On Wed, Mar 20, 2019 at 10:13:33PM +0100, Sebastian Andrzej Siewior wrote:
> 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.
[snip]
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0f31b79eb6761..05a1e42fdaf10 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -51,6 +51,12 @@
> #include <linux/tick.h>
> #include <linux/sysrq.h>
> #include <linux/kprobes.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"
> @@ -92,6 +98,9 @@ struct rcu_state rcu_state = {
> /* Dump rcu_node combining tree at boot to verify correct setup. */
> static bool dump_tree;
> module_param(dump_tree, bool, 0444);
> +/* Move RCU_SOFTIRQ to rcuc kthreads. */
> +static bool use_softirq = 1;
> +module_param(use_softirq, bool, 0444);
> /* Control rcu_node-tree auto-balancing at boot time. */
> static bool rcu_fanout_exact;
> module_param(rcu_fanout_exact, bool, 0444);
> @@ -2253,7 +2262,7 @@ void rcu_force_quiescent_state(void)
> EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
>
> /* Perform RCU core processing work for the current CPU. */
> -static __latent_entropy void rcu_core(struct softirq_action *unused)
> +static __latent_entropy void rcu_core(void)
> {
> unsigned long flags;
> struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> @@ -2295,6 +2304,34 @@ static __latent_entropy void rcu_core(struct
> softirq_action *unused)
> trace_rcu_utilization(TPS("End RCU core"));
> }
>
> +static void rcu_core_si(struct softirq_action *h)
> +{
> + rcu_core();
> +}
> +
> +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 void invoke_rcu_core_kthread(void)
> +{
> + struct task_struct *t;
> + unsigned long flags;
> +
> + local_irq_save(flags);
> + __this_cpu_write(rcu_data.rcu_cpu_has_work, 1);
> + t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task);
> + if (t != NULL && t != current)
> + rcu_wake_cond(t,
> __this_cpu_read(rcu_data.rcu_cpu_kthread_status));
> + local_irq_restore(flags);
> +}
> +
> /*
> * Schedule RCU callback invocation. If the running implementation of RCU
> * does not support RCU priority boosting, just do a direct call, otherwise
> @@ -2306,19 +2343,95 @@ 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();
> + if (rcu_state.boost || !use_softirq)
> + invoke_rcu_core_kthread();
> + rcu_do_batch(rdp);
Shouldn't there be an else before the rcu_do_batch? If we are waking up the
rcuc thread, then that will do the rcu_do_batch when it runs right?
Something like:
if (rcu_state.boost || !use_softirq)
invoke_rcu_core_kthread();
else
rcu_do_batch(rdp);
Previous code similarly had a return; also.
> }
>
> +/*
> + * Wake up this CPU's rcuc kthread to do RCU core processing.
> + */
> static void invoke_rcu_core(void)
> {
> - if (cpu_online(smp_processor_id()))
> + if (!cpu_online(smp_processor_id()))
> + return;
> + if (use_softirq)
> raise_softirq(RCU_SOFTIRQ);
> + else
> + invoke_rcu_core_kthread();
> }
>
> +static void rcu_cpu_kthread_park(unsigned int cpu)
> +{
> + per_cpu(rcu_data.rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
> +}
> +
> +static int rcu_cpu_kthread_should_run(unsigned int cpu)
> +{
> + return __this_cpu_read(rcu_data.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_data.rcu_cpu_kthread_status);
> + char work, *workp = this_cpu_ptr(&rcu_data.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;
> + local_irq_disable();
> + work = *workp;
> + *workp = 0;
> + local_irq_enable();
> + if (work)
> + rcu_core();
> + 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;
> +}
> +
[snip]
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index e253d11af3c49..a1a72a1ecb026 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -407,8 +407,8 @@ 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 f46b4af96ab95..b807204ffd83f 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -11,29 +11,7 @@
> * Paul E. McKenney <paul...@linux.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"
> -#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. */
> @@ -94,6 +72,8 @@ static void __init rcu_bootup_announce_oddness(void)
> pr_info("\tRCU debug GP init slowdown %d jiffies.\n",
> gp_init_delay);
> if (gp_cleanup_delay)
> pr_info("\tRCU debug GP init slowdown %d jiffies.\n",
> gp_cleanup_delay);
> + if (!use_softirq)
> + pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
> if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
> pr_info("\tRCU debug extended QS entry/exit.\n");
> rcupdate_announce_bootup_oddness();
> @@ -629,7 +609,10 @@ 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 (!use_softirq)
> + raise_softirq_irqoff(RCU_SOFTIRQ);
I believe this exclamation has been corrected in Paul's tree so that's Ok.
> + else
> + invoke_rcu_core();
But why not just directly call invoke_rcu_core() here? That will do the
appropriate use_softirq check right?
thanks,
- Joel
> } else {
> /* Enabling BH or preempt does reschedule, so... */
> set_tsk_need_resched(current);
> @@ -944,18 +927,21 @@ 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
> @@ -1093,23 +1079,6 @@ 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_data.rcu_cpu_has_work, 1);
> - if (__this_cpu_read(rcu_data.rcu_cpu_kthread_task) != NULL &&
> - current != __this_cpu_read(rcu_data.rcu_cpu_kthread_task)) {
> - rcu_wake_cond(__this_cpu_read(rcu_data.rcu_cpu_kthread_task),
> - __this_cpu_read(rcu_data.rcu_cpu_kthread_status));
> - }
> - local_irq_restore(flags);
> -}
> -
> /*
> * Is the current CPU running the RCU-callbacks kthread?
> * Caller must have preemption disabled.
> @@ -1163,59 +1132,6 @@ static int rcu_spawn_one_boost_kthread(struct rcu_node
> *rnp)
> return 0;
> }
>
> -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_data.rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
> -}
> -
> -static int rcu_cpu_kthread_should_run(unsigned int cpu)
> -{
> - return __this_cpu_read(rcu_data.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_data.rcu_cpu_kthread_status);
> - char work, *workp = this_cpu_ptr(&rcu_data.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;
> - local_irq_disable();
> - work = *workp;
> - *workp = 0;
> - local_irq_enable();
> - if (work)
> - rcu_do_batch(this_cpu_ptr(&rcu_data));
> - 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
> @@ -1246,27 +1162,13 @@ 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_data.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_data.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);
> }
> @@ -1289,11 +1191,6 @@ 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
>
