Re: [PATCH 2/3] nohz: Ensure full dynticks CPUs are RCU nocbs

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Apr 17, 2013 at 06:24:48PM +0200, Frederic Weisbecker wrote:
> We need full dynticks CPU to also be RCU nocb so
> that we don't have to keep the tick to handle RCU
> callbacks.
> 
> Make sure the range passed to nohz_full= boot
> parameter is a subset of rcu_nocbs=
> 
> The CPUs that fail to meet this requirement will be
> excluded from the nohz_full range. This is checked
> early in boot time, before any CPU has the opportunity
> to stop its tick.
> 
> Suggested-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Chris Metcalf <cmetcalf@tilera.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Geoff Levand <geoff@infradead.org>
> Cc: Gilad Ben Yossef <gilad@benyossef.com>
> Cc: Hakan Akkan <hakanakkan@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Kevin Hilman <khilman@linaro.org>
> Cc: Li Zhong <zhong@linux.vnet.ibm.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

(And again, also updated documentation.)

> ---
>  Documentation/kernel-parameters.txt |    2 ++
>  include/linux/rcupdate.h            |    7 +++++++
>  include/linux/tick.h                |    2 ++
>  init/main.c                         |    1 +
>  kernel/rcutree.c                    |    6 +++---
>  kernel/rcutree.h                    |    1 -
>  kernel/rcutree_plugin.h             |   13 ++++---------
>  kernel/time/tick-sched.c            |   22 ++++++++++++++++------
>  8 files changed, 35 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 887b297..4865e9b 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -1918,6 +1918,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  			the specified list of CPUs whose tick will be stopped
>  			whenever possible. The boot CPU will be forced outside
>  			the range to maintain the timekeeping.
> +			The CPUs in this range must also be included in the
> +			rcu_nocbs= set.
> 
>  	noiotrap	[SH] Disables trapped I/O port accesses.
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index b758ce1..8e0948c 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -999,4 +999,11 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
>  #define kfree_rcu(ptr, rcu_head)					\
>  	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
> 
> +#ifdef CONFIG_RCU_NOCB_CPU
> +extern bool rcu_is_nocb_cpu(int cpu);
> +#else
> +static inline bool rcu_is_nocb_cpu(int cpu) { return false; }
> +#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
> +
> +
>  #endif /* __LINUX_RCUPDATE_H */
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index b4e3b0c..0b6873c 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -158,8 +158,10 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
>  # endif /* !CONFIG_NO_HZ_COMMON */
> 
>  #ifdef CONFIG_NO_HZ_FULL
> +extern void tick_nohz_init(void);
>  extern int tick_nohz_full_cpu(int cpu);
>  #else
> +static inline void tick_nohz_init(void) { }
>  static inline int tick_nohz_full_cpu(int cpu) { return 0; }
>  #endif
> 
> diff --git a/init/main.c b/init/main.c
> index 63534a1..2acb5bb 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -547,6 +547,7 @@ asmlinkage void __init start_kernel(void)
>  	idr_init_cache();
>  	perf_event_init();
>  	rcu_init();
> +	tick_nohz_init();
>  	radix_tree_init();
>  	/* init some links before init_ISA_irqs() */
>  	early_irq_init();
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index f5ab502..1d4ceff 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1695,7 +1695,7 @@ rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
>  			  struct rcu_node *rnp, struct rcu_data *rdp)
>  {
>  	/* No-CBs CPUs do not have orphanable callbacks. */
> -	if (is_nocb_cpu(rdp->cpu))
> +	if (rcu_is_nocb_cpu(rdp->cpu))
>  		return;
> 
>  	/*
> @@ -2757,10 +2757,10 @@ static void _rcu_barrier(struct rcu_state *rsp)
>  	 * corresponding CPU's preceding callbacks have been invoked.
>  	 */
>  	for_each_possible_cpu(cpu) {
> -		if (!cpu_online(cpu) && !is_nocb_cpu(cpu))
> +		if (!cpu_online(cpu) && !rcu_is_nocb_cpu(cpu))
>  			continue;
>  		rdp = per_cpu_ptr(rsp->rda, cpu);
> -		if (is_nocb_cpu(cpu)) {
> +		if (rcu_is_nocb_cpu(cpu)) {
>  			_rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
>  					   rsp->n_barrier_done);
>  			atomic_inc(&rsp->barrier_cpu_count);
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index f993c0a..38acc49 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -529,7 +529,6 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
>  static void print_cpu_stall_info_end(void);
>  static void zero_cpu_stall_ticks(struct rcu_data *rdp);
>  static void increment_cpu_stall_ticks(void);
> -static bool is_nocb_cpu(int cpu);
>  static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
>  			    bool lazy);
>  static bool rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index a5745e9..0cd91cc 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -2167,7 +2167,7 @@ static int __init parse_rcu_nocb_poll(char *arg)
>  early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
> 
>  /* Is the specified CPU a no-CPUs CPU? */
> -static bool is_nocb_cpu(int cpu)
> +bool rcu_is_nocb_cpu(int cpu)
>  {
>  	if (have_rcu_nocb_mask)
>  		return cpumask_test_cpu(cpu, rcu_nocb_mask);
> @@ -2225,7 +2225,7 @@ static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
>  			    bool lazy)
>  {
> 
> -	if (!is_nocb_cpu(rdp->cpu))
> +	if (!rcu_is_nocb_cpu(rdp->cpu))
>  		return 0;
>  	__call_rcu_nocb_enqueue(rdp, rhp, &rhp->next, 1, lazy);
>  	return 1;
> @@ -2242,7 +2242,7 @@ static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
>  	long qll = rsp->qlen_lazy;
> 
>  	/* If this is not a no-CBs CPU, tell the caller to do it the old way. */
> -	if (!is_nocb_cpu(smp_processor_id()))
> +	if (!rcu_is_nocb_cpu(smp_processor_id()))
>  		return 0;
>  	rsp->qlen = 0;
>  	rsp->qlen_lazy = 0;
> @@ -2282,7 +2282,7 @@ static bool nocb_cpu_expendable(int cpu)
>  	 * If there are no no-CB CPUs or if this CPU is not a no-CB CPU,
>  	 * then offlining this CPU is harmless.  Let it happen.
>  	 */
> -	if (!have_rcu_nocb_mask || is_nocb_cpu(cpu))
> +	if (!have_rcu_nocb_mask || rcu_is_nocb_cpu(cpu))
>  		return 1;
> 
>  	/* If no memory, play it safe and keep the CPU around. */
> @@ -2464,11 +2464,6 @@ static void __init rcu_init_nocb(void)
> 
>  #else /* #ifdef CONFIG_RCU_NOCB_CPU */
> 
> -static bool is_nocb_cpu(int cpu)
> -{
> -	return false;
> -}
> -
>  static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
>  			    bool lazy)
>  {
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 2bac5ea..d71a5f2 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -203,17 +203,27 @@ static int __cpuinit tick_nohz_cpu_down_callback(struct notifier_block *nfb,
>   */
>  static char __initdata nohz_full_buf[NR_CPUS + 1];
> 
> -static int __init init_tick_nohz_full(void)
> +void __init tick_nohz_init(void)
>  {
> -	if (have_nohz_full_mask)
> -		cpu_notifier(tick_nohz_cpu_down_callback, 0);
> +	int cpu;
> +
> +	if (!have_nohz_full_mask)
> +		return;
> +
> +	cpu_notifier(tick_nohz_cpu_down_callback, 0);
> +
> +	/* Make sure full dynticks CPU are also RCU nocbs */
> +	for_each_cpu(cpu, nohz_full_mask) {
> +		if (!rcu_is_nocb_cpu(cpu)) {
> +			pr_warning("NO_HZ: CPU %d is not RCU nocb: "
> +				   "cleared from nohz_full range", cpu);
> +			cpumask_clear_cpu(cpu, nohz_full_mask);
> +		}
> +	}
> 
>  	cpulist_scnprintf(nohz_full_buf, sizeof(nohz_full_buf), nohz_full_mask);
>  	pr_info("NO_HZ: Full dynticks CPUs: %s.\n", nohz_full_buf);
> -
> -	return 0;
>  }
> -core_initcall(init_tick_nohz_full);
>  #else
>  #define have_nohz_full_mask (0)
>  #endif
> -- 
> 1.7.5.4
>