linux-kernel - Re: [PATCH 14/32] nohz/cpuset: Don't turn off the tick if rcu needs it

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Date:	Tue, 16 Aug 2011 13:13:42 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [PATCH 14/32] nohz/cpuset: Don't turn off the tick if rcu
 needs it

On Mon, Aug 15, 2011 at 05:52:11PM +0200, Frederic Weisbecker wrote:
> If RCU is waiting for the current CPU to complete a grace
> period, don't turn off the tick. Unlike dynctik-idle, we

s/dynctik/dyntick/  ;-)

> are not necessarily going to enter into rcu extended quiescent
> state, so we may need to keep the tick to note current CPU's
> quiescent states.

One question below...

							Thanx, Paul

> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Anton Blanchard <anton@....ibm.com>
> Cc: Avi Kivity <avi@...hat.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Lai Jiangshan <laijs@...fujitsu.com>
> Cc: Paul E . McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Paul Menage <menage@...gle.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Stephen Hemminger <shemminger@...tta.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Tim Pepper <lnxninja@...ux.vnet.ibm.com>
> ---
>  include/linux/rcupdate.h |    1 +
>  kernel/rcutree.c         |    3 +--
>  kernel/sched.c           |   14 ++++++++++++++
>  3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 99f9aa7..55a482a 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -133,6 +133,7 @@ static inline int rcu_preempt_depth(void)
>  extern void rcu_sched_qs(int cpu);
>  extern void rcu_bh_qs(int cpu);
>  extern void rcu_check_callbacks(int cpu, int user);
> +extern int rcu_pending(int cpu);
>  struct notifier_block;
> 
>  #ifdef CONFIG_NO_HZ
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index ba06207..0009bfc 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -205,7 +205,6 @@ int rcu_cpu_stall_suppress __read_mostly;
>  module_param(rcu_cpu_stall_suppress, int, 0644);
> 
>  static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
> -static int rcu_pending(int cpu);
> 
>  /*
>   * Return the number of RCU-sched batches processed thus far for debug & stats.
> @@ -1729,7 +1728,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
>   * by the current CPU, returning 1 if so.  This function is part of the
>   * RCU implementation; it is -not- an exported member of the RCU API.
>   */
> -static int rcu_pending(int cpu)
> +int rcu_pending(int cpu)
>  {
>  	return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) ||
>  	       __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) ||
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 0e1aa4e..353a66f 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2439,6 +2439,7 @@ DEFINE_PER_CPU(int, task_nohz_mode);
>  bool cpuset_nohz_can_stop_tick(void)
>  {
>  	struct rq *rq;
> +	int cpu;
> 
>  	rq = this_rq();
> 
> @@ -2446,6 +2447,19 @@ bool cpuset_nohz_can_stop_tick(void)
>  	if (rq->nr_running > 1)
>  		return false;
> 
> +	cpu = smp_processor_id();
> +
> +	/*
> +	 * FIXME: will probably be removed soon as it's
> +	 * already checked from tick_nohz_stop_sched_tick()
> +	 */
> +	if (rcu_needs_cpu(cpu))
> +		return false;
> +
> +	/* Is there a grace period to complete ? */
> +	if (rcu_pending(cpu))

This is from a quiescent state for both RCU and RCU-bh, right?
Or can their be RCU or RCU-bh read-side critical sections held
across here?  (It would be mildly bad if so.)

But force_quiescent_state() will catch cases where RCU needs
quiescent states from CPUs, so is this check really needed?

> +		return false;
> +
>  	return true;
>  }
> 
> -- 
> 1.7.5.4
> 
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