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 <t...@linutronix.de>
Tested-by: Mike Galbraith <efa...@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
---
v2…v3: - Ensure that with RCU_BOOST=y the callback is invoked in thread
context. Pointed out by Joel Fernandes.
- Swap the init logic so it initializes the rcuc thread with
RCU_BOOST=n
- Move the parameter switch to rcutree.use_softirq. Default is 1
which means SOFTIRQ processing (default, old behaviour). By
setting to 0 the softirq processing is disabled and rcuc
thread do the work.
- Add a few words to kernel-parameters.txt, added by Paul E.
McKenney.
v1…v2:
- rebased to Paul's rcu/dev tree/branch
.../admin-guide/kernel-parameters.txt | 5 +
kernel/rcu/tree.c | 130 +++++++++++++++-
kernel/rcu/tree.h | 2 +-
kernel/rcu/tree_plugin.h | 139 +++---------------
4 files changed, 146 insertions(+), 130 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt
b/Documentation/admin-guide/kernel-parameters.txt
index d377a2166b79c..7a44624c13219 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3672,6 +3672,11 @@
the propagation of recent CPU-hotplug changes up
the rcu_node combining tree.
+ rcutree.use_softirq= [KNL]
+ If set to zero, move all RCU_SOFTIRQ processing to
+ per-CPU rcuc kthreads. Defaults to a non-zero
+ value using RCU_SOFTIRQ.
+
rcutree.rcu_fanout_exact= [KNL]
Disable autobalancing of the rcu_node combining
tree. This is used by rcutorture, and might
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);
}
+/*
+ * 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;
+}
+
+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 per-CPU RCU core processing kthreads.
+ */
+static int __init rcu_spawn_core_kthreads(void)
+{
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
+ if (!IS_ENABLED(CONFIG_RCU_BOOST) && use_softirq)
+ return 0;
+ WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec),
+ "%s: Could not start rcuc 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.
*/
@@ -3355,7 +3468,8 @@ void __init rcu_init(void)
rcu_init_one();
if (dump_tree)
rcu_dump_rcu_node_tree();
- open_softirq(RCU_SOFTIRQ, rcu_core);
+ if (use_softirq)
+ open_softirq(RCU_SOFTIRQ, rcu_core_si);
/*
* We don't need protection against CPU-hotplug here because
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);
+ else
+ invoke_rcu_core();
} 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
