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Subject[Patch 6/7] rcu: Use smp_hotplug_thread facility for RCUs per-CPU kthread
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

Bring RCU into the new-age CPU-hotplug fold by modifying RCU's per-CPU
kthread code to use the new smp_hotplug_thread facility.

[ tglx: Adapted it to use callbacks and to the simplified rcu yield ]

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
kernel/rcutree.c | 4
kernel/rcutree.h | 8 -
kernel/rcutree_plugin.h | 203 +++++++++---------------------------------------
kernel/rcutree_trace.c | 3
4 files changed, 41 insertions(+), 177 deletions(-)
Index: tip/kernel/rcutree.c
===================================================================
--- tip.orig/kernel/rcutree.c
+++ tip/kernel/rcutree.c
@@ -125,7 +125,6 @@ static int rcu_scheduler_fully_active __
*/
static DEFINE_PER_CPU(struct task_struct *, rcu_cpu_kthread_task);
DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
-DEFINE_PER_CPU(int, rcu_cpu_kthread_cpu);
DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
DEFINE_PER_CPU(char, rcu_cpu_has_work);

@@ -1458,7 +1457,6 @@ static void rcu_cleanup_dead_cpu(int cpu
struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */

/* Adjust any no-longer-needed kthreads. */
- rcu_stop_cpu_kthread(cpu);
rcu_boost_kthread_setaffinity(rnp, -1);

/* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
@@ -2514,11 +2512,9 @@ static int __cpuinit rcu_cpu_notify(stru
case CPU_ONLINE:
case CPU_DOWN_FAILED:
rcu_boost_kthread_setaffinity(rnp, -1);
- rcu_cpu_kthread_setrt(cpu, 1);
break;
case CPU_DOWN_PREPARE:
rcu_boost_kthread_setaffinity(rnp, cpu);
- rcu_cpu_kthread_setrt(cpu, 0);
break;
case CPU_DYING:
case CPU_DYING_FROZEN:
Index: tip/kernel/rcutree.h
===================================================================
--- tip.orig/kernel/rcutree.h
+++ tip/kernel/rcutree.h
@@ -178,12 +178,6 @@ struct rcu_node {
/* Refused to boost: not sure why, though. */
/* This can happen due to race conditions. */
#endif /* #ifdef CONFIG_RCU_BOOST */
- struct task_struct *node_kthread_task;
- /* kthread that takes care of this rcu_node */
- /* structure, for example, awakening the */
- /* per-CPU kthreads as needed. */
- unsigned int node_kthread_status;
- /* State of node_kthread_task for tracing. */
} ____cacheline_internodealigned_in_smp;

/*
@@ -434,7 +428,6 @@ static int rcu_preempt_blocked_readers_c
#ifdef CONFIG_HOTPLUG_CPU
static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp,
unsigned long flags);
-static void rcu_stop_cpu_kthread(int cpu);
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
static void rcu_print_detail_task_stall(struct rcu_state *rsp);
static int rcu_print_task_stall(struct rcu_node *rnp);
@@ -467,7 +460,6 @@ static void rcu_preempt_do_callbacks(voi
static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
struct rcu_node *rnp);
#endif /* #ifdef CONFIG_RCU_BOOST */
-static void rcu_cpu_kthread_setrt(int cpu, int to_rt);
static void __cpuinit rcu_prepare_kthreads(int cpu);
static void rcu_prepare_for_idle_init(int cpu);
static void rcu_cleanup_after_idle(int cpu);
Index: tip/kernel/rcutree_plugin.h
===================================================================
--- tip.orig/kernel/rcutree_plugin.h
+++ tip/kernel/rcutree_plugin.h
@@ -25,6 +25,7 @@
*/

#include <linux/delay.h>
+#include <linux/smpboot.h>

#define RCU_KTHREAD_PRIO 1

@@ -1440,25 +1441,6 @@ static int __cpuinit rcu_spawn_one_boost
return 0;
}

-#ifdef CONFIG_HOTPLUG_CPU
-
-/*
- * Stop the RCU's per-CPU kthread when its CPU goes offline,.
- */
-static void rcu_stop_cpu_kthread(int cpu)
-{
- struct task_struct *t;
-
- /* Stop the CPU's kthread. */
- t = per_cpu(rcu_cpu_kthread_task, cpu);
- if (t != NULL) {
- per_cpu(rcu_cpu_kthread_task, cpu) = NULL;
- kthread_stop(t);
- }
-}
-
-#endif /* #ifdef CONFIG_HOTPLUG_CPU */
-
static void rcu_kthread_do_work(void)
{
rcu_do_batch(&rcu_sched_state, &__get_cpu_var(rcu_sched_data));
@@ -1466,59 +1448,22 @@ static void rcu_kthread_do_work(void)
rcu_preempt_do_callbacks();
}

-/*
- * Set the specified CPU's kthread to run RT or not, as specified by
- * the to_rt argument. The CPU-hotplug locks are held, so the task
- * is not going away.
- */
-static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
+static void rcu_cpu_kthread_setup(unsigned int cpu)
{
- int policy;
struct sched_param sp;
- struct task_struct *t;

- t = per_cpu(rcu_cpu_kthread_task, cpu);
- if (t == NULL)
- return;
- if (to_rt) {
- policy = SCHED_FIFO;
- sp.sched_priority = RCU_KTHREAD_PRIO;
- } else {
- policy = SCHED_NORMAL;
- sp.sched_priority = 0;
- }
- sched_setscheduler_nocheck(t, policy, &sp);
+ sp.sched_priority = RCU_KTHREAD_PRIO;
+ sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
}

-/*
- * Handle cases where the rcu_cpu_kthread() ends up on the wrong CPU.
- * This can happen while the corresponding CPU is either coming online
- * or going offline. We cannot wait until the CPU is fully online
- * before starting the kthread, because the various notifier functions
- * can wait for RCU grace periods. So we park rcu_cpu_kthread() until
- * the corresponding CPU is online.
- *
- * Return 1 if the kthread needs to stop, 0 otherwise.
- *
- * Caller must disable bh. This function can momentarily enable it.
- */
-static int rcu_cpu_kthread_should_stop(int cpu)
+static void rcu_cpu_kthread_park(unsigned int cpu)
{
- while (cpu_is_offline(cpu) ||
- !cpumask_equal(&current->cpus_allowed, cpumask_of(cpu)) ||
- smp_processor_id() != cpu) {
- if (kthread_should_stop())
- return 1;
- per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
- per_cpu(rcu_cpu_kthread_cpu, cpu) = raw_smp_processor_id();
- local_bh_enable();
- schedule_timeout_uninterruptible(1);
- if (!cpumask_equal(&current->cpus_allowed, cpumask_of(cpu)))
- set_cpus_allowed_ptr(current, cpumask_of(cpu));
- local_bh_disable();
- }
- per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
- return 0;
+ per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
+}
+
+static int rcu_cpu_kthread_should_run(unsigned int cpu)
+{
+ return __get_cpu_var(rcu_cpu_has_work);
}

/*
@@ -1526,96 +1471,35 @@ static int rcu_cpu_kthread_should_stop(i
* RCU softirq used in flavors and configurations of RCU that do not
* support RCU priority boosting.
*/
-static int rcu_cpu_kthread(void *arg)
+static void rcu_cpu_kthread(unsigned int cpu)
{
- int cpu = (int)(long)arg;
- unsigned long flags;
- int spincnt = 0;
- unsigned int *statusp = &per_cpu(rcu_cpu_kthread_status, cpu);
- char work;
- char *workp = &per_cpu(rcu_cpu_has_work, cpu);
+ unsigned int *statusp = &__get_cpu_var(rcu_cpu_kthread_status);
+ char work, *workp = &__get_cpu_var(rcu_cpu_has_work);
+ int spincnt;

- trace_rcu_utilization("Start CPU kthread@init");
- for (;;) {
- *statusp = RCU_KTHREAD_WAITING;
- trace_rcu_utilization("End CPU kthread@rcu_wait");
- rcu_wait(*workp != 0 || kthread_should_stop());
+ for (spincnt = 0; spincnt < 10; spincnt++) {
trace_rcu_utilization("Start CPU kthread@rcu_wait");
local_bh_disable();
- if (rcu_cpu_kthread_should_stop(cpu)) {
- local_bh_enable();
- break;
- }
*statusp = RCU_KTHREAD_RUNNING;
- per_cpu(rcu_cpu_kthread_loops, cpu)++;
- local_irq_save(flags);
+ this_cpu_inc(rcu_cpu_kthread_loops);
+ local_irq_disable();
work = *workp;
*workp = 0;
- local_irq_restore(flags);
+ local_irq_enable();
if (work)
rcu_kthread_do_work();
local_bh_enable();
- if (*workp != 0)
- spincnt++;
- else
- spincnt = 0;
- if (spincnt > 10) {
- *statusp = RCU_KTHREAD_YIELDING;
- trace_rcu_utilization("End CPU kthread@rcu_yield");
- schedule_timeout_interruptible(2);
- trace_rcu_utilization("Start CPU kthread@rcu_yield");
- spincnt = 0;
+ if (*workp == 0) {
+ trace_rcu_utilization("End CPU kthread@rcu_wait");
+ *statusp = RCU_KTHREAD_WAITING;
+ return;
}
}
- *statusp = RCU_KTHREAD_STOPPED;
- trace_rcu_utilization("End CPU kthread@term");
- return 0;
-}
-
-/*
- * Spawn a per-CPU kthread, setting up affinity and priority.
- * Because the CPU hotplug lock is held, no other CPU will be attempting
- * to manipulate rcu_cpu_kthread_task. There might be another CPU
- * attempting to access it during boot, but the locking in kthread_bind()
- * will enforce sufficient ordering.
- *
- * Please note that we cannot simply refuse to wake up the per-CPU
- * kthread because kthreads are created in TASK_UNINTERRUPTIBLE state,
- * which can result in softlockup complaints if the task ends up being
- * idle for more than a couple of minutes.
- *
- * However, please note also that we cannot bind the per-CPU kthread to its
- * CPU until that CPU is fully online. We also cannot wait until the
- * CPU is fully online before we create its per-CPU kthread, as this would
- * deadlock the system when CPU notifiers tried waiting for grace
- * periods. So we bind the per-CPU kthread to its CPU only if the CPU
- * is online. If its CPU is not yet fully online, then the code in
- * rcu_cpu_kthread() will wait until it is fully online, and then do
- * the binding.
- */
-static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
-{
- struct sched_param sp;
- struct task_struct *t;
-
- if (!rcu_scheduler_fully_active ||
- per_cpu(rcu_cpu_kthread_task, cpu) != NULL)
- return 0;
- t = kthread_create_on_node(rcu_cpu_kthread,
- (void *)(long)cpu,
- cpu_to_node(cpu),
- "rcuc/%d", cpu);
- if (IS_ERR(t))
- return PTR_ERR(t);
- if (cpu_online(cpu))
- kthread_bind(t, cpu);
- per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
- WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
- sp.sched_priority = RCU_KTHREAD_PRIO;
- sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
- per_cpu(rcu_cpu_kthread_task, cpu) = t;
- wake_up_process(t); /* Get to TASK_INTERRUPTIBLE quickly. */
- return 0;
+ *statusp = RCU_KTHREAD_YIELDING;
+ trace_rcu_utilization("Start CPU kthread@rcu_yield");
+ schedule_timeout_interruptible(2);
+ trace_rcu_utilization("End CPU kthread@rcu_yield");
+ *statusp = RCU_KTHREAD_WAITING;
}

/*
@@ -1651,6 +1535,15 @@ static void rcu_boost_kthread_setaffinit
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 all kthreads -- called as soon as the scheduler is running.
*/
@@ -1660,11 +1553,9 @@ static int __init rcu_spawn_kthreads(voi
int cpu;

rcu_scheduler_fully_active = 1;
- for_each_possible_cpu(cpu) {
+ for_each_possible_cpu(cpu)
per_cpu(rcu_cpu_has_work, cpu) = 0;
- if (cpu_online(cpu))
- (void)rcu_spawn_one_cpu_kthread(cpu);
- }
+ BUG_ON(smpboot_register_percpu_thread(&rcu_cpu_thread_spec));
rnp = rcu_get_root(rcu_state);
(void)rcu_spawn_one_boost_kthread(rcu_state, rnp);
if (NUM_RCU_NODES > 1) {
@@ -1681,10 +1572,8 @@ static void __cpuinit rcu_prepare_kthrea
struct rcu_node *rnp = rdp->mynode;

/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
- if (rcu_scheduler_fully_active) {
- (void)rcu_spawn_one_cpu_kthread(cpu);
+ if (rcu_scheduler_fully_active)
(void)rcu_spawn_one_boost_kthread(rcu_state, rnp);
- }
}

#else /* #ifdef CONFIG_RCU_BOOST */
@@ -1708,22 +1597,10 @@ static void rcu_preempt_boost_start_gp(s
{
}

-#ifdef CONFIG_HOTPLUG_CPU
-
-static void rcu_stop_cpu_kthread(int cpu)
-{
-}
-
-#endif /* #ifdef CONFIG_HOTPLUG_CPU */
-
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
}

-static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
-{
-}
-
static int __init rcu_scheduler_really_started(void)
{
rcu_scheduler_fully_active = 1;
Index: tip/kernel/rcutree_trace.c
===================================================================
--- tip.orig/kernel/rcutree_trace.c
+++ tip/kernel/rcutree_trace.c
@@ -83,11 +83,10 @@ static void print_one_rcu_data(struct se
rdp->nxttail[RCU_WAIT_TAIL]],
".D"[&rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL]]);
#ifdef CONFIG_RCU_BOOST
- seq_printf(m, " kt=%d/%c/%d ktl=%x",
+ seq_printf(m, " kt=%d/%c ktl=%x",
per_cpu(rcu_cpu_has_work, rdp->cpu),
convert_kthread_status(per_cpu(rcu_cpu_kthread_status,
rdp->cpu)),
- per_cpu(rcu_cpu_kthread_cpu, rdp->cpu),
per_cpu(rcu_cpu_kthread_loops, rdp->cpu) & 0xffff);
#endif /* #ifdef CONFIG_RCU_BOOST */
seq_printf(m, " b=%ld", rdp->blimit);



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