linux-kernel - Re: [PATCH 3/4] printk: Defer printing to irq work when we printed too much

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Date:	Thu, 7 Nov 2013 18:37:17 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH 3/4] printk: Defer printing to irq work when we printed
 too much

On Fri, 8 Nov 2013 00:21:51 +0100
Frederic Weisbecker <fweisbec@...il.com> wrote:

> Ok I see now.
> 
> But then this irq_work based solution won't work if, say, you run in full dynticks
> mode. Also the hook on the timer interrupt is something that I wish we get rid
> of on archs that can trigger self-IPIs.

Do we really want that? What about users that use LAZY? That is, the
work isn't that important to trigger right now (added interrupt
expense).

> 
> Notwithstanding it's going to have scalibility issues as irq work then converges
> to a single list for unbound works.

Well, it doesn't seem to be something that would be called often. All
CPUs checking a variable that is seldom changed should not have any
scalability issues. Unless of course it happens to share a cache line
with a variable that does change often.

> 
> Offloading to a workqueue would be perhaps better, and writing to the serial
> console could then be done with interrupts enabled, preemptible context, etc...

Oh God no ;-)  Adding workqueue logic into printk just spells a
nightmare of much more complexity for a critical kernel infrastructure.

-- Steve
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