[PATCH 10/10] rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread

Frederic Weisbecker
2021-10-19T00:09:03Z

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>
---
kernel/rcu/tree.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0ca4f4ec724d..8270e58cd0f3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2470,7 +2470,6 @@ 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;
@@ -2592,9 +2591,6 @@ 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);
}

@@ -2781,8 +2777,12 @@ 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);
--
2.25.1