linux-kernel - Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation

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Date:	Tue, 2 Sep 2008 08:18:06 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	cl@...ux-foundation.org, mingo@...e.hu, akpm@...ux-foundation.org,
	manfred@...orfullife.com, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, schamp@....com, niv@...ibm.com,
	dvhltc@...ibm.com, ego@...ibm.com, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org
Subject: Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation

> > It would be better if you hung rcu_irq_enter in the irq_enter() if 
> > statement that checks if the task was idle or not. This way it would
> > be zero overhead for interruptions of non busy CPUs, keeping 

Sorry that should have been "non idle CPUs" of course.

> > it out of many fast paths.
> > 
> > Haven't read everything, sorry.
> 
> So that I lose the #else above, and so that irq_enter() and irq_exit()
> look something like the following (with additional adjustments to suit)?

Yes looks good.

BTW I wonder if the compiler CSEs the idle_cpu() check properly.

> void irq_enter(void)
> {
> #ifdef CONFIG_NO_HZ
> 	int cpu = smp_processor_id();
> 	if (idle_cpu(cpu) && !in_interrupt())
> 		tick_nohz_stop_idle(cpu);
> #endif
> 	__irq_enter();
> #ifdef CONFIG_NO_HZ
> 	if (idle_cpu(cpu)) {
> 		rcu_irq_enter();
> 		tick_nohz_update_jiffies();
> 	}
> #endif


-Andi
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