nohz: New option to default all CPUs in full dynticks range

Provide a new kernel config that defaults all CPUs to be part
of the full dynticks range, except the boot one for timekeeping.

This default setting is overriden by the nohz_full= boot option
if passed by the user.

This is helpful for those who don't need a finegrained range
of full dynticks CPU and also for automated testing.
Suggested-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

kernel/time/Kconfig

@@ -128,6 +128,16 @@ config NO_HZ_FULL
 
 endchoice
 
+config NO_HZ_FULL_ALL
+       bool "Full dynticks system on all CPUs by default"
+       depends on NO_HZ_FULL
+       help
+         If the user doesn't pass the nohz_full boot option to
+	 define the range of full dynticks CPUs, consider that all
+	 CPUs in the system are full dynticks by default.
+	 Note the boot CPU will still be kept outside the range to
+	 handle the timekeeping duty.
+
 config NO_HZ
 	bool "Old Idle dynticks config"
 	depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS

kernel/time/tick-sched.c

@@ -203,12 +203,31 @@ static int __cpuinit tick_nohz_cpu_down_callback(struct notifier_block *nfb,
  */
 static char __initdata nohz_full_buf[NR_CPUS + 1];
 
+static int tick_nohz_init_all(void)
+{
+	int err = -1;
+
+#ifdef CONFIG_NO_HZ_FULL_ALL
+	if (!alloc_cpumask_var(&nohz_full_mask, GFP_KERNEL)) {
+		pr_err("NO_HZ: Can't allocate full dynticks cpumask\n");
+		return err;
+	}
+	err = 0;
+	cpumask_setall(nohz_full_mask);
+	cpumask_clear_cpu(smp_processor_id(), nohz_full_mask);
+	have_nohz_full_mask = true;
+#endif
+	return err;
+}
+
 void __init tick_nohz_init(void)
 {
 	int cpu;
 
-	if (!have_nohz_full_mask)
-		return;
+	if (!have_nohz_full_mask) {
+		if (tick_nohz_init_all() < 0)
+			return;
+	}
 
 	cpu_notifier(tick_nohz_cpu_down_callback, 0);