[PATCH] rcu/nocb: Add an option to ON/OFF an offloading from RT context

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

Introduce a RCU_NOCB_CPU_CB_BOOST kernel option. So a user can
decide if an offloading has to be done in a high-prio context or
not. Please note an option depends on RCU_NOCB_CPU and RCU_BOOST
parameters and by default it is off.

This patch splits the boosting preempted RCU readers and those
kthreads which directly responsible for driving expedited grace
periods forward with enabling/disabling the offloading from/to
SCHED_FIFO/SCHED_OTHER contexts.

The main reason of such split is, for example on Android there
are some workloads which require fast expedited grace period to
be done whereas offloading in RT context can lead to starvation
and hogging a CPU for a long time what is not acceptable for
latency sensitive environment. For instance:

<snip>
<...>-60 [006] d..1 2979.028717: rcu_batch_start: rcu_preempt CBs=34619 bl=270
<snip>

invoking 34 619 callbacks will take time thus making other CFS
tasks waiting in run-queue to be starved due to such behaviour.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/Kconfig     | 14 ++++++++++++++
 kernel/rcu/tree.c      |  5 ++++-
 kernel/rcu/tree_nocb.h |  3 ++-
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 27aab870ae4c..074630b94902 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -275,6 +275,20 @@ config RCU_NOCB_CPU_DEFAULT_ALL
 	  Say Y here if you want offload all CPUs by default on boot.
 	  Say N here if you are unsure.
 
+config RCU_NOCB_CPU_CB_BOOST
+	bool "Perform offloading from real-time kthread"
+	depends on RCU_NOCB_CPU && RCU_BOOST
+	default n
+	help
+	  Use this option to offload callbacks from the SCHED_FIFO context
+	  to make the process faster. As a side effect of this approach is
+	  a latency especially for the SCHED_OTHER tasks which will not be
+	  able to preempt an offloading kthread. That latency depends on a
+	  number of callbacks to be invoked.
+
+	  Say Y here if you want to set RT priority for offloading kthreads.
+	  Say N here if you are unsure.
+
 config TASKS_TRACE_RCU_READ_MB
 	bool "Tasks Trace RCU readers use memory barriers in user and idle"
 	depends on RCU_EXPERT && TASKS_TRACE_RCU
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9dc4c4e82db6..d769a15bc0e3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -154,7 +154,10 @@ 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/rcuop kthread realtime priority */
+/*
+ * rcuc/rcub/rcuop kthread realtime priority. The former
+ * depends on if CONFIG_RCU_NOCB_CPU_CB_BOOST is set.
+ */
 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 60cc92cc6655..a2823be9b1d0 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1315,8 +1315,9 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
 	if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
 		goto end;
 
-	if (kthread_prio)
+	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_CB_BOOST))
 		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);
 	return;
-- 
2.30.2