linux-kernel - Re: Removal of printk safe buffers delays NMI context printk

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Message-ID: <87h7cqg0xk.fsf@jogness.linutronix.de>
Date:   Fri, 05 Nov 2021 15:03:27 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Laurent Dufour <ldufour@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>
Subject: Re: Removal of printk safe buffers delays NMI context printk

On 2021-11-05, Nicholas Piggin <npiggin@...il.com> wrote:
>> What was removed from 93d102f094b was irq_work triggering on all
>> CPUs.
>
> No, it was the caller executing the flush for all remote CPUs itself.
> irq work was not involved (and irq work can't be raised in a remote
> CPU from NMI context).

Maybe I am missing something. In 93d102f094b~1 I see:

watchdog_smp_panic
  printk_safe_flush
    __printk_safe_flush
      printk_safe_flush_buffer
        printk_safe_flush_line
          printk_deferred
            vprintk_deferred
              vprintk_emit (but no direct printing)
              defer_console_output
                irq_work_queue

AFAICT, using defer_console_output() instead of your new printk_flush()
should cause the exact behavior as before.

> but we do need that printk flush capability back there and for
> nmi_backtrace.

Agreed. I had not considered this necessary side-effect when I removed
the NMI safe buffers.

I am just wondering if we should fix the regression by going back to
using irq_work (such as defer_console_output()) or if we want to
introduce something new that introduces direct printing.

John Ogness