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From: John Ogness <john.ogness@linutronix.de>
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To: Petr Mladek <pmladek@suse.com>, Andrew Murray
 <amurray@thegoodpenguin.co.uk>
Cc: senozhatsky@chromium.org, rostedt@goodmis.org,
 linux-serial@vger.kernel.org, Michael Cobb <mcobb@thegoodpenguin.co.uk>
Subject: Re: Lock contention with console kthreads
In-Reply-To: <aMFBWJWqv3KsaaZU@pathway.suse.cz>
References: <CALqELGwr=XWA2iD-7wa1jOigx3pfEnTnaGWUMQX_A5MWe3DN_g@mail.gmail.com>
 <aMFBWJWqv3KsaaZU@pathway.suse.cz>
Date: Wed, 10 Sep 2025 11:45:06 +0206
Message-ID: <84a5326405.fsf@jogness.linutronix.de>
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On 2025-09-10, Petr Mladek <pmladek@suse.com> wrote:
>> Since commit 5f53ca3ff83b ("printk: Implement legacy printer kthread
>> for PREEMPT_RT"), and only when PREEMPT_RT is enabled, legacy consoles
>> use a kthread to write out to the device. This is beneficial for boot
>> time when using a slow console such as a serial port. However, this
>> gain can be short-lived as console_on_rootfs in init/main.c appears to
>> act as a synchronisation point.
>> 
>> legacy_kthread_func holds console_lock when flushing, and is held for
>> a long period of time during boot.

[...]

> I would suggest to modify legacy_kthread_func() and flush one
> record-per-cycle and get/release the console_lock() in each cycle.

Agreed. This is an excellent suggestion!

John