[RFC PATCH 11/18] tick/nohz: Make nohz_full parameter optional

[Waiman Long <longman@redhat.com>]

To provide nohz_full tick support, there is a set of tick dependency
masks that need to be evaluated on every IRQ and context switch.
Switching on nohz_full tick support at runtime will be problematic
as some of the tick dependency masks may not be properly set causing
problem down the road.

Allow nohz_full boot option to be specified without any
parameter to force enable nohz_full tick support without any
CPU in the tick_nohz_full_mask yet. The context_tracking_key and
tick_nohz_full_running flag will be enabled in this case to make
tick_nohz_full_enabled() return true.

There is still a small performance overhead by force enable nohz_full
this way. So it should only be used if there is a chance that some
CPUs may become isolated later via the cpuset isolated partition
functionality and better CPU isolation closed to nohz_full is desired.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 .../admin-guide/kernel-parameters.txt         | 19 ++++++++++++-------
 include/linux/context_tracking.h              |  7 ++++++-
 kernel/context_tracking.c                     |  4 +++-
 kernel/sched/isolation.c                      | 13 ++++++++++++-
 kernel/time/tick-sched.c                      | 11 +++++++++--
 5 files changed, 42 insertions(+), 12 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 747a55abf494..89a8161475b5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4260,15 +4260,20 @@
 			Valid arguments: on, off
 			Default: on
 
-	nohz_full=	[KNL,BOOT,SMP,ISOL]
-			The argument is a cpu list, as described above.
+	nohz_full[=cpu-list]
+			[KNL,BOOT,SMP,ISOL]
 			In kernels built with CONFIG_NO_HZ_FULL=y, set
-			the specified list of CPUs whose tick will be stopped
-			whenever possible. The boot CPU will be forced outside
-			the range to maintain the timekeeping.  Any CPUs
-			in this list will have their RCU callbacks offloaded,
+			the specified list of CPUs whose tick will be
+			stopped whenever possible.  If the argument is
+			not specified, nohz_full will be forced enabled
+			without any CPU in the nohz_full list yet.
+			The boot CPU will be forced outside the range
+			to maintain the timekeeping.  Any CPUs in this
+			list will have their RCU callbacks offloaded,
 			just as if they had also been called out in the
-			rcu_nocbs= boot parameter.
+			rcu_nocbs= boot parameter.  There is no need
+			to use rcu_nocbs= boot parameter if nohz_full
+			has been set which will override rcu_nocbs.
 
 			Note that this argument takes precedence over
 			the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index af9fe87a0922..a3fea7f9fef6 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -9,8 +9,13 @@
 
 #include <asm/ptrace.h>
 
-
 #ifdef CONFIG_CONTEXT_TRACKING_USER
+/*
+ * Pass CONTEXT_TRACKING_FORCE_ENABLE to ct_cpu_track_user() to force enable
+ * user context tracking.
+ */
+#define CONTEXT_TRACKING_FORCE_ENABLE	(-1)
+
 extern void ct_cpu_track_user(int cpu);
 
 /* Called with interrupts disabled.  */
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index fb5be6e9b423..734354bbfdbb 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -698,7 +698,9 @@ void __init ct_cpu_track_user(int cpu)
 {
 	static __initdata bool initialized = false;
 
-	if (!per_cpu(context_tracking.active, cpu)) {
+	if (cpu == CONTEXT_TRACKING_FORCE_ENABLE) {
+		static_branch_inc(&context_tracking_key);
+	} else if (!per_cpu(context_tracking.active, cpu)) {
 		per_cpu(context_tracking.active, cpu) = true;
 		static_branch_inc(&context_tracking_key);
 	}
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index f26708667754..2bed4b2f9ec5 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -146,6 +146,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 	}
 
 	alloc_bootmem_cpumask_var(&non_housekeeping_mask);
+
 	if (cpulist_parse(str, non_housekeeping_mask) < 0) {
 		pr_warn("Housekeeping: nohz_full= or isolcpus= incorrect CPU range\n");
 		goto free_non_housekeeping_mask;
@@ -155,6 +156,13 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 	cpumask_andnot(housekeeping_staging,
 		       cpu_possible_mask, non_housekeeping_mask);
 
+	/*
+	 * Allow "nohz_full" without parameter to force enable nohz_full
+	 * at boot time without any CPUs in the nohz_full list yet.
+	 */
+	if ((flags & HK_FLAG_KERNEL_NOISE) && !*str)
+		goto setup_housekeeping_staging;
+
 	first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging);
 	if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
 		__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
@@ -168,6 +176,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 	if (cpumask_empty(non_housekeeping_mask))
 		goto free_housekeeping_staging;
 
+setup_housekeeping_staging:
 	if (!housekeeping.flags) {
 		/* First setup call ("nohz_full=" or "isolcpus=") */
 		enum hk_type type;
@@ -212,10 +221,12 @@ static int __init housekeeping_nohz_full_setup(char *str)
 	unsigned long flags;
 
 	flags = HK_FLAG_KERNEL_NOISE;
+	if (*str == '=')
+		str++;
 
 	return housekeeping_setup(str, flags);
 }
-__setup("nohz_full=", housekeeping_nohz_full_setup);
+__setup("nohz_full", housekeeping_nohz_full_setup);
 
 static int __init housekeeping_isolcpus_setup(char *str)
 {
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index c527b421c865..87b26a4471e7 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -651,8 +651,15 @@ void __init tick_nohz_init(void)
 		}
 	}
 
-	for_each_cpu(cpu, tick_nohz_full_mask)
-		ct_cpu_track_user(cpu);
+	/*
+	 * Force enable context_tracking_key if tick_nohz_full_mask empty
+	 */
+	if (cpumask_empty(tick_nohz_full_mask)) {
+		ct_cpu_track_user(CONTEXT_TRACKING_FORCE_ENABLE);
+	} else {
+		for_each_cpu(cpu, tick_nohz_full_mask)
+			ct_cpu_track_user(cpu);
+	}
 
 	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
 					"kernel/nohz:predown", NULL,
-- 
2.50.0