linux-kernel - Re: [PATCH RT v2] timer: Raise softirq if there's irq

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Date:	Fri, 24 Jan 2014 21:20:39 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	John Kacur <jkacur@...hat.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Joakim Hernberg <jbh@...hemy.lu>,
	Joe Korty <joe.korty@...r.com>,
	Muli Baron <muli.baron@...il.com>
Subject: Re: [PATCH RT v2] timer: Raise softirq if there's irq_work

* Steven Rostedt | 2014-01-24 15:09:33 [-0500]:

>[ Talking with Sebastian on IRC, it seems that doing the irq_work_run()
>  from the interrupt in -rt is a bad thing. Here we simply raise the
>  softirq if there's irq work to do. This too boots on my i7 ]

It is okay in general because most of the users should not run in bare
interrupt context. The only exception here is the nohz_full_kick_work
thing.

>After trying hard to figure out why my i7 box was locking up with the
>new active_timers code, that does not run the timer softirq if there
>are no active timers, I took an extra look at the softirq handler and
>noticed that it doesn't just run timer softirqs, it also runs irq work.
>
>This was the bug that was locking up the system. It wasn't missing a
>timer, it was missing irq work. By always doing the irq work callbacks,
>the system boots fine.
>
>No need to check for defined(CONFIG_IRQ_WORK). When that's not set the
>"irq_work_needs_cpu()" is a static inline that returns false.
>
>Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Thank you Steven, this makes sense.

Sebastian
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