2 files changed, 17 insertions, 41 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 82365dde00a8..887b29708447 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1916,8 +1916,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
         nohz_full=      [KNL,BOOT]
                         In kernels built with CONFIG_NO_HZ_FULL=y, set
                         the specified list of CPUs whose tick will be stopped
-                        whenever possible. You need to keep at least one online
-                        CPU outside the range to maintain the timekeeping.
+                        whenever possible. The boot CPU will be forced outside
+                        the range to maintain the timekeeping.
 
         noiotrap        [SH] Disables trapped I/O port accesses.
 
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 369b5769fc97..2bac5ea2c9af 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -158,11 +158,21 @@ int tick_nohz_full_cpu(int cpu)
 /* Parse the boot-time nohz CPU list from the kernel parameters. */
 static int __init tick_nohz_full_setup(char *str)
 {
+        int cpu;
+
         alloc_bootmem_cpumask_var(&nohz_full_mask);
-        if (cpulist_parse(str, nohz_full_mask) < 0)
+        if (cpulist_parse(str, nohz_full_mask) < 0) {
                 pr_warning("NOHZ: Incorrect nohz_full cpumask\n");
-        else
-                have_nohz_full_mask = true;
+                return 1;
+        }
+
+        cpu = smp_processor_id();
+        if (cpumask_test_cpu(cpu, nohz_full_mask)) {
+                pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
+                cpumask_clear_cpu(cpu, nohz_full_mask);
+        }
+        have_nohz_full_mask = true;
+
         return 1;
 }
 __setup("nohz_full=", tick_nohz_full_setup);
@@ -195,42 +205,8 @@ static char __initdata nohz_full_buf[NR_CPUS + 1];
 
 static int __init init_tick_nohz_full(void)
 {
-        cpumask_var_t online_nohz;
-        int cpu;
-
-        if (!have_nohz_full_mask)
-                return 0;
-
-        cpu_notifier(tick_nohz_cpu_down_callback, 0);
-
-        if (!zalloc_cpumask_var(&online_nohz, GFP_KERNEL)) {
-                pr_warning("NO_HZ: Not enough memory to check full nohz mask\n");
-                return -ENOMEM;
-        }
-
-        /*
-         * CPUs can probably not be concurrently offlined on initcall time.
-         * But we are paranoid, aren't we?
-         */
-        get_online_cpus();
-
-        /* Ensure we keep a CPU outside the dynticks range for timekeeping */
-        cpumask_and(online_nohz, cpu_online_mask, nohz_full_mask);
-        if (cpumask_equal(online_nohz, cpu_online_mask)) {
-                pr_warning("NO_HZ: Must keep at least one online CPU "
-                           "out of nohz_full range\n");
-                /*
-                 * We know the current CPU doesn't have its tick stopped.
-                 * Let's use it for the timekeeping duty.
-                 */
-                preempt_disable();
-                cpu = smp_processor_id();
-                pr_warning("NO_HZ: Clearing %d from nohz_full range\n", cpu);
-                cpumask_clear_cpu(cpu, nohz_full_mask);
-                preempt_enable();
-        }
-        put_online_cpus();
-        free_cpumask_var(online_nohz);
+        if (have_nohz_full_mask)
+                cpu_notifier(tick_nohz_cpu_down_callback, 0);
 
         cpulist_scnprintf(nohz_full_buf, sizeof(nohz_full_buf), nohz_full_mask);
         pr_info("NO_HZ: Full dynticks CPUs: %s.\n", nohz_full_buf);