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

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Message-ID: <20130226163911.GA3309@linux.vnet.ibm.com>
Date:	Tue, 26 Feb 2013 08:39:11 -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 04:14:43PM +0100, Thomas Gleixner wrote:
> 
> On Tue, 26 Feb 2013, Frederic Weisbecker wrote:
> 
> > 2013/2/26 Thomas Gleixner <tglx@...utronix.de>:
> > > On Fri, 22 Feb 2013, Linus Torvalds wrote:
> > >> On Fri, Feb 22, 2013 at 7:06 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > >> >>
> > >> >> I prefer to let you guys have the final word on this patch. Whether you
> > >> >> apply it or not, I fear I'll never be entirely happy either way :)
> > >> >> That's the sad fate of dealing with circular dependencies...
> > >> >
> > >> > plus the butt ugly softirq semantics or the lack thereof ...
> > >>
> > >> The softirq semantics are perfectly fine. Don't blame softirq for the
> > >> fact that irq_exit() has had shit-for-brains for a long time.
> > >>
> > >> Just move the whole "invoke_softirq()" thing down to *after* the
> > >> tick_nohz_irq_exit() stuff.
> > >
> > > We can't move tick_nohz_irq_exit() before invoke_softirq() simply
> > > because we need to take the timers into account for NOHZ and those can
> > > change when the softirq code runs.
> > >
> > > So no, we need an extra check after invoke_softirq() and the same is
> > > true for RCU.
> > 
> > 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().

Or am I missing some adjustment that is to be made when moving rcu_irq_exit()
to do_softirq()?

							Thanx, Paul

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