From: Nicholas Piggin <npiggin@gmail.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: "Laurent\
Dufour" <ldufour@linux.ibm.com>,
linux-kernel@vger.kernel.org, "Petr Mladek" <pmladek@suse.com>
Subject: Re: Removal of printk safe buffers delays NMI context printk
Date: Sat, 06 Nov 2021 09:48:09 +1000 [thread overview]
Message-ID: <1636155848.5e41htngh5.astroid@bobo.none> (raw)
In-Reply-To: <87h7cqg0xk.fsf@jogness.linutronix.de>
Excerpts from John Ogness's message of November 5, 2021 11:57 pm:
> On 2021-11-05, Nicholas Piggin <npiggin@gmail.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
Oh I thought you meant irq_work triggering on all CPUs (i.e., including
remote CPUs) was the key.
> AFAICT, using defer_console_output() instead of your new printk_flush()
> should cause the exact behavior as before.
It does.
>> 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.
irq_work works for this situation so for a minimal fix I think it's
fine. When you do the big rework it would be okay to do it directly
if you have such a facility for other reaons.
Thanks,
Nick
next prev parent reply other threads:[~2021-11-05 23:48 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-04 15:54 Removal of printk safe buffers delays NMI context printk Nicholas Piggin
2021-11-04 16:18 ` John Ogness
2021-11-05 1:26 ` Nicholas Piggin
2021-11-05 9:55 ` John Ogness
2021-11-05 11:43 ` Nicholas Piggin
2021-11-05 13:57 ` John Ogness
2021-11-05 16:23 ` Petr Mladek
2021-11-05 16:44 ` John Ogness
2021-11-06 0:26 ` Nicholas Piggin
2021-11-06 20:05 ` John Ogness
2021-11-05 23:57 ` Nicholas Piggin
2021-11-05 23:48 ` Nicholas Piggin [this message]
2021-11-05 16:01 ` Petr Mladek
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