Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

Paul E. McKenney
2014-08-07T17:48:39Z

On Thu, Aug 07, 2014 at 06:32:28PM +0200, Peter Zijlstra wrote:
> On Thu, Aug 07, 2014 at 08:43:58AM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 07, 2014 at 10:49:21AM +0200, Peter Zijlstra wrote:
> > > On Tue, Aug 05, 2014 at 02:55:10PM -0700, Paul E. McKenney wrote:
> > > > +/* Check for nohz_full CPUs executing in userspace. */
> > > > +static void check_no_hz_full_tasks(void)
> > > > +{
> > > > +#ifdef CONFIG_NO_HZ_FULL
> > > > + int cpu;
> > > > + struct task_struct *t;
> > > > +
> > > > + for_each_online_cpu(cpu) {
> > > > + cond_resched();
> > > > + rcu_read_lock();
> > > > + t = rcu_dynticks_task_cur(cpu);
> > > > + if (t == NULL || is_idle_task(t)) {
> > > > + rcu_read_unlock();
> > > > + continue;
> > > > + }
> > > > + if (ACCESS_ONCE(t->rcu_tasks_holdout))
> > > > + ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
> > > > + rcu_read_unlock();
> > > > + }
> > > > +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > > > +}
> > >
> > > That's not hotplug safe afaict, and I've no idea if someone pointed that
> > > out already because people refuse to trim email and I can't be arsed to
> > > wade through pages and pages of quoting.
> >
> > Hmmm... That does look a bit suspicious, now that you mention it.
> > After all, if a CPU is offline, its idle tasks cannot be on a
> > trampoline.
>
> But what about a CPU that in the process of getting on-line, it started
> when there still were trampolines but hasn't quite made it to full
> 'online' status when you do this check.

Well, one easy approach would be to exclude CPU hotplug during that time.

> Similar for going offline I suppose, we start to go offline when there
> were still trampolines and got missed in that check because we already
> cleared the 'online' bit but we're not quite dead yet.

Yep, same issue here.

> I couldn't find any serialization against either on or off lining of
> CPUs.

Yep, I need to fix this. The most straightforward approach seems to be
use of cpu_maps_update_begin() like in the following patch.

Thoughts?

Thanx, Paul

------------------------------------------------------------------------

rcu: Make RCU-tasks wait for idle tasks

Because idle-task code may need to be patched, RCU-tasks need to wait
for idle tasks to schedule. This commit therefore detects this case
via RCU's dyntick-idle counters. Block CPU hotplug during this time
to avoid sending IPIs to offline CPUs.

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

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0294fc180508..70f2b953c392 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -1138,14 +1138,14 @@ static inline void rcu_sysidle_force_exit(void)

#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */

-#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
+#if defined(CONFIG_TASKS_RCU) && !defined(CONFIG_TINY_RCU)
struct task_struct *rcu_dynticks_task_cur(int cpu);
-#else /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
+#else /* #if defined(CONFIG_TASKS_RCU) && !defined(CONFIG_TINY_RCU) */
static inline struct task_struct *rcu_dynticks_task_cur(int cpu)
{
return NULL;
}
-#endif /* #else #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
+#endif /* #else #if defined(CONFIG_TASKS_RCU) && !defined(CONFIG_TINY_RCU) */

#endif /* __LINUX_RCUPDATE_H */
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index d40a6a451330..b882c27cd314 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -154,7 +154,11 @@ static inline bool rcu_is_watching(void)
return true;
}

-
#endif /* #else defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) */

+static inline unsigned int rcu_dynticks_ctr(int cpu)
+{
+ return 0;
+}
+
#endif /* __LINUX_RCUTINY_H */
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 3e2f5d432743..0d8fdfcb4f0b 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -97,4 +97,6 @@ extern int rcu_scheduler_active __read_mostly;

bool rcu_is_watching(void);

+unsigned int rcu_dynticks_ctr(int cpu);
+
#endif /* __LINUX_RCUTREE_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3cf124389ec7..db4e6cb8fb77 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1277,6 +1277,7 @@ struct task_struct {
unsigned long rcu_tasks_nvcsw;
int rcu_tasks_holdout;
struct list_head rcu_tasks_holdout_list;
+ unsigned int rcu_tasks_dynticks;
#endif /* #ifdef CONFIG_TASKS_RCU */

#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 86a0a7d5bbbd..6298a66118e5 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -493,6 +493,16 @@ cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
}

/*
+ * rcu_dynticks_ctr - return value of the specified CPU's dynticks counter
+ */
+unsigned int rcu_dynticks_ctr(int cpu)
+{
+ struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+
+ return atomic_add_return(0, &rdtp->dynticks);
+}
+
+/*
* rcu_eqs_enter_common - current CPU is moving towards extended quiescent state
*
* If the new value of the ->dynticks_nesting counter now is zero,
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 1e79fa1b7cbf..e373f8ddc60a 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -88,9 +88,9 @@ struct rcu_dynticks {
/* Process level is worth LLONG_MAX/2. */
int dynticks_nmi_nesting; /* Track NMI nesting level. */
atomic_t dynticks; /* Even value for idle, else odd. */
-#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
+#if defined(CONFIG_TASKS_RCU)
struct task_struct *dynticks_tsk;
-#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
+#endif /* #if defined(CONFIG_TASKS_RCU) */
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
long long dynticks_idle_nesting;
/* irq/process nesting level from idle. */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 442d62edc564..381cb93ad3fa 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2868,7 +2868,7 @@ static void rcu_dynticks_task_exit(struct rcu_dynticks *rdtp)
rcu_dynticks_task_enter(rdtp, NULL);
}

-#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
+#if defined(CONFIG_TASKS_RCU) && !defined(CONFIG_TINY_RCU)
struct task_struct *rcu_dynticks_task_cur(int cpu)
{
struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
@@ -2877,4 +2877,4 @@ struct task_struct *rcu_dynticks_task_cur(int cpu)
smp_read_barrier_depends(); /* Dereferences after fetch of "t". */
return t;
}
-#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
+#endif /* #if defined(CONFIG_TASKS_RCU) && !defined(CONFIG_TINY_RCU) */
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 069c06fd6a79..637f8c7fc0c2 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -48,6 +48,7 @@
#include <linux/delay.h>
#include <linux/module.h>
#include <linux/kthread.h>
+#include "../sched/sched.h" /* cpu_rq()->idle */

#define CREATE_TRACE_POINTS

@@ -484,7 +485,6 @@ static void check_holdout_task(struct task_struct *t,
/* Check for nohz_full CPUs executing in userspace. */
static void check_no_hz_full_tasks(void)
{
-#ifdef CONFIG_NO_HZ_FULL
int cpu;
struct task_struct *t;

@@ -492,7 +492,11 @@ static void check_no_hz_full_tasks(void)
cond_resched();
rcu_read_lock();
t = rcu_dynticks_task_cur(cpu);
- if (t == NULL || is_idle_task(t)) {
+ if (t == NULL ||
+ (is_idle_task(t) &&
+ t->rcu_tasks_dynticks == rcu_dynticks_ctr(cpu))) {
+ if (t != NULL)
+ resched_cpu(cpu); /* Kick idle task. */
rcu_read_unlock();
continue;
}
@@ -500,12 +504,12 @@ static void check_no_hz_full_tasks(void)
ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
rcu_read_unlock();
}
-#endif /* #ifdef CONFIG_NO_HZ_FULL */
}

/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
static int __noreturn rcu_tasks_kthread(void *arg)
{
+ int cpu;
unsigned long flags;
struct task_struct *g, *t;
unsigned long lastreport;
@@ -566,8 +570,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
*/
rcu_read_lock();
for_each_process_thread(g, t) {
- if (t != current && ACCESS_ONCE(t->on_rq) &&
- !is_idle_task(t)) {
+ if (t != current && ACCESS_ONCE(t->on_rq)) {
get_task_struct(t);
t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw);
ACCESS_ONCE(t->rcu_tasks_holdout) = 1;
@@ -578,6 +581,26 @@ static int __noreturn rcu_tasks_kthread(void *arg)
rcu_read_unlock();

/*
+ * Next, queue up any currently running idle tasks.
+ * Exclude CPU hotplug during the time we are working
+ * with idle tasks, as it is considered bad form to
+ * send IPIs to offline CPUs.
+ */
+ cpu_maps_update_begin();
+ for_each_online_cpu(cpu) {
+ t = cpu_rq(cpu)->idle;
+ if (t == rcu_dynticks_task_cur(cpu)) {
+ list_add(&t->rcu_tasks_holdout_list,
+ &rcu_tasks_holdouts);
+ t->rcu_tasks_dynticks = rcu_dynticks_ctr(cpu);
+ t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw);
+ ACCESS_ONCE(t->rcu_tasks_holdout) = 1;
+ list_add(&t->rcu_tasks_holdout_list,
+ &rcu_tasks_holdouts);
+ }
+ }
+
+ /*
* Wait for tasks that are in the process of exiting.
* This does only part of the job, ensuring that all
* tasks that were previously exiting reach the point
@@ -613,6 +636,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
cond_resched();
}
}
+ cpu_maps_update_done();

/*
* Because ->on_rq and ->nvcsw are not guaranteed