Re: [PATCH,RFC] Add call_rcu_sched()

From: Mathieu Desnoyers
Date: Mon Mar 24 2008 - 01:07:20 EST

* Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> Hello!
>
> Second cut of patch to provide the call_rcu_sched() needed for Mathieu's
> markers implementation. This is again to synchronize_sched() as
> call_rcu() is to synchronize_rcu().
>
> Should be fine for experimental use, but not ready for inclusion.
>
> Passes short rcutorture sessions, but should be treated with some caution
> given that very little of it is more than 24 hours old. Fixes since the
> first version include a bug that could result in indefinite blocking
> (spotted by Gautham Shenoy), better resiliency against CPU-hotplug
> operations, and other minor fixes.
>
> Known/suspected shortcomings:
>
> o Only moderately tested -- only short rcutorture sessions.
>
> o Need to add call_rcu_sched() testing to rcutorture.
>
> o If I remember correctly, an rcu_barrier_sched() is required
> (Mathieu?).
>

Hi Paul,

Thanks for this work, I'll give it a try (I'm just back from a weekend
away from the city). Yes, my code needs a rcu_barrier_sched() so it can
wait for call_rcu_sched completion before it tries to re-use the data
structures at the next modification of the same marker.

I think rcu_barrier_sched should be quite straightforward to implement
if we derive it from kernel/rcupdate.c:rcu_barrier. Actually, couldn't
we just rename rcu_barrier into something else made static (_rcu_barrier)
and call it with a different parameter telling which of call_rcu or
call_rcu_sched to use ?

Something like this :

Add rcu_barrier_sched

Adds rcu_barrier_sched, which uses call_rcu_sched. It wait for each in flight
call_rcu_sched to be completed before it returns.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
---
include/linux/rcupdate.h | 1 +
kernel/rcupdate.c | 40 +++++++++++++++++++++++++++++++++-------
2 files changed, 34 insertions(+), 7 deletions(-)

Index: linux-2.6-lttng/include/linux/rcupdate.h
===================================================================
--- linux-2.6-lttng.orig/include/linux/rcupdate.h 2008-03-24 00:13:26.000000000 -0400
+++ linux-2.6-lttng/include/linux/rcupdate.h 2008-03-24 00:13:36.000000000 -0400
@@ -260,6 +260,7 @@ extern void call_rcu_bh(struct rcu_head
/* Exported common interfaces */
extern void synchronize_rcu(void);
extern void rcu_barrier(void);
+extern void rcu_barrier_sched(void);
extern long rcu_batches_completed(void);
extern long rcu_batches_completed_bh(void);

Index: linux-2.6-lttng/kernel/rcupdate.c
===================================================================
--- linux-2.6-lttng.orig/kernel/rcupdate.c 2008-03-24 00:07:15.000000000 -0400
+++ linux-2.6-lttng/kernel/rcupdate.c 2008-03-24 00:17:01.000000000 -0400
@@ -45,6 +45,11 @@
#include <linux/mutex.h>
#include <linux/module.h>

+enum rcu_barrier {
+ RCU_BARRIER_STD,
+ RCU_BARRIER_SCHED,
+};
+
static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
static atomic_t rcu_barrier_cpu_count;
static DEFINE_MUTEX(rcu_barrier_mutex);
@@ -83,19 +88,23 @@ static void rcu_barrier_callback(struct
/*
* Called with preemption disabled, and from cross-cpu IRQ context.
*/
-static void rcu_barrier_func(void *notused)
+static void rcu_barrier_func(void *type)
{
int cpu = smp_processor_id();
struct rcu_head *head = &per_cpu(rcu_barrier_head, cpu);

atomic_inc(&rcu_barrier_cpu_count);
- call_rcu(head, rcu_barrier_callback);
+ switch((enum rcu_barrier)type) {
+ case RCU_BARRIER_STD:
+ call_rcu(head, rcu_barrier_callback);
+ break;
+ case RCU_BARRIER_SCHED:
+ call_rcu_sched(head, rcu_barrier_callback);
+ break;
+ }
}

-/**
- * rcu_barrier - Wait until all the in-flight RCUs are complete.
- */
-void rcu_barrier(void)
+static void _rcu_barrier(enum rcu_barrier type)
{
BUG_ON(in_interrupt());
/* Take cpucontrol mutex to protect against CPU hotplug */
@@ -111,13 +120,30 @@ void rcu_barrier(void)
* until all the callbacks are queued.
*/
rcu_read_lock();
- on_each_cpu(rcu_barrier_func, NULL, 0, 1);
+ on_each_cpu(rcu_barrier_func, (void *)type, 0, 1);
rcu_read_unlock();
wait_for_completion(&rcu_barrier_completion);
mutex_unlock(&rcu_barrier_mutex);
}
+
+/**
+ * rcu_barrier - Wait until all the in-flight RCUs are complete.
+ */
+void rcu_barrier(void)
+{
+ _rcu_barrier(RCU_BARRIER_STD);
+}
EXPORT_SYMBOL_GPL(rcu_barrier);

+/**
+ * rcu_barrier_sched - Wait until all the in-flight call_rcu_sched are complete.
+ */
+void rcu_barrier_sched(void)
+{
+ _rcu_barrier(RCU_BARRIER_SCHED);
+}
+EXPORT_SYMBOL_GPL(rcu_barrier_sched);
+
void __init rcu_init(void)
{
__rcu_init();


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

Fix call_rcu_sched wait

> o If there are no synchronize_sched() calls for more than two
> minutes, one can see messages of the form "INFO: task
> rcu_sched_grace:924 blocked for more than 120 seconds."
> Any thoughts on how to avoid this message? Should I be using
> something other than __wait_event() and wake_up(), which sleep
> uninterruptibly, thus triggering this message?
>

Could you use __wait_event_interruptible and wake_up_interruptible
instead ? softlockup.c only seems to complain when uninterruptible tasks
are not scheduled for 2 minutes. I guess that when we receive a signal
we could simply go through another loop.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
---
kernel/rcupreempt.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

Index: linux-2.6-lttng/kernel/rcupreempt.c
===================================================================
--- linux-2.6-lttng.orig/kernel/rcupreempt.c 2008-03-24 00:26:27.000000000 -0400
+++ linux-2.6-lttng/kernel/rcupreempt.c 2008-03-24 00:33:47.000000000 -0400
@@ -1074,7 +1074,7 @@ void call_rcu_sched(struct rcu_head *hea
rcu_ctrlblk.sched_sleep = rcu_sched_not_sleeping;
spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags);
if (wake_gp)
- wake_up(&rcu_ctrlblk.sched_wq);
+ wake_up_interruptible(&rcu_ctrlblk.sched_wq);
}
}
EXPORT_SYMBOL_GPL(call_rcu_sched);
@@ -1097,6 +1097,7 @@ rcu_sched_grace_period(void *arg)
int couldsleep; /* might sleep after current pass. */
int couldsleepnext = 0; /* might sleep after next pass. */
int cpu;
+ int ret;
long err;
unsigned long flags;
int needsoftirq;
@@ -1242,8 +1243,10 @@ retry:

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);
+ ret = 0;
+ __wait_event_interruptible(rcu_ctrlblk.sched_wq,
+ rcu_ctrlblk.sched_sleep != rcu_sched_sleeping,
+ ret);
couldsleepnext = 0;

} while (!kthread_should_stop());

> One other thing -- this patch also fixes a long-standing bug in the
> earlier preemptable-RCU implementation of synchronize_rcu() that could
> result in loss of concurrent external changes to a task's CPU affinity
> mask. I have lost track of who reported this...
>

That's always good :)

Mathieu

> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>
> 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 | 276 +++++++++++++++++++++++++++++++++++++++++++--
> 6 files changed, 308 insertions(+), 29 deletions(-)
>
> diff -urpNa -X dontdiff linux-2.6.25-rc6/include/linux/rcuclassic.h linux-2.6.25-rc6-C1-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-C1-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-C1-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-C1-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-C1-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-C1-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-C1-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-C1-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-C1-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-C1-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-C1-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-C1-call_rcu_sched/kernel/rcupreempt.c 2008-03-21 12:44:24.000000000 -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 +1042,214 @@ 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)
> + wake_gp = 0;
> + rcu_ctrlblk.sched_sleep = rcu_sched_not_sleeping;
> + 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; /* might sleep after current pass. */
> + int couldsleepnext = 0; /* might sleep after next pass. */
> int cpu;
> + long err;
> + 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) {
> +
> +retry:
> +
> + /* Initialize and schedule onto current CPU. */
> +
> + needsoftirq = 0;
> + err = sched_setaffinity(0, cpumask_of_cpu(cpu));
> + if (err < 0) {
> + printk(KERN_WARNING "sched_setaffinity(%d) error: %ld, cpu_is_offline: %ld\n", cpu, err, cpu_is_offline(cpu));
> + schedule_timeout_interruptible(HZ);
> + continue;
> + }
> +
> + /*
> + * 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;
> + }
> +
> + /*
> + * If we didn't end up on the CPU we expected
> + * to, try again. This can happen if a CPU
> + * goes offline before we attempt to schedule
> + * on it, but comes back online before we get
> + * to this check.
> + */
> +
> + if (smp_processor_id() != cpu) {
> + spin_unlock_irqrestore(&rdp->lock, flags);
> + goto retry;
> + }
> +
> + /*
> + * 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;
> + couldsleep = 0;
> + couldsleepnext = 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 +1344,11 @@ 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 +1371,18 @@ 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)

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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