rcu: Eliminate NOCBs CPU-state Kconfig options

The CONFIG_RCU_NOCB_CPU_ALL, CONFIG_RCU_NOCB_CPU_NONE, and
CONFIG_RCU_NOCB_CPU_ZERO Kconfig options are used only in testing and
are redundant with the rcu_nocbs= boot parameter.  This commit therefore
removes these three Kconfig options and adjusts the rcutorture scripts
to use the boot parameter instead.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/kernel-per-CPU-kthreads.txt

@@ -109,13 +109,12 @@ SCHED_SOFTIRQ: Do all of the following:
 	on that CPU.  If a thread that expects to run on the de-jittered
 	CPU awakens, the scheduler will send an IPI that can result in
 	a subsequent SCHED_SOFTIRQ.
-2.	Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
-	CONFIG_NO_HZ_FULL=y, and, in addition, ensure that the CPU
-	to be de-jittered is marked as an adaptive-ticks CPU using the
-	"nohz_full=" boot parameter.  This reduces the number of
-	scheduler-clock interrupts that the de-jittered CPU receives,
-	minimizing its chances of being selected to do the load balancing
-	work that runs in SCHED_SOFTIRQ context.
+2.	CONFIG_NO_HZ_FULL=y and ensure that the CPU to be de-jittered
+	is marked as an adaptive-ticks CPU using the "nohz_full="
+	boot parameter.  This reduces the number of scheduler-clock
+	interrupts that the de-jittered CPU receives, minimizing its
+	chances of being selected to do the load balancing work that
+	runs in SCHED_SOFTIRQ context.
 3.	To the extent possible, keep the CPU out of the kernel when it
 	is non-idle, for example, by avoiding system calls and by
 	forcing both kernel threads and interrupts to execute elsewhere.
@@ -135,11 +134,10 @@ HRTIMER_SOFTIRQ:  Do all of the following:
 RCU_SOFTIRQ:  Do at least one of the following:
 1.	Offload callbacks and keep the CPU in either dyntick-idle or
 	adaptive-ticks state by doing all of the following:
-	a.	Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
-		CONFIG_NO_HZ_FULL=y, and, in addition ensure that the CPU
-		to be de-jittered is marked as an adaptive-ticks CPU using
-		the "nohz_full=" boot parameter.  Bind the rcuo kthreads
-		to housekeeping CPUs, which can tolerate OS jitter.
+	a.	CONFIG_NO_HZ_FULL=y and ensure that the CPU to be
+		de-jittered is marked as an adaptive-ticks CPU using the
+		"nohz_full=" boot parameter.  Bind the rcuo kthreads to
+		housekeeping CPUs, which can tolerate OS jitter.
 	b.	To the extent possible, keep the CPU out of the kernel
 		when it is non-idle, for example, by avoiding system
 		calls and by forcing both kernel threads and interrupts
@@ -236,11 +234,10 @@ To reduce its OS jitter, do at least one of the following:
 	is feasible only if your workload never requires RCU priority
 	boosting, for example, if you ensure frequent idle time on all
 	CPUs that might execute within the kernel.
-3.	Build with CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y,
-	which offloads all RCU callbacks to kthreads that can be moved
-	off of CPUs susceptible to OS jitter.  This approach prevents the
-	rcuc/%u kthreads from having any work to do, so that they are
-	never awakened.
+3.	Build with CONFIG_RCU_NOCB_CPU=y and boot with the rcu_nocbs=
+	boot parameter offloading RCU callbacks from all CPUs susceptible
+	to OS jitter.  This approach prevents the rcuc/%u kthreads from
+	having any work to do, so that they are never awakened.
 4.	Ensure that the CPU never enters the kernel, and, in particular,
 	avoid initiating any CPU hotplug operations on this CPU.  This is
 	another way of preventing any callbacks from being queued on the

Documentation/timers/NO_HZ.txt

@@ -194,32 +194,9 @@ that the RCU callbacks are processed in a timely fashion.
 
 Another approach is to offload RCU callback processing to "rcuo" kthreads
 using the CONFIG_RCU_NOCB_CPU=y Kconfig option.  The specific CPUs to
-offload may be selected via several methods:
-
-1.	One of three mutually exclusive Kconfig options specify a
-	build-time default for the CPUs to offload:
-
-	a.	The CONFIG_RCU_NOCB_CPU_NONE=y Kconfig option results in
-		no CPUs being offloaded.
-
-	b.	The CONFIG_RCU_NOCB_CPU_ZERO=y Kconfig option causes
-		CPU 0 to be offloaded.
-
-	c.	The CONFIG_RCU_NOCB_CPU_ALL=y Kconfig option causes all
-		CPUs to be offloaded.  Note that the callbacks will be
-		offloaded to "rcuo" kthreads, and that those kthreads
-		will in fact run on some CPU.  However, this approach
-		gives fine-grained control on exactly which CPUs the
-		callbacks run on, along with their scheduling priority
-		(including the default of SCHED_OTHER), and it further
-		allows this control to be varied dynamically at runtime.
-
-2.	The "rcu_nocbs=" kernel boot parameter, which takes a comma-separated
-	list of CPUs and CPU ranges, for example, "1,3-5" selects CPUs 1,
-	3, 4, and 5.  The specified CPUs will be offloaded in addition to
-	any CPUs specified as offloaded by CONFIG_RCU_NOCB_CPU_ZERO=y or
-	CONFIG_RCU_NOCB_CPU_ALL=y.  This means that the "rcu_nocbs=" boot
-	parameter has no effect for kernels built with RCU_NOCB_CPU_ALL=y.
+offload may be selected using The "rcu_nocbs=" kernel boot parameter,
+which takes a comma-separated list of CPUs and CPU ranges, for example,
+"1,3-5" selects CPUs 1, 3, 4, and 5.
 
 The offloaded CPUs will never queue RCU callbacks, and therefore RCU
 never prevents offloaded CPUs from entering either dyntick-idle mode

init/Kconfig

@@ -709,59 +709,6 @@ config RCU_NOCB_CPU
 	  Say Y here if you want to help to debug reduced OS jitter.
 	  Say N here if you are unsure.
 
-choice
-	prompt "Build-forced no-CBs CPUs"
-	default RCU_NOCB_CPU_NONE
-	depends on RCU_NOCB_CPU
-	help
-	  This option allows no-CBs CPUs (whose RCU callbacks are invoked
-	  from kthreads rather than from softirq context) to be specified
-	  at build time.  Additional no-CBs CPUs may be specified by
-	  the rcu_nocbs= boot parameter.
-
-config RCU_NOCB_CPU_NONE
-	bool "No build_forced no-CBs CPUs"
-	help
-	  This option does not force any of the CPUs to be no-CBs CPUs.
-	  Only CPUs designated by the rcu_nocbs= boot parameter will be
-	  no-CBs CPUs, whose RCU callbacks will be invoked by per-CPU
-	  kthreads whose names begin with "rcuo".  All other CPUs will
-	  invoke their own RCU callbacks in softirq context.
-
-	  Select this option if you want to choose no-CBs CPUs at
-	  boot time, for example, to allow testing of different no-CBs
-	  configurations without having to rebuild the kernel each time.
-
-config RCU_NOCB_CPU_ZERO
-	bool "CPU 0 is a build_forced no-CBs CPU"
-	help
-	  This option forces CPU 0 to be a no-CBs CPU, so that its RCU
-	  callbacks are invoked by a per-CPU kthread whose name begins
-	  with "rcuo".	Additional CPUs may be designated as no-CBs
-	  CPUs using the rcu_nocbs= boot parameter will be no-CBs CPUs.
-	  All other CPUs will invoke their own RCU callbacks in softirq
-	  context.
-
-	  Select this if CPU 0 needs to be a no-CBs CPU for real-time
-	  or energy-efficiency reasons, but the real reason it exists
-	  is to ensure that randconfig testing covers mixed systems.
-
-config RCU_NOCB_CPU_ALL
-	bool "All CPUs are build_forced no-CBs CPUs"
-	help
-	  This option forces all CPUs to be no-CBs CPUs.  The rcu_nocbs=
-	  boot parameter will be ignored.  All CPUs' RCU callbacks will
-	  be executed in the context of per-CPU rcuo kthreads created for
-	  this purpose.  Assuming that the kthreads whose names start with
-	  "rcuo" are bound to "housekeeping" CPUs, this reduces OS jitter
-	  on the remaining CPUs, but might decrease memory locality during
-	  RCU-callback invocation, thus potentially degrading throughput.
-
-	  Select this if all CPUs need to be no-CBs CPUs for real-time
-	  or energy-efficiency reasons.
-
-endchoice
-
 endmenu # "RCU Subsystem"
 
 config BUILD_BIN2C

kernel/rcu/rcu.h

@@ -564,9 +564,7 @@ void rcu_bh_force_quiescent_state(void);
 void rcu_sched_force_quiescent_state(void);
 #endif /* #else #ifdef CONFIG_TINY_RCU */
 
-#if defined(CONFIG_RCU_NOCB_CPU_ALL)
-static inline bool rcu_is_nocb_cpu(int cpu) { return true; }
-#elif defined(CONFIG_RCU_NOCB_CPU)
+#ifdef CONFIG_RCU_NOCB_CPU
 bool rcu_is_nocb_cpu(int cpu);
 #else
 static inline bool rcu_is_nocb_cpu(int cpu) { return false; }

kernel/rcu/tree_plugin.h

@@ -1296,8 +1296,7 @@ static void rcu_prepare_kthreads(int cpu)
 int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 {
 	*nextevt = KTIME_MAX;
-	return IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL)
-	       ? 0 : rcu_cpu_has_callbacks(NULL);
+	return rcu_cpu_has_callbacks(NULL);
 }
 
 /*
@@ -1409,10 +1408,6 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 	unsigned long dj;
 
 	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_needs_cpu() invoked with irqs enabled!!!");
-	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL)) {
-		*nextevt = KTIME_MAX;
-		return 0;
-	}
 
 	/* Snapshot to detect later posting of non-lazy callback. */
 	rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted;
@@ -1462,8 +1457,7 @@ static void rcu_prepare_for_idle(void)
 	int tne;
 
 	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_prepare_for_idle() invoked with irqs enabled!!!");
-	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) ||
-	    rcu_is_nocb_cpu(smp_processor_id()))
+	if (rcu_is_nocb_cpu(smp_processor_id()))
 		return;
 
 	/* Handle nohz enablement switches conservatively. */
@@ -1518,8 +1512,7 @@ static void rcu_prepare_for_idle(void)
 static void rcu_cleanup_after_idle(void)
 {
 	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_cleanup_after_idle() invoked with irqs enabled!!!");
-	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) ||
-	    rcu_is_nocb_cpu(smp_processor_id()))
+	if (rcu_is_nocb_cpu(smp_processor_id()))
 		return;
 	if (rcu_try_advance_all_cbs())
 		invoke_rcu_core();
@@ -1786,7 +1779,6 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
 	init_swait_queue_head(&rnp->nocb_gp_wq[1]);
 }
 
-#ifndef CONFIG_RCU_NOCB_CPU_ALL
 /* Is the specified CPU a no-CBs CPU? */
 bool rcu_is_nocb_cpu(int cpu)
 {
@@ -1794,7 +1786,6 @@ bool rcu_is_nocb_cpu(int cpu)
 		return cpumask_test_cpu(cpu, rcu_nocb_mask);
 	return false;
 }
-#endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
 
 /*
  * Kick the leader kthread for this NOCB group.
@@ -2253,10 +2244,6 @@ void __init rcu_init_nohz(void)
 	bool need_rcu_nocb_mask = true;
 	struct rcu_state *rsp;
 
-#ifdef CONFIG_RCU_NOCB_CPU_NONE
-	need_rcu_nocb_mask = false;
-#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
-
 #if defined(CONFIG_NO_HZ_FULL)
 	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
 		need_rcu_nocb_mask = true;
@@ -2272,14 +2259,6 @@ void __init rcu_init_nohz(void)
 	if (!have_rcu_nocb_mask)
 		return;
 
-#ifdef CONFIG_RCU_NOCB_CPU_ZERO
-	pr_info("\tOffload RCU callbacks from CPU 0\n");
-	cpumask_set_cpu(0, rcu_nocb_mask);
-#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
-#ifdef CONFIG_RCU_NOCB_CPU_ALL
-	pr_info("\tOffload RCU callbacks from all CPUs\n");
-	cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
-#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
 #if defined(CONFIG_NO_HZ_FULL)
 	if (tick_nohz_full_running)
 		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);

tools/testing/selftests/rcutorture/configs/rcu/TREE01

@@ -12,7 +12,6 @@ CONFIG_HOTPLUG_CPU=y
 CONFIG_MAXSMP=y
 CONFIG_CPUMASK_OFFSTACK=y
 CONFIG_RCU_NOCB_CPU=y
-CONFIG_RCU_NOCB_CPU_ZERO=y
 CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_RCU_BOOST=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n

tools/testing/selftests/rcutorture/configs/rcu/TREE01.boot

@@ -2,3 +2,4 @@ rcutorture.torture_type=rcu_bh maxcpus=8
 rcutree.gp_preinit_delay=3
 rcutree.gp_init_delay=3
 rcutree.gp_cleanup_delay=3
+rcu_nocbs=0

tools/testing/selftests/rcutorture/configs/rcu/TREE05

@@ -13,7 +13,6 @@ CONFIG_HOTPLUG_CPU=y
 CONFIG_RCU_FANOUT=6
 CONFIG_RCU_FANOUT_LEAF=6
 CONFIG_RCU_NOCB_CPU=y
-CONFIG_RCU_NOCB_CPU_NONE=y
 CONFIG_DEBUG_LOCK_ALLOC=y
 CONFIG_PROVE_LOCKING=y
 #CHECK#CONFIG_PROVE_RCU=y

tools/testing/selftests/rcutorture/configs/rcu/TREE08

@@ -15,7 +15,6 @@ CONFIG_HIBERNATION=n
 CONFIG_RCU_FANOUT=3
 CONFIG_RCU_FANOUT_LEAF=2
 CONFIG_RCU_NOCB_CPU=y
-CONFIG_RCU_NOCB_CPU_ALL=y
 CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_PROVE_LOCKING=n
 CONFIG_RCU_BOOST=n

tools/testing/selftests/rcutorture/configs/rcu/TREE08.boot

@@ -2,3 +2,4 @@ rcutorture.torture_type=sched
 rcupdate.rcu_self_test=1
 rcupdate.rcu_self_test_sched=1
 rcutree.rcu_fanout_exact=1
+rcu_nocbs=0-7

tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt

@@ -16,11 +16,9 @@ CONFIG_PROVE_RCU -- Hardwired to CONFIG_PROVE_LOCKING.
 CONFIG_RCU_BOOST -- one of PREEMPT_RCU.
 CONFIG_RCU_FANOUT -- Cover hierarchy, but overlap with others.
 CONFIG_RCU_FANOUT_LEAF -- Do one non-default.
-CONFIG_RCU_FAST_NO_HZ -- Do one, but not with CONFIG_RCU_NOCB_CPU_ALL.
-CONFIG_RCU_NOCB_CPU -- Do three, see below.
-CONFIG_RCU_NOCB_CPU_ALL -- Do one.
-CONFIG_RCU_NOCB_CPU_NONE -- Do one.
-CONFIG_RCU_NOCB_CPU_ZERO -- Do one.
+CONFIG_RCU_FAST_NO_HZ -- Do one, but not with all nohz_full CPUs.
+CONFIG_RCU_NOCB_CPU -- Do three, one with no rcu_nocbs CPUs, one with
+	rcu_nocbs=0, and one with all rcu_nocbs CPUs.
 CONFIG_RCU_TRACE -- Do half.
 CONFIG_SMP -- Need one !SMP for PREEMPT_RCU.
 CONFIG_RCU_EXPERT=n -- Do a few, but these have to be vanilla configurations.