linux-kernel - Re: [PATCH 01/10] alpha: Add missing RCU idle APIs on idle loop

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Message-ID: <20120822171930.GE2447@linux.vnet.ibm.com>
Date:	Wed, 22 Aug 2012 10:19:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Matt Turner <mattst88@...il.com>,
	alpha <linux-alpha@...r.kernel.org>,
	"3.2.x.." <stable@...nel.org>
Subject: Re: [PATCH 01/10] alpha: Add missing RCU idle APIs on idle loop

On Wed, Aug 22, 2012 at 06:23:39PM +0200, Frederic Weisbecker wrote:
> In the old times, the whole idle task was considered
> as an RCU quiescent state. But as RCU became more and
> more successful overtime, some RCU read side critical
> section have been added even in the code of some
> architectures idle tasks, for tracing for example.
> 
> So nowadays, rcu_idle_enter() and rcu_idle_exit() must
> be called by the architecture to tell RCU about the part
> in the idle loop that doesn't make use of rcu read side
> critical sections, typically the part that puts the CPU
> in low power mode.
> 
> This is necessary for RCU to find the quiescent states in
> idle in order to complete grace periods.
> 
> Add this missing pair of calls in the Alpha's idle loop.
> 
> Reported-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Richard Henderson <rth@...ddle.net>
> Cc: Ivan Kokshaysky <ink@...assic.park.msu.ru>
> Cc: Matt Turner <mattst88@...il.com>
> Cc: alpha <linux-alpha@...r.kernel.org>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: 3.2.x.. <stable@...nel.org>
> ---
>  arch/alpha/kernel/process.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/alpha/kernel/process.c b/arch/alpha/kernel/process.c
> index 153d3fc..2ebf7b5 100644
> --- a/arch/alpha/kernel/process.c
> +++ b/arch/alpha/kernel/process.c
> @@ -28,6 +28,7 @@
>  #include <linux/tty.h>
>  #include <linux/console.h>
>  #include <linux/slab.h>
> +#include <linux/rcupdate.h>
> 
>  #include <asm/reg.h>
>  #include <asm/uaccess.h>
> @@ -50,13 +51,16 @@ cpu_idle(void)
>  {
>  	set_thread_flag(TIF_POLLING_NRFLAG);
> 
> +	preempt_disable();

I don't understand the above preempt_disable() not having a matching
preempt_enable() at exit, but the rest of the patches in this series
look good to me.

							Thanx, Paul

>  	while (1) {
>  		/* FIXME -- EV6 and LCA45 know how to power down
>  		   the CPU.  */
> 
> +		rcu_idle_enter();
>  		while (!need_resched())
>  			cpu_relax();
> -		schedule();
> +		rcu_idle_exit();
> +		schedule_preempt_disabled();
>  	}
>  }
> 
> -- 
> 1.7.5.4
> 

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