[PATCH 3/4] RCU: elevate priority of offloaded callback threads

[Alison Chaiken]

When CONFIG_PREEMPT_RT=y, the rcutree.kthread_prio command-line
parameter signals initialization code to boost the priority of rcuc
callbacks to the designated value.  With the additional
CONFIG_RCU_NOCB_CPU=y configuration and an additional rcu_nocbs
command-line parameter, the callbacks on the listed cores are
offloaded to new rcuop threads that are not pinned to the cores whose
post-grace-period work is performed.  While the rcuop threads perform
the same function as the rcuc threads they offload, the kthread_prio
parameter only boosts the priority of the rcuc threads.  Fix this
inconsistency by elevating rcuop threads to the same priority as rcuc
ones.

Signed-off-by: Alison Chaiken <achaiken@xxxxxxxxxxx>
---
 kernel/rcu/tree.c      | 2 +-
 kernel/rcu/tree_nocb.h | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 15cabb288a1c..ba9eaa9a72b0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -153,7 +153,7 @@ static void sync_sched_exp_online_cleanup(int cpu);
 static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);
 static bool rcu_rdp_is_offloaded(struct rcu_data *rdp);
 
-/* rcuc/rcub kthread realtime priority */
+/* rcuc/rcub/rcuop kthread realtime priority */
 static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
 module_param(kthread_prio, int, 0444);
 
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 08b5c0042979..3343e42685dc 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1210,6 +1210,9 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
 			"rcuo%c/%d", rcu_state.abbr, cpu);
 	if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
 		return;
+
+	if (kthread_prio)
+		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
 	WRITE_ONCE(rdp->nocb_cb_kthread, t);
 	WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
 }
-- 
2.32.0