linux-kernel - Re: [PATCH 1/7] printk: Hand over printing to console if printing too long

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Message-ID: <CAK1hOcOzWiO6LR0JJ-MhAL4Js_534WpCX=gcY44=hQ9+3Yfmsw@mail.gmail.com>
Date:	Tue, 1 Mar 2016 18:22:25 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Jan Kara <jack@...e.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, pmladek@...e.com,
	KY Srinivasan <kys@...rosoft.com>,
	Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing too long

On Mon, Oct 26, 2015 at 5:52 AM, Jan Kara <jack@...e.com> wrote:
> This patch implements a mechanism where after printing specified number
> of characters (tunable as a kernel parameter printk.offload_chars), CPU
> doing printing asks for help by waking up one of dedicated kthreads.  As
> soon as the printing CPU notices kthread got scheduled and is spinning
> on print_lock dedicated for that purpose, it drops console_sem,
> print_lock, and exits console_unlock(). Kthread then takes over printing
> instead. This way no CPU should spend printing too long even if there
> is heavy printk traffic.


> +/*
> + * Number of kernel threads for offloading printing. We need at least two so
> + * that they can hand over printing from one to another one and thus switch
> + * CPUs.
> + */
> +#define PRINTING_TASKS 2
> +
> +/* Wait queue printing kthreads sleep on when idle */
> +static DECLARE_WAIT_QUEUE_HEAD(print_queue);

Having two tasks, not one, for printking for the case
when console output is slow... sounds wasteful.

Can this be improved so that only one task is needed?