rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread

rcu_core() tries to ensure that its self-invocation in case of callbacks
overload only happen in softirq/rcuc mode. Indeed it doesn't make sense
to trigger local RCU core from nocb_cb kthread since it can execute
on a CPU different from the target rdp. Also in case of overload, the
nocb_cb kthread simply iterates a new loop of callbacks processing.

However the "offloaded" check that aims at preventing misplaced
rcu_core() invocations is wrong. First of all that state is volatile
and second: softirq/rcuc can execute while the target rdp is offloaded.
As a result rcu_core() can be invoked on the wrong CPU while in the
process of (de-)offloading.

Fix that with moving the rcu_core() self-invocation to rcu_core() itself,
irrespective of the rdp offloaded state.

Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1b31fad9f1a0a5b282eb9bdd8fab973401f03b61..03c038f324a097a991b3b8955f31c40f3265d47c 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2460,7 +2460,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
        int div;
        bool __maybe_unused empty;
        unsigned long flags;
-       const bool offloaded = rcu_rdp_is_offloaded(rdp);
        struct rcu_head *rhp;
        struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
        long bl, count = 0;
@@ -2582,9 +2581,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
 
        rcu_nocb_unlock_irqrestore(rdp, flags);
 
-       /* Re-invoke RCU core processing if there are callbacks remaining. */
-       if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist))
-               invoke_rcu_core();
        tick_dep_clear_task(current, TICK_DEP_BIT_RCU);
 }
 
@@ -2771,8 +2767,12 @@ static __latent_entropy void rcu_core(void)
 
        /* If there are callbacks ready, invoke them. */
        if (do_batch && rcu_segcblist_ready_cbs(&rdp->cblist) &&
-           likely(READ_ONCE(rcu_scheduler_fully_active)))
+           likely(READ_ONCE(rcu_scheduler_fully_active))) {
                rcu_do_batch(rdp);
+               /* Re-invoke RCU core processing if there are callbacks remaining. */
+               if (rcu_segcblist_ready_cbs(&rdp->cblist))
+                       invoke_rcu_core();
+       }
 
        /* Do any needed deferred wakeups of rcuo kthreads. */
        do_nocb_deferred_wakeup(rdp);