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SubjectRe: [PATCH] rcu: Allow to eliminate softirq processing from rcutree
On Tue, Mar 19, 2019 at 08:26:13PM -0400, Joel Fernandes wrote:
> Adding the rcu@vger.kernel.org list as well, more comment below:
>
> On Fri, Mar 15, 2019 at 12:11:30PM +0100, Sebastian Andrzej Siewior wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> >
> > 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.
> >
> > Reported-by: Thomas Gleixner <tglx@linutronix.de>
> > Tested-by: Mike Galbraith <bitbucket@online.de>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > [bigeasy: add rcunosoftirq option]
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> > 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);
>
> Looks like a nice change, but one question...
>
> Consider the case where rcunosoftirq boot option is not passed.
>
> Before, if RCU_BOOST=y, then callbacks would be invoked in rcuc threads if
> possible, by those threads being woken up from within the softirq context
> (in invoke_rcu_callbacks).
>
> Now, if RCU_BOOST=y, then callbacks would only be invoked in softirq context
> and not in the threads at all. Because rcu_softirq_enabled = false, so the
> path executes:
> rcu_read_unlock_special() ->
> raise_softirq_irqsoff() ->
> rcu_process_callbacks_si() ->
> rcu_process_callbacks() ->
> invoke_rcu_callbacks() ->
> rcu_do_batch()
>
> This seems like a behavioral change to me. This makes the callbacks always
> execute from the softirq context and not the threads when boosting is
> configured. IMO in the very least, such behavioral change should be
> documented in the change.
>
> One way to fix this I think could be, if boosting is enabled, then set
> rcu_softirq_enabled to false by default so the callbacks are still executed
> in the rcuc threads.
>
> Did I miss something? Sorry if I did, thanks!

This did pass light rcutorture testing without rcunosoftirq set, but it
is quite possible that it was just getting lucky. Then again, there are
a few different versions of this patch in flight, as Sebastian notes in
his later email. The one I used is on -rcu.

Thanx, Paul

> - Joel
>
>
> > }
> >
> > +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@linux.vnet.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"
> >
> > /*
> > @@ -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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