Date:   Sun, 17 Mar 2019 19:24:34 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, tglx@...utronix.de,
        Mike Galbraith <bitbucket@...ine.de>
Subject: Re: [PATCH] rcu: Allow to eliminate softirq processing from rcutree

On Fri, Mar 15, 2019 at 12:11:30PM +0100, Sebastian Andrzej Siewior wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> Running RCU out of softirq is a problem for some workloads that would
> like to manage RCU core processing independently of other softirq work,
> for example, setting kthread priority.
> This commit therefore introduces the `rcunosoftirq' option which moves
> the RCU core work from softirq to a per-CPU/per-flavor SCHED_OTHER
> kthread named rcuc.
> The SCHED_OTHER approach avoids the scalability problems that appeared
> with the earlier attempt to move RCU core processing to from softirq to
> kthreads.
> That said, kernels built with RCU_BOOST=y will run the rcuc kthreads at
> the RCU-boosting priority.

Is this against -rt, mainline, or some such?  I was going to start up
a test overnight, but it does not apply against -rcu.

Either way, I will take a closer look at the patch early this week.

							Thanx, Paul

> Reported-by: Thomas Gleixner <tglx@...utronix.de>
> Tested-by: Mike Galbraith <bitbucket@...ine.de>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> [bigeasy: add rcunosoftirq option]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/rcu/tree.c        | 132 ++++++++++++++++++++++++++++++++---
>  kernel/rcu/tree.h        |   4 +-
>  kernel/rcu/tree_plugin.h | 145 +++++----------------------------------
>  3 files changed, 141 insertions(+), 140 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9180158756d2c..498dc5e9287d0 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -62,6 +62,12 @@
>  #include <linux/suspend.h>
>  #include <linux/ftrace.h>
>  #include <linux/tick.h>
> +#include <linux/gfp.h>
> +#include <linux/oom.h>
> +#include <linux/smpboot.h>
> +#include <linux/jiffies.h>
> +#include <linux/sched/isolation.h>
> +#include "../time/tick-internal.h"
> 
>  #include "tree.h"
>  #include "rcu.h"
> @@ -2716,7 +2722,7 @@ EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
>   * structures.  This may be called only from the CPU to whom the rdp
>   * belongs.
>   */
> -static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused)
> +static __latent_entropy void rcu_process_callbacks(void)
>  {
>  	unsigned long flags;
>  	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> @@ -2758,6 +2764,13 @@ static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused
>  	trace_rcu_utilization(TPS("End RCU core"));
>  }
> 
> +static void rcu_process_callbacks_si(struct softirq_action *h)
> +{
> +	rcu_process_callbacks();
> +}
> +
> +static DEFINE_PER_CPU(struct task_struct *, rcu_cpu_kthread_task);
> +
>  /*
>   * Schedule RCU callback invocation.  If the running implementation of RCU
>   * does not support RCU priority boosting, just do a direct call, otherwise
> @@ -2769,19 +2782,121 @@ static void invoke_rcu_callbacks(struct rcu_data *rdp)
>  {
>  	if (unlikely(!READ_ONCE(rcu_scheduler_fully_active)))
>  		return;
> -	if (likely(!rcu_state.boost)) {
> -		rcu_do_batch(rdp);
> -		return;
> -	}
> -	invoke_rcu_callbacks_kthread();
> +	rcu_do_batch(rdp);
>  }
> 
> +static void rcu_wake_cond(struct task_struct *t, int status)
> +{
> +	/*
> +	 * If the thread is yielding, only wake it when this
> +	 * is invoked from idle
> +	 */
> +	if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current)))
> +		wake_up_process(t);
> +}
> +
> +static bool rcu_softirq_enabled = true;
> +
> +static int __init rcunosoftirq_setup(char *str)
> +{
> +	rcu_softirq_enabled = false;
> +	return 0;
> +}
> +__setup("rcunosoftirq", rcunosoftirq_setup);
> +
> +/*
> + * Wake up this CPU's rcuc kthread to do RCU core processing.
> + */
>  static void invoke_rcu_core(void)
>  {
> -	if (cpu_online(smp_processor_id()))
> +	unsigned long flags;
> +	struct task_struct *t;
> +
> +	if (!cpu_online(smp_processor_id()))
> +		return;
> +	if (rcu_softirq_enabled) {
>  		raise_softirq(RCU_SOFTIRQ);
> +	} else {
> +		local_irq_save(flags);
> +		__this_cpu_write(rcu_cpu_has_work, 1);
> +		t = __this_cpu_read(rcu_cpu_kthread_task);
> +		if (t != NULL && current != t)
> +			rcu_wake_cond(t, __this_cpu_read(rcu_cpu_kthread_status));
> +		local_irq_restore(flags);
> +	}
>  }
> 
> +static void rcu_cpu_kthread_park(unsigned int cpu)
> +{
> +	per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
> +}
> +
> +static int rcu_cpu_kthread_should_run(unsigned int cpu)
> +{
> +	return __this_cpu_read(rcu_cpu_has_work);
> +}
> +
> +/*
> + * Per-CPU kernel thread that invokes RCU callbacks.  This replaces
> + * the RCU softirq used in configurations of RCU that do not support RCU
> + * priority boosting.
> + */
> +static void rcu_cpu_kthread(unsigned int cpu)
> +{
> +	unsigned int *statusp = this_cpu_ptr(&rcu_cpu_kthread_status);
> +	char work, *workp = this_cpu_ptr(&rcu_cpu_has_work);
> +	int spincnt;
> +
> +	for (spincnt = 0; spincnt < 10; spincnt++) {
> +		trace_rcu_utilization(TPS("Start CPU kthread@..._wait"));
> +		local_bh_disable();
> +		*statusp = RCU_KTHREAD_RUNNING;
> +		this_cpu_inc(rcu_cpu_kthread_loops);
> +		local_irq_disable();
> +		work = *workp;
> +		*workp = 0;
> +		local_irq_enable();
> +		if (work)
> +			rcu_process_callbacks();
> +		local_bh_enable();
> +		if (*workp == 0) {
> +			trace_rcu_utilization(TPS("End CPU kthread@..._wait"));
> +			*statusp = RCU_KTHREAD_WAITING;
> +			return;
> +		}
> +	}
> +	*statusp = RCU_KTHREAD_YIELDING;
> +	trace_rcu_utilization(TPS("Start CPU kthread@..._yield"));
> +	schedule_timeout_interruptible(2);
> +	trace_rcu_utilization(TPS("End CPU kthread@..._yield"));
> +	*statusp = RCU_KTHREAD_WAITING;
> +}
> +
> +static struct smp_hotplug_thread rcu_cpu_thread_spec = {
> +	.store			= &rcu_cpu_kthread_task,
> +	.thread_should_run	= rcu_cpu_kthread_should_run,
> +	.thread_fn		= rcu_cpu_kthread,
> +	.thread_comm		= "rcuc/%u",
> +	.setup			= rcu_cpu_kthread_setup,
> +	.park			= rcu_cpu_kthread_park,
> +};
> +
> +/*
> + * Spawn per-CPU RCU core processing kthreads.
> + */
> +static int __init rcu_spawn_core_kthreads(void)
> +{
> +	int cpu;
> +
> +	for_each_possible_cpu(cpu)
> +		per_cpu(rcu_cpu_has_work, cpu) = 0;
> +	if (!IS_ENABLED(CONFIG_RCU_BOOST) && !rcu_softirq_enabled)
> +		return 0;
> +	WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__);
> +	return 0;
> +}
> +early_initcall(rcu_spawn_core_kthreads);
> +
>  /*
>   * Handle any core-RCU processing required by a call_rcu() invocation.
>   */
> @@ -3777,7 +3892,8 @@ void __init rcu_init(void)
>  	rcu_init_one();
>  	if (dump_tree)
>  		rcu_dump_rcu_node_tree();
> -	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
> +	if (rcu_softirq_enabled)
> +		open_softirq(RCU_SOFTIRQ, rcu_process_callbacks_si);
> 
>  	/*
>  	 * We don't need protection against CPU-hotplug here because
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index d90b02b53c0ec..fb8fc6ecc391b 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -402,12 +402,10 @@ static const char *tp_rcu_varname __used __tracepoint_string = rcu_name;
> 
>  int rcu_dynticks_snap(struct rcu_data *rdp);
> 
> -#ifdef CONFIG_RCU_BOOST
>  DECLARE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
>  DECLARE_PER_CPU(int, rcu_cpu_kthread_cpu);
>  DECLARE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
>  DECLARE_PER_CPU(char, rcu_cpu_has_work);
> -#endif /* #ifdef CONFIG_RCU_BOOST */
> 
>  /* Forward declarations for rcutree_plugin.h */
>  static void rcu_bootup_announce(void);
> @@ -425,8 +423,8 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func);
>  static void dump_blkd_tasks(struct rcu_node *rnp, int ncheck);
>  static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags);
>  static void rcu_preempt_boost_start_gp(struct rcu_node *rnp);
> -static void invoke_rcu_callbacks_kthread(void);
>  static bool rcu_is_callbacks_kthread(void);
> +static void rcu_cpu_kthread_setup(unsigned int cpu);
>  static void __init rcu_spawn_boost_kthreads(void);
>  static void rcu_prepare_kthreads(int cpu);
>  static void rcu_cleanup_after_idle(void);
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 1b3dd2fc0cd64..b440d6ef45d16 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -24,17 +24,6 @@
>   *	   Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>   */
> 
> -#include <linux/delay.h>
> -#include <linux/gfp.h>
> -#include <linux/oom.h>
> -#include <linux/sched/debug.h>
> -#include <linux/smpboot.h>
> -#include <linux/sched/isolation.h>
> -#include <uapi/linux/sched/types.h>
> -#include "../time/tick-internal.h"
> -
> -#ifdef CONFIG_RCU_BOOST
> -
>  #include "../locking/rtmutex_common.h"
> 
>  /*
> @@ -45,19 +34,6 @@ DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
>  DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
>  DEFINE_PER_CPU(char, rcu_cpu_has_work);
> 
> -#else /* #ifdef CONFIG_RCU_BOOST */
> -
> -/*
> - * Some architectures do not define rt_mutexes, but if !CONFIG_RCU_BOOST,
> - * all uses are in dead code.  Provide a definition to keep the compiler
> - * happy, but add WARN_ON_ONCE() to complain if used in the wrong place.
> - * This probably needs to be excluded from -rt builds.
> - */
> -#define rt_mutex_owner(a) ({ WARN_ON_ONCE(1); NULL; })
> -#define rt_mutex_futex_unlock(x) WARN_ON_ONCE(1)
> -
> -#endif /* #else #ifdef CONFIG_RCU_BOOST */
> -
>  #ifdef CONFIG_RCU_NOCB_CPU
>  static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
>  static bool __read_mostly rcu_nocb_poll;    /* Offload kthread are to poll. */
> @@ -652,7 +628,10 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  		/* Need to defer quiescent state until everything is enabled. */
>  		if (irqs_were_disabled) {
>  			/* Enabling irqs does not reschedule, so... */
> -			raise_softirq_irqoff(RCU_SOFTIRQ);
> +			if (rcu_softirq_enabled)
> +				raise_softirq_irqoff(RCU_SOFTIRQ);
> +			else
> +				invoke_rcu_core();
>  		} else {
>  			/* Enabling BH or preempt does reschedule, so... */
>  			set_tsk_need_resched(current);
> @@ -1150,18 +1129,21 @@ dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
> 
>  #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
> 
> -#ifdef CONFIG_RCU_BOOST
> -
> -static void rcu_wake_cond(struct task_struct *t, int status)
> +/*
> + * If boosting, set rcuc kthreads to realtime priority.
> + */
> +static void rcu_cpu_kthread_setup(unsigned int cpu)
>  {
> -	/*
> -	 * If the thread is yielding, only wake it when this
> -	 * is invoked from idle
> -	 */
> -	if (status != RCU_KTHREAD_YIELDING || is_idle_task(current))
> -		wake_up_process(t);
> +#ifdef CONFIG_RCU_BOOST
> +	struct sched_param sp;
> +
> +	sp.sched_priority = kthread_prio;
> +	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
> +#endif /* #ifdef CONFIG_RCU_BOOST */
>  }
> 
> +#ifdef CONFIG_RCU_BOOST
> +
>  /*
>   * Carry out RCU priority boosting on the task indicated by ->exp_tasks
>   * or ->boost_tasks, advancing the pointer to the next task in the
> @@ -1299,23 +1281,6 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
>  	}
>  }
> 
> -/*
> - * Wake up the per-CPU kthread to invoke RCU callbacks.
> - */
> -static void invoke_rcu_callbacks_kthread(void)
> -{
> -	unsigned long flags;
> -
> -	local_irq_save(flags);
> -	__this_cpu_write(rcu_cpu_has_work, 1);
> -	if (__this_cpu_read(rcu_cpu_kthread_task) != NULL &&
> -	    current != __this_cpu_read(rcu_cpu_kthread_task)) {
> -		rcu_wake_cond(__this_cpu_read(rcu_cpu_kthread_task),
> -			      __this_cpu_read(rcu_cpu_kthread_status));
> -	}
> -	local_irq_restore(flags);
> -}
> -
>  /*
>   * Is the current CPU running the RCU-callbacks kthread?
>   * Caller must have preemption disabled.
> @@ -1369,65 +1334,6 @@ static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
>  	return 0;
>  }
> 
> -static void rcu_kthread_do_work(void)
> -{
> -	rcu_do_batch(this_cpu_ptr(&rcu_data));
> -}
> -
> -static void rcu_cpu_kthread_setup(unsigned int cpu)
> -{
> -	struct sched_param sp;
> -
> -	sp.sched_priority = kthread_prio;
> -	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
> -}
> -
> -static void rcu_cpu_kthread_park(unsigned int cpu)
> -{
> -	per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
> -}
> -
> -static int rcu_cpu_kthread_should_run(unsigned int cpu)
> -{
> -	return __this_cpu_read(rcu_cpu_has_work);
> -}
> -
> -/*
> - * Per-CPU kernel thread that invokes RCU callbacks.  This replaces
> - * the RCU softirq used in configurations of RCU that do not support RCU
> - * priority boosting.
> - */
> -static void rcu_cpu_kthread(unsigned int cpu)
> -{
> -	unsigned int *statusp = this_cpu_ptr(&rcu_cpu_kthread_status);
> -	char work, *workp = this_cpu_ptr(&rcu_cpu_has_work);
> -	int spincnt;
> -
> -	for (spincnt = 0; spincnt < 10; spincnt++) {
> -		trace_rcu_utilization(TPS("Start CPU kthread@..._wait"));
> -		local_bh_disable();
> -		*statusp = RCU_KTHREAD_RUNNING;
> -		this_cpu_inc(rcu_cpu_kthread_loops);
> -		local_irq_disable();
> -		work = *workp;
> -		*workp = 0;
> -		local_irq_enable();
> -		if (work)
> -			rcu_kthread_do_work();
> -		local_bh_enable();
> -		if (*workp == 0) {
> -			trace_rcu_utilization(TPS("End CPU kthread@..._wait"));
> -			*statusp = RCU_KTHREAD_WAITING;
> -			return;
> -		}
> -	}
> -	*statusp = RCU_KTHREAD_YIELDING;
> -	trace_rcu_utilization(TPS("Start CPU kthread@..._yield"));
> -	schedule_timeout_interruptible(2);
> -	trace_rcu_utilization(TPS("End CPU kthread@..._yield"));
> -	*statusp = RCU_KTHREAD_WAITING;
> -}
> -
>  /*
>   * Set the per-rcu_node kthread's affinity to cover all CPUs that are
>   * served by the rcu_node in question.  The CPU hotplug lock is still
> @@ -1458,27 +1364,13 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
>  	free_cpumask_var(cm);
>  }
> 
> -static struct smp_hotplug_thread rcu_cpu_thread_spec = {
> -	.store			= &rcu_cpu_kthread_task,
> -	.thread_should_run	= rcu_cpu_kthread_should_run,
> -	.thread_fn		= rcu_cpu_kthread,
> -	.thread_comm		= "rcuc/%u",
> -	.setup			= rcu_cpu_kthread_setup,
> -	.park			= rcu_cpu_kthread_park,
> -};
> -
>  /*
>   * Spawn boost kthreads -- called as soon as the scheduler is running.
>   */
>  static void __init rcu_spawn_boost_kthreads(void)
>  {
>  	struct rcu_node *rnp;
> -	int cpu;
> 
> -	for_each_possible_cpu(cpu)
> -		per_cpu(rcu_cpu_has_work, cpu) = 0;
> -	if (WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__))
> -		return;
>  	rcu_for_each_leaf_node(rnp)
>  		(void)rcu_spawn_one_boost_kthread(rnp);
>  }
> @@ -1501,11 +1393,6 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
>  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  }
> 
> -static void invoke_rcu_callbacks_kthread(void)
> -{
> -	WARN_ON_ONCE(1);
> -}
> -
>  static bool rcu_is_callbacks_kthread(void)
>  {
>  	return false;
> -- 
> 2.20.1
> 

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