Date:   Tue, 19 Mar 2019 20:26:13 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
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>, tglx@...utronix.de,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Mike Galbraith <bitbucket@...ine.de>, rcu@...r.kernel.org
Subject: Re: [PATCH] rcu: Allow to eliminate softirq processing from rcutree

Adding the rcu@...r.kernel.org list as well, more comment below:

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.
> 
> 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);

Looks like a nice change, but one question...

Consider the case where rcunosoftirq boot option is not passed.

Before, if RCU_BOOST=y, then callbacks would be invoked in rcuc threads if
possible, by those threads being woken up from within the softirq context
(in invoke_rcu_callbacks).

Now, if RCU_BOOST=y, then callbacks would only be invoked in softirq context
and not in the threads at all. Because rcu_softirq_enabled = false, so the
path executes:
  rcu_read_unlock_special() ->
        raise_softirq_irqsoff() ->
                rcu_process_callbacks_si() ->
                        rcu_process_callbacks() ->
                                invoke_rcu_callbacks() ->
                                        rcu_do_batch()

This seems like a behavioral change to me. This makes the callbacks always
execute from the softirq context and not the threads when boosting is
configured. IMO in the very least, such behavioral change should be
documented in the change.

One way to fix this I think could be, if boosting is enabled, then set
rcu_softirq_enabled to false by default so the callbacks are still executed
in the rcuc threads.

Did I miss something? Sorry if I did, thanks!

 - Joel


>  }
>  
> +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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