nohz: Ensure full dynticks CPUs are RCU nocbs

We need full dynticks CPU to also be RCU nocb so
that we don't have to keep the tick to handle RCU
callbacks.

Make sure the range passed to nohz_full= boot
parameter is a subset of rcu_nocbs=

The CPUs that fail to meet this requirement will be
excluded from the nohz_full range. This is checked
early in boot time, before any CPU has the opportunity
to stop its tick.

Suggested-by: Steven Rostedt <rostedt@goodmis.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>

1 files changed, 16 insertions, 6 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 2bac5ea2c9af..d71a5f2bd7b2 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -203,17 +203,27 @@ static int __cpuinit tick_nohz_cpu_down_callback(struct notifier_block *nfb,
  */
 static char __initdata nohz_full_buf[NR_CPUS + 1];
 
-static int __init init_tick_nohz_full(void)
+void __init tick_nohz_init(void)
 {
-        if (have_nohz_full_mask)
-                cpu_notifier(tick_nohz_cpu_down_callback, 0);
+        int cpu;
+
+        if (!have_nohz_full_mask)
+                return;
+
+        cpu_notifier(tick_nohz_cpu_down_callback, 0);
+
+        /* Make sure full dynticks CPU are also RCU nocbs */
+        for_each_cpu(cpu, nohz_full_mask) {
+                if (!rcu_is_nocb_cpu(cpu)) {
+                        pr_warning("NO_HZ: CPU %d is not RCU nocb: "
+                                   "cleared from nohz_full range", cpu);
+                        cpumask_clear_cpu(cpu, nohz_full_mask);
+                }
+        }
 
         cpulist_scnprintf(nohz_full_buf, sizeof(nohz_full_buf), nohz_full_mask);
         pr_info("NO_HZ: Full dynticks CPUs: %s.\n", nohz_full_buf);
-
-        return 0;
 }
-core_initcall(init_tick_nohz_full);
 #else
 #define have_nohz_full_mask (0)
 #endif