linux-kernel - [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might not be NOCB

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Date:   Wed, 13 May 2020 18:47:11 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>
Subject: [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might not be NOCB

So far nohz_full CPUs had to be nocb. This requirement may change
temporarily as we are working on preparing RCU to be able to toggle the
nocb state of a CPU. Once that is done and nohz_full can be toggled as
well dynamically, we'll restore that initial requirement.

Thus for now as a temporary state, make rcu_nohz_full_cpu() aware of
nohz_full CPUs that are not nocb so that they can handle the callbacks
locally.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
---
 kernel/rcu/tree.c        | 2 +-
 kernel/rcu/tree.h        | 2 +-
 kernel/rcu/tree_plugin.h | 7 ++++---
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cc95419f6491..74b6798309ef 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3223,7 +3223,7 @@ static int rcu_pending(int user)
 		return 1;
 
 	/* Is this a nohz_full CPU in userspace or idle?  (Ignore RCU if so.) */
-	if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu())
+	if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu(rdp))
 		return 0;
 
 	/* Is the RCU core waiting for a quiescent state from this CPU? */
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 9dc2ec021da5..4b9643d9f5e0 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -451,7 +451,7 @@ do {									\
 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
 
 static void rcu_bind_gp_kthread(void);
-static bool rcu_nohz_full_cpu(void);
+static bool rcu_nohz_full_cpu(struct rcu_data *rdp);
 static void rcu_dynticks_task_enter(void);
 static void rcu_dynticks_task_exit(void);
 
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 43ecc047af26..f19e81e0c691 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2532,13 +2532,14 @@ static void show_rcu_nocb_state(struct rcu_data *rdp)
  * The idea is to avoid waking up RCU core processing on such a
  * CPU unless the grace period has extended for too long.
  *
- * This code relies on the fact that all NO_HZ_FULL CPUs are also
- * CONFIG_RCU_NOCB_CPU CPUs.
+ * This code relies on the fact that NO_HZ_FULL CPUs might not
+ * be CONFIG_RCU_NOCB_CPU CPUs (temporary development state).
  */
-static bool rcu_nohz_full_cpu(void)
+static bool rcu_nohz_full_cpu(struct rcu_data *rdp)
 {
 #ifdef CONFIG_NO_HZ_FULL
 	if (tick_nohz_full_cpu(smp_processor_id()) &&
+	    rcu_segcblist_is_offloaded(&rdp->cblist) &&
 	    (!rcu_gp_in_progress() ||
 	     ULONG_CMP_LT(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
 		return true;
-- 
2.25.0