Re: [PATCH] rcu: Allow to eliminate softirq processing from rcutree

From: Paul E. McKenney
Date: Mon Mar 18 2019 - 10:09:57 EST

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On Fri, Mar 15, 2019 at 12:11:30PM +0100, Sebastian Andrzej Siewior wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> Tested-by: Mike Galbraith <bitbucket@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> [bigeasy: add rcunosoftirq option]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> 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@rcu_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@rcu_wait"));
> + *statusp = RCU_KTHREAD_WAITING;
> + return;
> + }
> + }
> + *statusp = RCU_KTHREAD_YIELDING;
> + trace_rcu_utilization(TPS("Start CPU kthread@rcu_yield"));
> + schedule_timeout_interruptible(2);
> + trace_rcu_utilization(TPS("End CPU kthread@rcu_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@xxxxxxxxxxxxxxxxxx>
> */
>
> -#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@rcu_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@rcu_wait"));
> - *statusp = RCU_KTHREAD_WAITING;
> - return;
> - }
> - }
> - *statusp = RCU_KTHREAD_YIELDING;
> - trace_rcu_utilization(TPS("Start CPU kthread@rcu_yield"));
> - schedule_timeout_interruptible(2);
> - trace_rcu_utilization(TPS("End CPU kthread@rcu_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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