[PATCH rcu 0/7] RCU Tasks updates for v6.1

[PATCH rcu 0/7] RCU Tasks updates for v6.1

[Paul E. McKenney]

Hello!

This series provides some RCU Tasks updates:

1.	rcu-tasks: Convert RCU_LOCKDEP_WARN() to WARN_ONCE(), courtesy
	of Zqiang.

2.	rcu-tasks: Ensure RCU Tasks Trace loops have quiescent states.

3.	rcu-tasks: Make RCU Tasks Trace check for userspace execution,
	courtesy of Zqiang.

						Thanx, Paul

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

 b/kernel/rcu/tasks.h       |    2 +-
 b/kernel/rcu/tree.c        |    4 ++--
 b/kernel/rcu/tree_plugin.h |    4 ----
 kernel/rcu/tasks.h         |    3 +++
 4 files changed, 6 insertions(+), 7 deletions(-)

[PATCH rcu 1/3] rcu-tasks: Convert RCU_LOCKDEP_WARN() to WARN_ONCE()

[Paul E. McKenney]

From: Zqiang <qiang1.zhang@intel.com>

Kernels built with CONFIG_PROVE_RCU=y and CONFIG_DEBUG_LOCK_ALLOC=y
attempt to emit a warning when the synchronize_rcu_tasks_generic()
function is called during early boot while the rcu_scheduler_active
variable is RCU_SCHEDULER_INACTIVE.  However the warnings is not
actually be printed because the debug_lockdep_rcu_enabled() returns
false, exactly because the rcu_scheduler_active variable is still equal
to RCU_SCHEDULER_INACTIVE.

This commit therefore replaces RCU_LOCKDEP_WARN() with WARN_ONCE()
to force these warnings to actually be printed.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@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 83c7e6620d403..469bf2a3b505e 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -560,7 +560,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
 {
 	/* Complain if the scheduler has not started.  */
-	RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
+	WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
 			 "synchronize_rcu_tasks called too soon");
 
 	// If the grace-period kthread is running, use it.
-- 
2.31.1.189.g2e36527f23

[PATCH rcu 2/3] rcu-tasks: Ensure RCU Tasks Trace loops have quiescent states

[Paul E. McKenney]

The RCU Tasks Trace grace-period kthread loops across all CPUs, and
there can be quite a few CPUs, with some commercially available systems
sporting well over a thousand of them.  Some of these loops can feature
IPIs, which can take some time.  This commit therefore places a call to
cond_resched_tasks_rcu_qs() in each such loop.

Link: https://docs.google.com/document/d/1V0YnG1HTWMt9WHJjroiJL9lf-hMrud4v8Fn3fhyY0cI/edit?usp=sharing
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tasks.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 469bf2a3b505e..f5bf6fb430dab 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1500,6 +1500,7 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop)
 		if (rcu_tasks_trace_pertask_prep(t, true))
 			trc_add_holdout(t, hop);
 		rcu_read_unlock();
+		cond_resched_tasks_rcu_qs();
 	}
 
 	// Only after all running tasks have been accounted for is it
@@ -1520,6 +1521,7 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop)
 			raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 		}
 		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+		cond_resched_tasks_rcu_qs();
 	}
 
 	// Re-enable CPU hotplug now that the holdout list is populated.
@@ -1619,6 +1621,7 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
 			trc_del_holdout(t);
 		else if (needreport)
 			show_stalled_task_trace(t, firstreport);
+		cond_resched_tasks_rcu_qs();
 	}
 
 	// Re-enable CPU hotplug now that the holdout list scan has completed.
-- 
2.31.1.189.g2e36527f23

[PATCH rcu 3/3] rcu-tasks: Make RCU Tasks Trace check for userspace execution

[Paul E. McKenney]

From: Zqiang <qiang1.zhang@intel.com>

Userspace execution is a valid quiescent state for RCU Tasks Trace,
but the scheduling-clock interrupt does not currently report such
quiescent states.

Of course, the scheduling-clock interrupt is not strictly speaking
userspace execution.  However, the only way that this code is not
in a quiescent state is if something invoked rcu_read_lock_trace(),
and that would be reflected in the ->trc_reader_nesting field in
the task_struct structure.  Furthermore, this field is checked by
rcu_tasks_trace_qs(), which is invoked by rcu_tasks_qs() which is in
turn invoked by rcu_note_voluntary_context_switch() in kernels building
at least one of the RCU Tasks flavors.  It is therefore safe to invoke
rcu_tasks_trace_qs() from the rcu_sched_clock_irq().

But rcu_tasks_qs() also invokes rcu_tasks_classic_qs() for RCU
Tasks, which lacks the read-side markers provided by RCU Tasks Trace.
This raises the possibility that an RCU Tasks grace period could start
after the interrupt from userspace execution, but before the call to
rcu_sched_clock_irq().  However, it turns out that this is safe because
the RCU Tasks grace period waits for an RCU grace period, which will
wait for the entire scheduling-clock interrupt handler, including any
RCU Tasks read-side critical section that this handler might contain.

This commit therefore updates the rcu_sched_clock_irq() function's
check for usermode execution and its call to rcu_tasks_classic_qs()
to instead check for both usermode execution and interrupt from idle,
and to instead call rcu_note_voluntary_context_switch().  This
consolidates code and provides more faster RCU Tasks Trace
reporting of quiescent states in kernels that do scheduling-clock
interrupts for userspace execution.

[ paulmck: Consolidate checks into rcu_sched_clock_irq(). ]

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree.c        | 4 ++--
 kernel/rcu/tree_plugin.h | 4 ----
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 79aea7df4345e..11d5aefd16961 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2341,8 +2341,8 @@ void rcu_sched_clock_irq(int user)
 	rcu_flavor_sched_clock_irq(user);
 	if (rcu_pending(user))
 		invoke_rcu_core();
-	if (user)
-		rcu_tasks_classic_qs(current, false);
+	if (user || rcu_is_cpu_rrupt_from_idle())
+		rcu_note_voluntary_context_switch(current);
 	lockdep_assert_irqs_disabled();
 
 	trace_rcu_utilization(TPS("End scheduler-tick"));
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 438ecae6bd7e7..aa64b035c24f2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -718,9 +718,6 @@ static void rcu_flavor_sched_clock_irq(int user)
 	struct task_struct *t = current;
 
 	lockdep_assert_irqs_disabled();
-	if (user || rcu_is_cpu_rrupt_from_idle()) {
-		rcu_note_voluntary_context_switch(current);
-	}
 	if (rcu_preempt_depth() > 0 ||
 	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
 		/* No QS, force context switch if deferred. */
@@ -972,7 +969,6 @@ static void rcu_flavor_sched_clock_irq(int user)
 		 * neither access nor modify, at least not while the
 		 * corresponding CPU is online.
 		 */
-
 		rcu_qs();
 	}
 }
-- 
2.31.1.189.g2e36527f23