Messages in this thread | | | Date | Fri, 22 Mar 2019 21:04:21 -0400 | From | Joel Fernandes <> | Subject | Re: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree |
| |
On Fri, Mar 22, 2019 at 05:25:19PM -0700, Paul E. McKenney wrote: > 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!
Thanks! Also I am sending some the lockdep dyntick checking patches shortly :)
> > > } > > > > > > +/* > > > + * 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. ;-)
Ah yes, you are right :-)
thanks,
- Joel
| |