nohz: Assert existing housekeepers when nohz full enabled

The code ensures that when nohz full is running, at least the
boot CPU serves as a housekeeper and it can't be later offlined.

Let's assert this assumption to make sure that we have CPUs to
handle unbound jobs like workqueues and timers while nohz full
CPUs run undisturbed.

Also improve the comments on housekeeper offlining prevention.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Vatika Harlalka <vatikaharlalka@gmail.com>
Link: http://lkml.kernel.org/r/1441119060-2230-3-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3319e16f31e58ed69534ab2fa7e5cd05d7b90d8f..7c7ec45159834a1b25576fbed037c9951f3c076f 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -290,16 +290,17 @@ static int __init tick_nohz_full_setup(char *str)
 __setup("nohz_full=", tick_nohz_full_setup);
 
 static int tick_nohz_cpu_down_callback(struct notifier_block *nfb,
-                                                unsigned long action,
-                                                void *hcpu)
+                                      unsigned long action,
+                                      void *hcpu)
 {
        unsigned int cpu = (unsigned long)hcpu;
 
        switch (action & ~CPU_TASKS_FROZEN) {
        case CPU_DOWN_PREPARE:
                /*
-                * If we handle the timekeeping duty for full dynticks CPUs,
-                * we can't safely shutdown that CPU.
+                * The boot CPU handles housekeeping duty (unbound timers,
+                * workqueues, timekeeping, ...) on behalf of full dynticks
+                * CPUs. It must remain online when nohz full is enabled.
                 */
                if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
                        return NOTIFY_BAD;
@@ -370,6 +371,12 @@ void __init tick_nohz_init(void)
        cpu_notifier(tick_nohz_cpu_down_callback, 0);
        pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n",
                cpumask_pr_args(tick_nohz_full_mask));
+
+       /*
+        * We need at least one CPU to handle housekeeping work such
+        * as timekeeping, unbound timers, workqueues, ...
+        */
+       WARN_ON_ONCE(cpumask_empty(housekeeping_mask));
 }
 #endif