Messages in this thread | | | Date | Fri, 22 Mar 2019 17:25:19 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree |
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On Fri, Mar 22, 2019 at 07:48:19PM -0400, Joel Fernandes wrote: > 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.
I believe that you are correct, so I will give it a shot. Good eyes!
> > } > > > > +/* > > + * 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 <paulmck@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?
It is -so- close! But it invokes raise_softirq() instead of the needed raise_softirq_irqoff().
Plus I bet that this has a few more changes to go before it is all the way there. ;-)
Thanx, Paul
> 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 > > >
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