linux-kernel - Re: [PATCH printk v6 15/17] printk: Implement legacy printer kthread for PREEMPT

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Message-ID: <ZthjSaN25qGud8wr@pathway.suse.cz>
Date: Wed, 4 Sep 2024 15:40:25 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v6 15/17] printk: Implement legacy printer kthread
 for PREEMPT_RT

On Wed 2024-09-04 14:11:34, John Ogness wrote:
> The write() callback of legacy consoles usually makes use of
> spinlocks. This is not permitted with PREEMPT_RT in atomic
> contexts.
> 
> For PREEMPT_RT, create a new kthread to handle printing of all
> the legacy consoles (and nbcon consoles if boot consoles are
> registered). This allows legacy consoles to work on PREEMPT_RT
> without requiring modification. (However they will not have
> the reliability properties guaranteed by nbcon atomic
> consoles.)
> 
> Use the existing printk_kthreads_check_locked() to start/stop
> the legacy kthread as needed.
> 
> Introduce the macro force_legacy_kthread() to query if the
> forced threading of legacy consoles is in effect. Although
> currently only enabled for PREEMPT_RT, this acts as a simple
> mechanism for the future to allow other preemption models to
> easily take advantage of the non-interference property provided
> by the legacy kthread.
> 
> When force_legacy_kthread() is true, the legacy kthread
> fulfills the role of the console_flush_type @legacy_offload by
> waking the legacy kthread instead of printing via the
> console_lock in the irq_work. If the legacy kthread is not
> yet available, no legacy printing takes place (unless in
> panic).
> 
> If for some reason the legacy kthread fails to create, any
> legacy consoles are unregistered. With force_legacy_kthread(),
> the legacy kthread is a critical component for legacy consoles.
> 
> These changes only affect CONFIG_PREEMPT_RT.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr