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    SubjectRe: [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


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