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Subject[PATCH,RFC] Add call_rcu_sched()
Hello!

Rough first cut of patch to provide the call_rcu_sched() needed for
Mathieu's markers implementation. This is to synchronize_sched()
as call_rcu() is to synchronize_rcu().

Not for inclusion. Passes light testing, but should be treated with
some caution given that no part of it is more than 24 hours old.
Known/suspected shortcomings:

o Only lightly tested -- haven't yet tried rcutorture.

o Need to add call_rcu_sched() testing to rcutorture.

o If I remember correctly, an rcu_barrier_sched() is required
(Mathieu?).

o Interaction of this patch with CPU hotplug should be viewed
with great suspicion.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

include/linux/rcuclassic.h | 3
include/linux/rcupdate.h | 22 +++
include/linux/rcupreempt.h | 15 ++
init/main.c | 1
kernel/rcupdate.c | 20 ---
kernel/rcupreempt.c | 256 +++++++++++++++++++++++++++++++++++++++++++--
6 files changed, 288 insertions(+), 29 deletions(-)

diff -urpNa -X dontdiff linux-2.6.25-rc6/include/linux/rcuclassic.h linux-2.6.25-rc6-call_rcu_sched/include/linux/rcuclassic.h
--- linux-2.6.25-rc6/include/linux/rcuclassic.h 2008-03-16 17:45:16.000000000 -0700
+++ linux-2.6.25-rc6-call_rcu_sched/include/linux/rcuclassic.h 2008-03-21 04:27:31.000000000 -0700
@@ -153,7 +153,10 @@ extern struct lockdep_map rcu_lock_map;

#define __synchronize_sched() synchronize_rcu()

+#define call_rcu_sched(head, func) call_rcu(head, func)
+
extern void __rcu_init(void);
+#define rcu_init_sched() do { } while (0)
extern void rcu_check_callbacks(int cpu, int user);
extern void rcu_restart_cpu(int cpu);

diff -urpNa -X dontdiff linux-2.6.25-rc6/include/linux/rcupdate.h linux-2.6.25-rc6-call_rcu_sched/include/linux/rcupdate.h
--- linux-2.6.25-rc6/include/linux/rcupdate.h 2008-03-16 17:45:16.000000000 -0700
+++ linux-2.6.25-rc6-call_rcu_sched/include/linux/rcupdate.h 2008-03-20 21:10:42.000000000 -0700
@@ -42,6 +42,7 @@
#include <linux/cpumask.h>
#include <linux/seqlock.h>
#include <linux/lockdep.h>
+#include <linux/completion.h>

/**
* struct rcu_head - callback structure for use with RCU
@@ -182,6 +183,27 @@ struct rcu_head {
(p) = (v); \
})

+/* Infrastructure to implement the synchronize_() primitives. */
+
+struct rcu_synchronize {
+ struct rcu_head head;
+ struct completion completion;
+};
+
+extern void wakeme_after_rcu(struct rcu_head *head);
+
+#define synchronize_rcu_xxx(name, func) \
+void name(void) \
+{ \
+ struct rcu_synchronize rcu; \
+ \
+ init_completion(&rcu.completion); \
+ /* Will wake me after RCU finished. */ \
+ func(&rcu.head, wakeme_after_rcu); \
+ /* Wait for it. */ \
+ wait_for_completion(&rcu.completion); \
+}
+
/**
* synchronize_sched - block until all CPUs have exited any non-preemptive
* kernel code sequences.
diff -urpNa -X dontdiff linux-2.6.25-rc6/include/linux/rcupreempt.h linux-2.6.25-rc6-call_rcu_sched/include/linux/rcupreempt.h
--- linux-2.6.25-rc6/include/linux/rcupreempt.h 2008-03-16 17:45:16.000000000 -0700
+++ linux-2.6.25-rc6-call_rcu_sched/include/linux/rcupreempt.h 2008-03-21 04:31:29.000000000 -0700
@@ -46,6 +46,20 @@
#define rcu_bh_qsctr_inc(cpu)
#define call_rcu_bh(head, rcu) call_rcu(head, rcu)

+/**
+ * call_rcu_sched - Queue RCU callback for invocation after sched grace period.
+ * @head: structure to be used for queueing the RCU updates.
+ * @func: actual update function to be invoked after the grace period
+ *
+ * The update function will be invoked some time after a full
+ * synchronize_sched()-style grace period elapses, in other words after
+ * all currently executing preempt-disabled sections of code (including
+ * hardirq handlers, NMI handlers, and local_irq_save() blocks) have
+ * completed.
+ */
+extern void call_rcu_sched(struct rcu_head *head,
+ void (*func)(struct rcu_head *head));
+
extern void __rcu_read_lock(void) __acquires(RCU);
extern void __rcu_read_unlock(void) __releases(RCU);
extern int rcu_pending(int cpu);
@@ -57,6 +71,7 @@ extern int rcu_needs_cpu(int cpu);
extern void __synchronize_sched(void);

extern void __rcu_init(void);
+extern void rcu_init_sched(void);
extern void rcu_check_callbacks(int cpu, int user);
extern void rcu_restart_cpu(int cpu);
extern long rcu_batches_completed(void);
diff -urpNa -X dontdiff linux-2.6.25-rc6/init/main.c linux-2.6.25-rc6-call_rcu_sched/init/main.c
--- linux-2.6.25-rc6/init/main.c 2008-03-16 17:45:17.000000000 -0700
+++ linux-2.6.25-rc6-call_rcu_sched/init/main.c 2008-03-21 04:31:31.000000000 -0700
@@ -736,6 +736,7 @@ static void __init do_basic_setup(void)
driver_init();
init_irq_proc();
do_initcalls();
+ rcu_init_sched();
}

static int __initdata nosoftlockup;
diff -urpNa -X dontdiff linux-2.6.25-rc6/kernel/rcupdate.c linux-2.6.25-rc6-call_rcu_sched/kernel/rcupdate.c
--- linux-2.6.25-rc6/kernel/rcupdate.c 2008-03-16 17:45:17.000000000 -0700
+++ linux-2.6.25-rc6-call_rcu_sched/kernel/rcupdate.c 2008-03-20 21:10:39.000000000 -0700
@@ -39,18 +39,12 @@
#include <linux/sched.h>
#include <asm/atomic.h>
#include <linux/bitops.h>
-#include <linux/completion.h>
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/mutex.h>
#include <linux/module.h>

-struct rcu_synchronize {
- struct rcu_head head;
- struct completion completion;
-};
-
static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
static atomic_t rcu_barrier_cpu_count;
static DEFINE_MUTEX(rcu_barrier_mutex);
@@ -60,7 +54,7 @@ static struct completion rcu_barrier_com
* Awaken the corresponding synchronize_rcu() instance now that a
* grace period has elapsed.
*/
-static void wakeme_after_rcu(struct rcu_head *head)
+void wakeme_after_rcu(struct rcu_head *head)
{
struct rcu_synchronize *rcu;

@@ -77,17 +71,7 @@ static void wakeme_after_rcu(struct rcu_
* sections are delimited by rcu_read_lock() and rcu_read_unlock(),
* and may be nested.
*/
-void synchronize_rcu(void)
-{
- struct rcu_synchronize rcu;
-
- init_completion(&rcu.completion);
- /* Will wake me after RCU finished */
- call_rcu(&rcu.head, wakeme_after_rcu);
-
- /* Wait for it */
- wait_for_completion(&rcu.completion);
-}
+synchronize_rcu_xxx(synchronize_rcu, call_rcu)
EXPORT_SYMBOL_GPL(synchronize_rcu);

static void rcu_barrier_callback(struct rcu_head *notused)
diff -urpNa -X dontdiff linux-2.6.25-rc6/kernel/rcupreempt.c linux-2.6.25-rc6-call_rcu_sched/kernel/rcupreempt.c
--- linux-2.6.25-rc6/kernel/rcupreempt.c 2008-03-16 17:45:17.000000000 -0700
+++ linux-2.6.25-rc6-call_rcu_sched/kernel/rcupreempt.c 2008-03-21 04:31:26.000000000 -0700
@@ -46,6 +46,7 @@
#include <asm/atomic.h>
#include <linux/bitops.h>
#include <linux/module.h>
+#include <linux/kthread.h>
#include <linux/completion.h>
#include <linux/moduleparam.h>
#include <linux/percpu.h>
@@ -87,9 +88,14 @@ struct rcu_data {
struct rcu_head **nexttail;
struct rcu_head *waitlist[GP_STAGES];
struct rcu_head **waittail[GP_STAGES];
- struct rcu_head *donelist;
+ struct rcu_head *donelist; /* from waitlist & waitschedlist */
struct rcu_head **donetail;
long rcu_flipctr[2];
+ struct rcu_head *nextschedlist;
+ struct rcu_head **nextschedtail;
+ struct rcu_head *waitschedlist;
+ struct rcu_head **waitschedtail;
+ int rcu_sched_sleeping;
#ifdef CONFIG_RCU_TRACE
struct rcupreempt_trace trace;
#endif /* #ifdef CONFIG_RCU_TRACE */
@@ -131,11 +137,24 @@ enum rcu_try_flip_states {
rcu_try_flip_waitmb_state,
};

+/*
+ * States for rcu_ctrlblk.rcu_sched_sleep.
+ */
+
+enum rcu_sched_sleep_states {
+ rcu_sched_not_sleeping, /* Not sleeping, callbacks need GP. */
+ rcu_sched_sleep_prep, /* Thinking of sleeping, rechecking. */
+ rcu_sched_sleeping, /* Sleeping, awaken if GP needed. */
+};
+
struct rcu_ctrlblk {
spinlock_t fliplock; /* Protect state-machine transitions. */
long completed; /* Number of last completed batch. */
enum rcu_try_flip_states rcu_try_flip_state; /* The current state of
the rcu state machine */
+ spinlock_t schedlock; /* Protect rcu_sched sleep state. */
+ enum rcu_sched_sleep_states sched_sleep; /* rcu_sched state. */
+ wait_queue_head_t sched_wq; /* Place for rcu_sched to sleep. */
};

static DEFINE_PER_CPU(struct rcu_data, rcu_data);
@@ -143,8 +162,12 @@ static struct rcu_ctrlblk rcu_ctrlblk =
.fliplock = __SPIN_LOCK_UNLOCKED(rcu_ctrlblk.fliplock),
.completed = 0,
.rcu_try_flip_state = rcu_try_flip_idle_state,
+ .schedlock = __SPIN_LOCK_UNLOCKED(rcu_ctrlblk.schedlock),
+ .sched_sleep = rcu_sched_not_sleeping,
+ .sched_wq = __WAIT_QUEUE_HEAD_INITIALIZER(rcu_ctrlblk.sched_wq),
};

+static struct task_struct *rcu_sched_grace_period_task;

#ifdef CONFIG_RCU_TRACE
static char *rcu_try_flip_state_names[] =
@@ -871,6 +894,8 @@ void rcu_offline_cpu(int cpu)
struct rcu_head *list = NULL;
unsigned long flags;
struct rcu_data *rdp = RCU_DATA_CPU(cpu);
+ struct rcu_head *schedlist = NULL;
+ struct rcu_head **schedtail = &schedlist;
struct rcu_head **tail = &list;

/*
@@ -884,6 +909,11 @@ void rcu_offline_cpu(int cpu)
rcu_offline_cpu_enqueue(rdp->waitlist[i], rdp->waittail[i],
list, tail);
rcu_offline_cpu_enqueue(rdp->nextlist, rdp->nexttail, list, tail);
+ rcu_offline_cpu_enqueue(rdp->waitschedlist, rdp->waitschedtail,
+ schedlist, schedtail);
+ rcu_offline_cpu_enqueue(rdp->nextschedlist, rdp->nextschedtail,
+ schedlist, schedtail);
+ rdp->rcu_sched_sleeping = 0;
spin_unlock_irqrestore(&rdp->lock, flags);
rdp->waitlistcount = 0;

@@ -924,16 +954,35 @@ void rcu_offline_cpu(int cpu)
*rdp->nexttail = list;
if (list)
rdp->nexttail = tail;
+ *rdp->nextschedtail = schedlist;
+ if (schedlist)
+ rdp->nextschedtail = schedtail;
spin_unlock_irqrestore(&rdp->lock, flags);
}

void __devinit rcu_online_cpu(int cpu)
{
unsigned long flags;
+ struct rcu_data *rdp;

spin_lock_irqsave(&rcu_ctrlblk.fliplock, flags);
cpu_set(cpu, rcu_cpu_online_map);
spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags);
+
+ /*
+ * The rcu_sched grace-period processing might have bypassed
+ * this CPU, given that it was not in the rcu_cpu_online_map
+ * when the grace-period scan started. This means that the
+ * grace-period task might sleep. So make sure that if this
+ * should happen, the first callback posted to this CPU will
+ * wake up the grace-period task if need be.
+ */
+
+ local_irq_save(flags);
+ rdp = RCU_DATA_ME();
+ spin_lock(&rdp->lock);
+ rdp->rcu_sched_sleeping = 1;
+ spin_unlock_irqrestore(&rdp->lock, flags);
}

#else /* #ifdef CONFIG_HOTPLUG_CPU */
@@ -993,26 +1042,194 @@ void call_rcu(struct rcu_head *head, voi
}
EXPORT_SYMBOL_GPL(call_rcu);

+void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
+{
+ unsigned long flags;
+ struct rcu_data *rdp;
+ int wake_gp = 0;
+
+ head->func = func;
+ head->next = NULL;
+ local_irq_save(flags);
+ rdp = RCU_DATA_ME();
+ spin_lock(&rdp->lock);
+ *rdp->nextschedtail = head;
+ rdp->nextschedtail = &head->next;
+ if (rdp->rcu_sched_sleeping) {
+
+ /* Grace-period processing might be sleeping... */
+
+ rdp->rcu_sched_sleeping = 0;
+ wake_gp = 1;
+ }
+ spin_unlock(&rdp->lock);
+ local_irq_restore(flags);
+ if (wake_gp) {
+
+ /* Wake up grace-period processing, unless someone beat us. */
+
+ spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags);
+ if (rcu_ctrlblk.sched_sleep == rcu_sched_sleeping)
+ rcu_ctrlblk.sched_sleep = rcu_sched_not_sleeping;
+ else
+ wake_gp = 0;
+ spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
+ if (wake_gp)
+ wake_up(&rcu_ctrlblk.sched_wq);
+ }
+}
+EXPORT_SYMBOL_GPL(call_rcu_sched);
+
/*
* Wait until all currently running preempt_disable() code segments
* (including hardware-irq-disable segments) complete. Note that
* in -rt this does -not- necessarily result in all currently executing
* interrupt -handlers- having completed.
*/
-void __synchronize_sched(void)
+synchronize_rcu_xxx(__synchronize_sched, call_rcu_sched)
+EXPORT_SYMBOL_GPL(__synchronize_sched);
+
+/*
+ * kthread function that manages call_rcu_sched grace periods.
+ */
+static int
+rcu_sched_grace_period(void *arg)
{
- cpumask_t oldmask;
+ int couldsleep;
+ int couldsleepnext = 0;
int cpu;
+ unsigned long flags;
+ int needsoftirq;
+ struct rcu_data *rdp;

- if (sched_getaffinity(0, &oldmask) < 0)
- oldmask = cpu_possible_map;
- for_each_online_cpu(cpu) {
- sched_setaffinity(0, cpumask_of_cpu(cpu));
- schedule();
- }
- sched_setaffinity(0, oldmask);
+ /*
+ * Each pass through the following loop handles one
+ * rcu_sched grace period cycle.
+ */
+
+ do {
+ /*
+ * Sleep for about an RCU grace-period's worth to
+ * allow better batching and to consume less CPU.
+ */
+
+ schedule_timeout_interruptible(HZ / 20);
+
+ /*
+ * If there was nothing to do last time, prepare to
+ * sleep at the end of the current grace period cycle.
+ */
+
+ couldsleep = couldsleepnext;
+ couldsleepnext = 1;
+ if (couldsleep) {
+ spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags);
+ rcu_ctrlblk.sched_sleep = rcu_sched_sleep_prep;
+ spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
+ }
+
+ /*
+ * Schedule on each CPU in turn, advancing callbacks
+ * as we go. We will have visited each CPU between
+ * the time we move a callback from the nextsched
+ * list and the time we move that callback to the
+ * done list. Now, a given CPU might come online
+ * during that interval, but that means that it
+ * was offline when we started, so we can safely
+ * ignore it.
+ */
+
+ for_each_online_cpu(cpu) {
+
+ /* Initialize and schedule onto current CPU. */
+
+ needsoftirq = 0;
+ sched_setaffinity(0, cpumask_of_cpu(cpu)); /*@@@fail?*/
+ schedule();
+
+ /*
+ * Get a reference to this CPU's rcu_data
+ * structure, lock it, and verify that this
+ * CPU is still online (skip it otherwise).
+ */
+
+ rdp = RCU_DATA_CPU(cpu);
+ spin_lock_irqsave(&rdp->lock, flags);
+ if (cpu_is_offline(cpu)) {
+ spin_unlock_irqrestore(&rdp->lock, flags);
+ continue;
+ }
+
+ /*
+ * We are running on the CPU irq-disabled, so it
+ * cannot go offline until we re-enable irqs.
+ *
+ * Advance the callbacks! We share normal RCU's
+ * donelist, since callbacks are invoked the
+ * same way in either case.
+ */
+
+ if (rdp->waitschedlist != NULL) {
+ *rdp->donetail = rdp->waitschedlist;
+ rdp->donetail = rdp->waitschedtail;
+ needsoftirq = 1;
+ }
+ if (rdp->nextschedlist != NULL) {
+ rdp->waitschedlist = rdp->nextschedlist;
+ rdp->waitschedtail = rdp->nextschedtail;
+ couldsleepnext = couldsleep = 0;
+ } else {
+ rdp->waitschedlist = NULL;
+ rdp->waitschedtail = &rdp->waitschedlist;
+ }
+ rdp->nextschedlist = NULL;
+ rdp->nextschedtail = &rdp->nextschedlist;
+
+ /* Mark sleep intention. */
+
+ rdp->rcu_sched_sleeping = couldsleep;
+
+ spin_unlock_irqrestore(&rdp->lock, flags);
+
+ /* If we added callbacks to donelist, process. */
+
+ if (needsoftirq)
+ raise_softirq(RCU_SOFTIRQ);
+ }
+
+ /* If we saw callbacks on the last scan, go deal with them. */
+
+ if (!couldsleep) {
+ continue;
+ }
+
+ /* Attempt to block... */
+
+ spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags);
+ if (rcu_ctrlblk.sched_sleep != rcu_sched_sleep_prep) {
+
+ /*
+ * Someone posted a callback after we scanned.
+ * Go take care of it.
+ */
+
+ spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
+ couldsleepnext = 0;
+ continue;
+ }
+
+ /* Block until the next person posts a callback. */
+
+ rcu_ctrlblk.sched_sleep = rcu_sched_sleeping;
+ spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
+ __wait_event(rcu_ctrlblk.sched_wq,
+ rcu_ctrlblk.sched_sleep != rcu_sched_sleeping);
+ couldsleepnext = 0;
+
+ } while (!kthread_should_stop());
+
+ return (0);
}
-EXPORT_SYMBOL_GPL(__synchronize_sched);

/*
* Check to see if any future RCU-related work will need to be done
@@ -1107,6 +1324,11 @@ void __init __rcu_init(void)
rdp->donetail = &rdp->donelist;
rdp->rcu_flipctr[0] = 0;
rdp->rcu_flipctr[1] = 0;
+ rdp->nextschedlist = NULL;
+ rdp->nextschedtail = &rdp->nextschedlist;
+ rdp->waitschedlist = NULL;
+ rdp->waitschedtail = &rdp->waitschedlist;
+ rdp->rcu_sched_sleeping = 0;
}
register_cpu_notifier(&rcu_nb);

@@ -1129,6 +1351,18 @@ void __init __rcu_init(void)
}

/*
+ * Late-boot-time RCU initialization that must wait until after scheduler
+ * has been initialized.
+ */
+void __init rcu_init_sched(void)
+{
+ rcu_sched_grace_period_task = kthread_run(rcu_sched_grace_period,
+ NULL,
+ "rcu_sched_grace_period");
+ WARN_ON(IS_ERR(rcu_sched_grace_period_task));
+}
+
+/*
* Deprecated, use synchronize_rcu() or synchronize_sched() instead.
*/
void synchronize_kernel(void)

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