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>

2 files changed, 31 insertions, 2 deletions

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 358d601a4fec..99c3f13dd478 100644
--- a/kernel/time/Kconfig
+++ b/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
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index d71a5f2bd7b2..a76e09044f9f 100644
--- a/kernel/time/tick-sched.c
+++ b/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);