linux-kernel - Re: [PATCH] prinkt/nbcon: Add a scheduling point to nbcon_kthread

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Date: Thu, 20 Jun 2024 12:18:37 -0500
From: Andrew Halaney <ahalaney@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Derek Barbosa <debarbos@...hat.com>, john.ogness@...utronix.de, 
	pmladek@...e.com, rostedt@...dmis.org, senozhatsky@...omium.org, 
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org, williams@...hat.com, 
	jlelli@...hat.com, lgoncalv@...hat.com, jwyatt@...hat.com, aubaker@...hat.com
Subject: Re: [PATCH] prinkt/nbcon: Add a scheduling point to
 nbcon_kthread_func().

nit: s/prinkt/printk

I make that typo so often :P

On Thu, Jun 20, 2024 at 11:43:00AM GMT, Sebastian Andrzej Siewior wrote:
> Constant printing can lead to a CPU hog in nbcon_kthread_func(). The
> context is preemptible but on !PREEMPT kernels there is no explicit
> preemption point which leads softlockup warnings.
> 
> Add an explicit preemption point in nbcon_kthread_func().
> 
> Reported-by: Derek Barbosa <debarbos@...hat.com>

Acked-by: Andrew Halaney <ahalaney@...hat.com>
Tested-by: Andrew Halaney <ahalaney@...hat.com>

This survived a bunch of tests that normally would cause some lockups
etc in PREEMPT_VOLUNTARY systems. I can see that the nbcon thread successfully
migrated NUMA nodes etc during periods of overwhelming the console backlog
successfully, which without this would not work prior.

Thanks!

> Link: https://lore.kernel.org/ZnHF5j1DUDjN1kkq@debarbos-thinkpadt14sgen2i.remote.csb
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/printk/nbcon.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index bb9689f94d302..0813ce88a49c5 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1119,6 +1119,7 @@ static int nbcon_kthread_func(void *__console)
>  		}
>  
>  		console_srcu_read_unlock(cookie);
> +		cond_resched();
>  
>  	} while (backlog);
>  
> -- 
> 2.45.2
>