[PATCH] rcu: unify boost and kthread priorities

[Clark Williams <clark.williams@gmail.com>]

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Paul,

Here's the result of our conversation Friday, regarding the per-CPU and
boost 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>
---
 Documentation/kernel-parameters.txt |  6 ++++++
 init/Kconfig                        | 23 ++++++++++++-----------
 kernel/rcu/tree_plugin.h            | 16 ++++++++--------
 3 files changed, 26 insertions(+), 19 deletions(-)

diff --git a/Documentation/kernel-parameters.txt
b/Documentation/kernel-parameters.txt index 10d51c2f10d7..471f885ab24d
100644 --- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2856,6 +2856,12 @@ 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.
+
 	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 e84c6423a2e5..beacc8bb1523 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -662,30 +662,31 @@ 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 a7997e272564..dfb3b793b80a 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -30,13 +30,10 @@
 #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
-#else
-#define RCU_BOOST_PRIO RCU_KTHREAD_PRIO
+/* rcuc/rcub kthread realtime priority */
+static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
+module_param(kthread_prio, int, 0644);
 #endif
 
 #ifdef CONFIG_RCU_NOCB_CPU
@@ -112,6 +109,9 @@ static void __init rcu_bootup_announce_oddness(void)
 			pr_info("\tPoll for callbacks from no-CBs
CPUs.\n"); }
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
+#ifdef CONFIG_RCU_BOOST
+	pr_info("\tRCU kthread priority: %d.\n", kthread_prio);
+#endif
 }
 
 #ifdef CONFIG_TREE_PREEMPT_RCU
@@ -1352,7 +1352,7 @@ 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;
@@ -1369,7 +1369,7 @@ 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);
 }
 
-- 
1.9.3


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