <pavel@ucw.cz> - 2017-04-07 09:20 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-07 09:50 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Pavel Machek <pavel@ucw.cz> - 2017-04-07 10:20 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-07 14:20 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Pavel Machek <pavel@ucw.cz> - 2017-04-07 14:50 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Steven Rostedt <rostedt@goodmis.org> - 2017-04-07 16:50 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-04-07 17:20 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Peter Zijlstra <peterz@infradead.org> - 2017-04-07 17:30 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-04-07 17:50 +0200
                Re: [printk]  fbc14616f4: BUG:kernel_reboot-without-warning_in_test_stage ebiederm@xmission.com (Eric W. Biederman) - 2017-04-09 20:30 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-10 06:50 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Pavel Machek <pavel@ucw.cz> - 2017-04-09 12:20 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-10 07:00 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Petr Mladek <pmladek@suse.com> - 2017-04-10 14:00 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Steven Rostedt <rostedt@goodmis.org> - 2017-04-07 16:40 +0200
                Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Pavel Machek <pavel@ucw.cz> - 2017-04-09 12:00 +0200
              Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-03 13:00 +0200
          Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Ye Xiaolong <xiaolong.ye@intel.com> - 2017-04-05 09:40 +0200
            Re: [printk]  fbc14616f4:  BUG:kernel_reboot-without-warning_in_test_stage Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-05 10:50 +0200
    Re: [RFC][PATCHv2 8/8] printk: enable printk offloading Petr Mladek <pmladek@suse.com> - 2017-04-03 17:50 +0200
      Re: [RFC][PATCHv2 8/8] printk: enable printk offloading Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-04-04 14:30 +0200
  [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe places Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-29 11:40 +0200
    Re: [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe  places Petr Mladek <pmladek@suse.com> - 2017-03-31 17:10 +0200
    Re: [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe  places Pavel Machek <pavel@ucw.cz> - 2017-04-06 19:30 +0200
      Re: [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe  places Andreas Mohr <andi@lisas.de> - 2017-04-09 13:00 +0200
        Re: [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe  places Petr Mladek <pmladek@suse.com> - 2017-04-10 14:30 +0200
          Re: [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe  places Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-04-10 16:40 +0200
  [RFC][PATCHv2 7/8] printk: add printk emergency_mode parameter Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-03-29 11:40 +0200
    Re: [RFC][PATCHv2 7/8] printk: add printk emergency_mode parameter Petr Mladek <pmladek@suse.com> - 2017-04-03 17:30 +0200
      Re: [RFC][PATCHv2 7/8] printk: add printk emergency_mode parameter Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-04-04 10:30 +0200

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