Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread

From	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread
Date	2017-04-07 10:20 +0200
Message-ID	<ttxhD-4AV-11@gated-at.bofh.it> (permalink)
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Hello,

On (04/07/17 09:21), Pavel Machek wrote:
> > spin_dump() and trigger_all_cpu_backtrace() result in a bunch of
> > additional printk()-s so CPU0 has even more job to do in console_unlock(),
> > while it still holds the contended spin_lock. and so on; there are
> > many other examples.
> > 
> > so should we declare a "we can spend only 2 seconds in direct printk()
> > and then must offload printing" rule? I don't think it's much better
> > than a simpler "we always offload, as long as we think it's safe".
> 
> I believe we should do the 2 seconds rule. It allows us to print "some
> messages delayed" message, so at least whoever is trying to debug the
> crash will have the hints that he needs to look at the printk system.

do you mean panic()? in panic() we call console_flush_on_panic(),
which immediately outputs all pending logbuf messages. printk()
offloading does not happen there.

> "we always offload, as long as we think it's safe" rule does not
> really work, as printk() can not detect if it is safe or not.

but "2 seconds" rule has that "as long as we think it's safe" string
attached as well. just because we do offloading. which is sometimes
un-safe. so regardless the timeout value (0 seconds or 2 seconds) we
still need some sort of a hint from the path that issues printk()
because that path (panic, kexec, sysrq, etc.) knows for sure when
things are abnormal. printk() is pretty clueless in this regard.
/* well, I still think that EMERG loglevel thing is not completely
 broken. */

	-ss

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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