[PATCH 0/3] RCU tasks for v6.7

[PATCH 0/3] RCU tasks for v6.7

[Frederic Weisbecker]

Hello,

Please find below the RCU tasks updates for the next merge window:

Jiapeng Chong (1):
  rcu-tasks: Make rcu_tasks_lazy_ms static

Paul E. McKenney (2):
  rcu-tasks: Add printk()s to localize boot-time self-test hang
  rcu-tasks: Pull sampling of ->percpu_dequeue_lim out of loop

Thanks,
        Frederic.

 kernel/rcu/tasks.h | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

-- 
2.34.1

[PATCH 1/3] rcu-tasks: Add printk()s to localize boot-time self-test hang

[Frederic Weisbecker]

From: "Paul E. McKenney" <paulmck@kernel.org>

Currently, rcu_tasks_initiate_self_tests() prints a message and then
initiates self tests on up to three different RCU Tasks flavors.  If one
of the flavors has a grace-period hang, it is not easy to work out which
of the three hung.  This commit therefore prints a message prior to each
individual test.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tasks.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 8d65f7d576a3..83049a893de5 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1979,20 +1979,22 @@ static void test_rcu_tasks_callback(struct rcu_head *rhp)
 
 static void rcu_tasks_initiate_self_tests(void)
 {
-	pr_info("Running RCU-tasks wait API self tests\n");
 #ifdef CONFIG_TASKS_RCU
+	pr_info("Running RCU Tasks wait API self tests\n");
 	tests[0].runstart = jiffies;
 	synchronize_rcu_tasks();
 	call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
 #endif
 
 #ifdef CONFIG_TASKS_RUDE_RCU
+	pr_info("Running RCU Tasks Rude wait API self tests\n");
 	tests[1].runstart = jiffies;
 	synchronize_rcu_tasks_rude();
 	call_rcu_tasks_rude(&tests[1].rh, test_rcu_tasks_callback);
 #endif
 
 #ifdef CONFIG_TASKS_TRACE_RCU
+	pr_info("Running RCU Tasks Trace wait API self tests\n");
 	tests[2].runstart = jiffies;
 	synchronize_rcu_tasks_trace();
 	call_rcu_tasks_trace(&tests[2].rh, test_rcu_tasks_callback);
-- 
2.34.1

[PATCH 2/3] rcu-tasks: Pull sampling of ->percpu_dequeue_lim out of loop

[Frederic Weisbecker]

From: "Paul E. McKenney" <paulmck@kernel.org>

The rcu_tasks_need_gpcb() samples ->percpu_dequeue_lim as part of the
condition clause of a "for" loop, which is a bit confusing.  This commit
therefore hoists this sampling out of the loop, using the result loaded
in the condition clause.

So why does this work in the face of a concurrent switch from single-CPU
queueing to per-CPU queueing?

o	The call_rcu_tasks_generic() that makes the change has already
	enqueued its callback, which means that all of the other CPU's
	callback queues are empty.

o	For the call_rcu_tasks_generic() that first notices
	the switch to per-CPU queues, the smp_store_release()
	used to update ->percpu_enqueue_lim pairs with the
	raw_spin_trylock_rcu_node()'s full barrier that is
	between the READ_ONCE(rtp->percpu_enqueue_shift) and the
	rcu_segcblist_enqueue() that enqueues the callback.

o	Because this CPU's queue is empty (unless it happens to
	be the original single queue, in which case there is no
	need for synchronization), this call_rcu_tasks_generic()
	will do an irq_work_queue() to schedule a handler for the
	needed rcuwait_wake_up() call.	This call will be ordered
	after the first call_rcu_tasks_generic() function's change to
	->percpu_dequeue_lim.

o	This rcuwait_wake_up() will either happen before or after the
	set_current_state() in rcuwait_wait_event().  If it happens
	before, the "condition" argument's call to rcu_tasks_need_gpcb()
	will be ordered after the original change, and all callbacks on
	all CPUs will be visible.  Otherwise, if it happens after, then
	the grace-period kthread's state will be set back to running,
	which will result in a later call to rcuwait_wait_event() and
	thus to rcu_tasks_need_gpcb(), which will again see the change.

So it all works out.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tasks.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 83049a893de5..94bb5abdbb37 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -432,6 +432,7 @@ static void rcu_barrier_tasks_generic(struct rcu_tasks *rtp)
 static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
 {
 	int cpu;
+	int dequeue_limit;
 	unsigned long flags;
 	bool gpdone = poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);
 	long n;
@@ -439,7 +440,8 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
 	long ncbsnz = 0;
 	int needgpcb = 0;
 
-	for (cpu = 0; cpu < smp_load_acquire(&rtp->percpu_dequeue_lim); cpu++) {
+	dequeue_limit = smp_load_acquire(&rtp->percpu_dequeue_lim);
+	for (cpu = 0; cpu < dequeue_limit; cpu++) {
 		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 
 		/* Advance and accelerate any new callbacks. */
-- 
2.34.1

[PATCH 3/3] rcu-tasks: Make rcu_tasks_lazy_ms static

[Frederic Weisbecker]

From: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>

The rcu_tasks_lazy_ms variable is not used outside the file tasks.h,
so this commit marks it static.

kernel/rcu/tasks.h:1085:5: warning: symbol 'rcu_tasks_lazy_ms' was not declared. Should it be static?

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=6086
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tasks.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 94bb5abdbb37..018f03f20629 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1086,7 +1086,7 @@ void rcu_barrier_tasks(void)
 }
 EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
 
-int rcu_tasks_lazy_ms = -1;
+static int rcu_tasks_lazy_ms = -1;
 module_param(rcu_tasks_lazy_ms, int, 0444);
 
 static int __init rcu_spawn_tasks_kthread(void)
-- 
2.34.1