[PATCH 2/6] rcu/nocb: Prepare nocb_cb_wait() to start with a non-offloaded rdp

Frederic Weisbecker
2021-11-17T15:56:55Z

In order to be able to toggle the offloaded state from cpusets, a nocb
kthread will need to be created for all possible CPUs whenever the
"rcu_nocbs=" parameter is passed.

Therefore nocb_cb_wait() kthread must prepare to start running on a
de-offloaded rdp. Simply move the sleeping condition in the beginning
of the kthread callback is enough to prevent from running callbacks
before the rdp ever becomes offloaded.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
---
kernel/rcu/tree_nocb.h | 23 ++++++++++++-----------
1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index e728128be02a..0543d5e913bb 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -789,6 +789,18 @@ static void nocb_cb_wait(struct rcu_data *rdp)
bool can_sleep = true;
struct rcu_node *rnp = rdp->mynode;

+ do {
+ swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
+ nocb_cb_wait_cond(rdp));
+
+ // VVV Ensure CB invocation follows _sleep test.
+ if (smp_load_acquire(&rdp->nocb_cb_sleep)) { // ^^^
+ WARN_ON(signal_pending(current));
+ trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
+ }
+ } while (!nocb_cb_can_run(rdp));
+
+
local_irq_save(flags);
rcu_momentary_dyntick_idle();
local_irq_restore(flags);
@@ -841,17 +853,6 @@ static void nocb_cb_wait(struct rcu_data *rdp)

if (needwake_state)
swake_up_one(&rdp->nocb_state_wq);
-
- do {
- swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
- nocb_cb_wait_cond(rdp));
-
- // VVV Ensure CB invocation follows _sleep test.
- if (smp_load_acquire(&rdp->nocb_cb_sleep)) { // ^^^
- WARN_ON(signal_pending(current));
- trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
- }
- } while (!nocb_cb_can_run(rdp));
}

/*
--
2.25.1