linux-kernel - Re: [PATCH 2/2] irq: Cleanup context state transitions in irq

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Message-ID: <20130226191758.GA28847@linux.vnet.ibm.com>
Date:	Tue, 26 Feb 2013 11:17:58 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] irq: Cleanup context state transitions in irq_exit()

On Tue, Feb 26, 2013 at 07:55:06PM +0100, Thomas Gleixner wrote:
> On Tue, 26 Feb 2013, Paul E. McKenney wrote:
> > On Tue, Feb 26, 2013 at 04:14:43PM +0100, Thomas Gleixner wrote:
> > > 
> > > On Tue, 26 Feb 2013, Frederic Weisbecker wrote:
> > > > And what do you think about Linus's idea to move tick_nohz_irq_exit()
> > > > to do_softirq()?
> > > > This sounds feasible and a right place to do this, I hope that won't
> > > > uglify do_softirq() though.
> > > > I can try something.
> > > 
> > > Yeah, looks doable. the rcu stuff needs to go there as well, right?
> > 
> > If it does, it needs to do so in such a way that rcu_irq_enter() and
> > rcu_irq_exit() nest properly.  One area of concern is the force_irqthreads
> > case, skips calling do_softirq().  Another area of concern is the
> > __ARCH_IRQ_EXIT_IRQS_DISABLED case, which calls __do_softirq() rather
> > than do_softirq().
> 
> That's sorted already. We disable interrupts in irq_exit().
> 
> > Or am I missing some adjustment that is to be made when moving rcu_irq_exit()
> > to do_softirq()?
> 
> Yeah, we need an extra parameter or such, so the other callers of
> __do_softirq() don't mess with it.
> 
> I also noticed that rcu_irq_enter/exit needs to be symetric and the
> NOHZ code will get confused as well if we call it asymetric.

Exactly!

							Thanx, Paul

> So that will become even more ugly than the extra check at the end of
> irq_exit().
> 
> Thanks,
> 
> 	tglx
> 
> 
> 

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