linux-kernel - [PATCH 1/2] rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU

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Date:   Tue,  6 Jul 2021 01:43:43 +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>,
        Peter Zijlstra <peterz@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 1/2] rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU

cond_resched() reports an RCU quiescent state only in non-preemptible
TREE RCU implementation. Provide an explanation for the different
behaviour in CONFIG_PREEMPT_RCU=y.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Uladzislau Rezki <urezki@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/core.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cf16f8fda9a6..db374cb38eb2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7780,6 +7780,19 @@ int __sched __cond_resched(void)
 		preempt_schedule_common();
 		return 1;
 	}
+	/*
+	 * A process spending a long time in the kernel space might
+	 * have too few opportunities to report quiescent states
+	 * when CONFIG_PREEMPT_RCU=n because then the tick can't know
+	 * if it's interrupting an RCU read side critical section. In the
+	 * absence of voluntary sleeps, the last resort resides in tracking
+	 * calls to cond_resched() which always imply quiescent states.
+	 *
+	 * On the other hand, preemptible RCU has a real RCU read side
+	 * tracking that allows the tick for reporting interrupted quiescent
+	 * states or, in the worst case, deferred quiescent states after
+	 * rcu_read_unlock().
+	 */
 #ifndef CONFIG_PREEMPT_RCU
 	rcu_all_qs();
 #endif
-- 
2.25.1