linux-kernel - Re: [PATCH tip/core/rcu 14/15] time: RCU permitted to stop idle entry via softirq

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Date:	Fri, 31 Aug 2012 11:51:41 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 14/15] time: RCU permitted to stop idle
 entry via softirq

On Thu, Aug 30, 2012 at 11:56:27AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> 
> The can_stop_idle_tick() function complains if a softirq vector is
> raised too late in the idle-entry process, presumably in order to
> prevent dangling softirq invocations from being delayed across the
> full idle period, which might be indefinitely long -- and if softirq
> was asserted any later than the call to this function, such a delay
> might well happen.
> 
> However, RCU needs to be able to use softirq to stop idle entry in
> order to be able to drain RCU callbacks from the current CPU, which in
> turn enables faster entry into dyntick-idle mode, which in turn reduces
> power consumption.  Because RCU takes this action at a well-defined
> point in the idle-entry path, it is safe for RCU to take this approach.
> 
> This commit therefore silences the error message that is sometimes
> produced when the going-idle CPU suddenly finds that it has an RCU_SOFTIRQ
> to process.  The error message will continue to be issued for other
> softirq vectors.
> 
> Reported-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>

Reviewed-by: Josh Triplett <josh@...htriplett.org>

> ---
>  include/linux/interrupt.h |    2 ++
>  kernel/time/tick-sched.c  |    3 ++-
>  2 files changed, 4 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> index c5f856a..5e4e617 100644
> --- a/include/linux/interrupt.h
> +++ b/include/linux/interrupt.h
> @@ -430,6 +430,8 @@ enum
>  	NR_SOFTIRQS
>  };
>  
> +#define SOFTIRQ_STOP_IDLE_MASK (~(1 << RCU_SOFTIRQ))
> +
>  /* map softirq index to softirq name. update 'softirq_to_name' in
>   * kernel/softirq.c when adding a new softirq.
>   */
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 024540f..4b1785a 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -436,7 +436,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
>  	if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
>  		static int ratelimit;
>  
> -		if (ratelimit < 10) {
> +		if (ratelimit < 10 &&
> +		    (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
>  			printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
>  			       (unsigned int) local_softirq_pending());
>  			ratelimit++;
> -- 
> 1.7.8
> 
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