linux-kernel - Re: [PATCH, RFC] v4 scalable classic RCU implementation

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Message-ID: <20080907172536.GA6941@linux.vnet.ibm.com>
Date:	Sun, 7 Sep 2008 10:25:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	mingo@...e.hu, akpm@...ux-foundation.org, 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, penberg@...helsinki.fi,
	andi@...stfloor.org
Subject: Re: [PATCH, RFC] v4 scalable classic RCU implementation

On Sat, Sep 06, 2008 at 06:37:38PM +0200, Manfred Spraul wrote:
> Hi Paul,
>
> Paul E. McKenney wrote:
>> o	The rcu_pending() and rcu_needs_cpu() primitives are now
>> 	much more aggressive about permitting CPUs to enter dynticks
>> 	idle mode.  Only CPUs that have RCU callbacks are kept out
>> 	of dynticks idle mode.
>>   
>
> I've noticed that right now rcu_enter_nohz() can be nested within 
> rcu_irq_enter():
> irq_exit() first calls tick_nohz_stop_sched_tick(), then rcu_irq_exit().
> And tick_nohz_stop_sched_tick() can switch into nohz mode.
>
> Is that intentional? Does rcupreempt support that? It broke my rcustate 
> code on x86-64.
>
> I would prefer if something like the attached patch is applied. What do you 
> think?
> Do you need the patch as well?

Good question -- when I tried splitting irqs from NMIs, things broke
badly, and this might well explain it.  Thank you very much for the
hint!!!

							Thanx, Paul

> --
>    Manfred

> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index ba20a90..cca5a83 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -284,10 +284,10 @@ void irq_exit(void)
>  		invoke_softirq();
> 
>  #ifdef CONFIG_NO_HZ
> +	rcu_irq_exit(0);
>  	/* Make sure that timer wheel updates are propagated */
>  	if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
>  		tick_nohz_stop_sched_tick(0);
> -	rcu_irq_exit(0);
>  #endif
>  	preempt_enable_no_resched();
>  }

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