Commit 7c8bb6cb authored Sep 01, 2015 by Frederic Weisbecker Committed by Ingo Molnar Sep 02, 2015

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>

parent 9642d18e

kernel/time/tick-sched.c

+11 −4

Original line number	Diff line number	Diff line
		@@ -298,8 +298,9 @@ static int tick_nohz_cpu_down_callback(struct notifier_block *nfb,
		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