rcu: Unify boost and kthread priorities

Rename CONFIG_RCU_BOOST_PRIO to CONFIG_RCU_KTHREAD_PRIO and use this
value for both the per-CPU kthreads (rcuc/N) and the rcu boosting
threads (rcub/n).

Also, create the module_parameter rcutree.kthread_prio to be used on
the kernel command line at boot to set a new value (rcutree.kthread_prio=N).

Signed-off-by: Clark Williams <clark.williams@gmail.com>
[ paulmck: Ported to rcu/dev, applied Paul Bolle and Peter Zijlstra feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

5 files changed, 32 insertions, 24 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 7dbe5ec9d9cd..6608520e9fa2 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2922,6 +2922,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         quiescent states.  Units are jiffies, minimum
                         value is one, and maximum value is HZ.
 
+        rcutree.kthread_prio=    [KNL,BOOT]
+                        Set the SCHED_FIFO priority of the RCU
+                        per-CPU kthreads (rcuc/N). This value is also
+                        used for the priority of the RCU boost threads
+                        (rcub/N). Valid values are 1-99 and the default
+                        is 1 (the least-favored priority).
+
         rcutree.rcu_nocb_leader_stride= [KNL]
                         Set the number of NOCB kthread groups, which
                         defaults to the square root of the number of
diff --git a/init/Kconfig b/init/Kconfig
index 0c9546ba56e8..15c299cc7c1e 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -672,30 +672,31 @@ config RCU_BOOST
           Say Y here if you are working with real-time apps or heavy loads
           Say N here if you are unsure.
 
-config RCU_BOOST_PRIO
-        int "Real-time priority to boost RCU readers to"
+config RCU_KTHREAD_PRIO
+        int "Real-time priority to use for RCU worker threads"
         range 1 99
         depends on RCU_BOOST
         default 1
         help
-          This option specifies the real-time priority to which long-term
-          preempted RCU readers are to be boosted.  If you are working
-          with a real-time application that has one or more CPU-bound
-          threads running at a real-time priority level, you should set
-          RCU_BOOST_PRIO to a priority higher then the highest-priority
-          real-time CPU-bound thread.  The default RCU_BOOST_PRIO value
-          of 1 is appropriate in the common case, which is real-time
+          This option specifies the SCHED_FIFO priority value that will be
+          assigned to the rcuc/n and rcub/n threads and is also the value
+          used for RCU_BOOST (if enabled). If you are working with a
+          real-time application that has one or more CPU-bound threads
+          running at a real-time priority level, you should set
+          RCU_KTHREAD_PRIO to a priority higher than the highest-priority
+          real-time CPU-bound application thread.  The default RCU_KTHREAD_PRIO
+          value of 1 is appropriate in the common case, which is real-time
           applications that do not have any CPU-bound threads.
 
           Some real-time applications might not have a single real-time
           thread that saturates a given CPU, but instead might have
           multiple real-time threads that, taken together, fully utilize
-          that CPU.  In this case, you should set RCU_BOOST_PRIO to
+          that CPU.  In this case, you should set RCU_KTHREAD_PRIO to
           a priority higher than the lowest-priority thread that is
           conspiring to prevent the CPU from running any non-real-time
           tasks.  For example, if one thread at priority 10 and another
           thread at priority 5 are between themselves fully consuming
-          the CPU time on a given CPU, then RCU_BOOST_PRIO should be
+          the CPU time on a given CPU, then RCU_KTHREAD_PRIO should be
           set to priority 6 or higher.
 
           Specify the real-time priority, or take the default if unsure.
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 357891104ea0..344f0e661515 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -30,12 +30,13 @@
 #include <linux/smpboot.h>
 #include "../time/tick-internal.h"
 
-#define RCU_KTHREAD_PRIO 1
-
 #ifdef CONFIG_RCU_BOOST
 
 #include "../locking/rtmutex_common.h"
-#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
+
+/* rcuc/rcub kthread realtime priority */
+static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
+module_param(kthread_prio, int, 0644);
 
 /*
  * Control variables for per-CPU and per-rcu_node kthreads.  These
@@ -46,11 +47,7 @@ DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
 DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
 DEFINE_PER_CPU(char, rcu_cpu_has_work);
 
-#else /* #ifdef CONFIG_RCU_BOOST */
-
-#define RCU_BOOST_PRIO RCU_KTHREAD_PRIO
-
-#endif /* #else #ifdef CONFIG_RCU_BOOST */
+#endif /* #ifdef CONFIG_RCU_BOOST */
 
 #ifdef CONFIG_RCU_NOCB_CPU
 static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
@@ -98,6 +95,9 @@ static void __init rcu_bootup_announce_oddness(void)
                 pr_info("\tBoot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);
         if (nr_cpu_ids != NR_CPUS)
                 pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
+#ifdef CONFIG_RCU_BOOST
+        pr_info("\tRCU kthread priority: %d.\n", kthread_prio);
+#endif
 }
 
 #ifdef CONFIG_TREE_PREEMPT_RCU
@@ -1339,7 +1339,7 @@ static int rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
         smp_mb__after_unlock_lock();
         rnp->boost_kthread_task = t;
         raw_spin_unlock_irqrestore(&rnp->lock, flags);
-        sp.sched_priority = RCU_BOOST_PRIO;
+        sp.sched_priority = kthread_prio;
         sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
         wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
         return 0;
@@ -1356,7 +1356,7 @@ static void rcu_cpu_kthread_setup(unsigned int cpu)
 {
         struct sched_param sp;
 
-        sp.sched_priority = RCU_KTHREAD_PRIO;
+        sp.sched_priority = kthread_prio;
         sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
 }
 
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE03 b/tools/testing/selftests/rcutorture/configs/rcu/TREE03
index f4567fb3e332..b95f62efd6f2 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE03
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE03
@@ -17,5 +17,5 @@ CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_RCU_CPU_STALL_INFO=n
 CONFIG_RCU_CPU_STALL_VERBOSE=n
 CONFIG_RCU_BOOST=y
-CONFIG_RCU_BOOST_PRIO=2
+CONFIG_RCU_KTHREAD_PRIO=2
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
diff --git a/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt b/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
index 3e588db86a17..ab6e7b4103ac 100644
--- a/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
+++ b/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
@@ -15,8 +15,8 @@ CONFIG_PREEMPT -- Do half.  (First three and #8.)
 CONFIG_PROVE_LOCKING -- Do all but two, covering CONFIG_PROVE_RCU and not.
 CONFIG_PROVE_RCU -- Do all but one under CONFIG_PROVE_LOCKING.
 CONFIG_RCU_BOOST -- one of TREE_PREEMPT_RCU.
-CONFIG_RCU_BOOST_PRIO -- set to 2 for _BOOST testing.
-CONFIG_RCU_CPU_STALL_INFO -- do one with and without _VERBOSE.
+CONFIG_RCU_KTHREAD_PRIO -- set to 2 for _BOOST testing.
+CONFIG_RCU_CPU_STALL_INFO -- Do one.
 CONFIG_RCU_CPU_STALL_VERBOSE -- do one with and without _INFO.
 CONFIG_RCU_FANOUT -- Cover hierarchy as currently, but overlap with others.
 CONFIG_RCU_FANOUT_EXACT -- Do one.