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From: John Ogness <john.ogness@linutronix.de>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Petr Mladek <pmladek@suse.com>
Cc: linux-kernel@vger.kernel.org,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v3] printk: Skip console drivers on PREEMPT_RT.
Date: Tue, 26 Jul 2022 10:03:39 +0206	[thread overview]
Message-ID: <871qu8i9vw.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <Yt6zwP9xSdUhsoQ9@linutronix.de>

On 2022-07-25, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> printk might be invoked in a context with disabled interrupts and or
> preemption and additionally disables interrupts before it invokes the
> console drivers. This behaviour is not desired on PREEMPT_RT:
> - The console driver are using spinlock_t based locking which become sleeping
>   locks on PREEMPT_RT and must not be acquired with disabled interrupts (or
>   preemption).
>
> - The locks within the console drivers must remain sleeping locks and they must
>   not disable interrupts. Printing (and polling for its completion) at 115200
>   baud on an UART takes too long for PREEMPT_RT in general and so raises the
>   latency of the IRQ-off time of the system beyond acceptable levels.
>
> Skip printing to the console as temporary workaround until the printing threads
> and atomic consoles have been introduced or another solution which is
> compatible with the PREEMPT_RT approach.
> With this change, the user will not see any kernel message printed to the
> console but can retrieve the printk buffer from userland (via the dmesg
> command). This allows enable PREEMPT_RT as a whole without disabling printk and
> loosing all kernel output.

Note that "the dmesg command" is not the only userspace tool to access
the kernel logs. Logging daemons (using /proc/kmsg or /dev/kmsg) also
have full access.

> Disable console printing on PREEMPT_RT.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: John Ogness <john.ogness@linutronix.de>

  parent reply	other threads:[~2022-07-26  7:57 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-07-20 15:48 [PATCH] printk: Skip console drivers on PREEMPT_RT Sebastian Andrzej Siewior
2022-07-20 16:26 ` John Ogness
2022-07-20 16:35   ` Sebastian Andrzej Siewior
2022-07-20 17:50     ` John Ogness
2022-07-21  6:50       ` [PATCH v2] " Sebastian Andrzej Siewior
2022-07-22 12:39         ` Petr Mladek
2022-07-22 17:03           ` Sebastian Andrzej Siewior
2022-07-25 12:30             ` Petr Mladek
2022-07-25 12:51               ` John Ogness
2022-07-25 13:55               ` Sebastian Andrzej Siewior
2022-07-25 14:24                 ` Petr Mladek
2022-07-25 15:16                   ` [PATCH v3] " Sebastian Andrzej Siewior
2022-07-26  7:39                     ` Petr Mladek
2022-07-26  7:57                     ` John Ogness [this message]
2022-07-26 13:11                       ` Petr Mladek

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