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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>
This commit is contained in:
Frederic Weisbecker 2015-09-01 16:51:00 +02:00 committed by Ingo Molnar
parent 9642d18eee
commit 7c8bb6cb95
1 changed files with 11 additions and 4 deletions
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15
kernel/time/tick-sched.c
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@ -290,16 +290,17 @@ static int __init tick_nohz_full_setup(char *str)
__setup("nohz_full=", tick_nohz_full_setup); __setup("nohz_full=", tick_nohz_full_setup);
static int tick_nohz_cpu_down_callback(struct notifier_block *nfb, static int tick_nohz_cpu_down_callback(struct notifier_block *nfb,
unsigned long action, unsigned long action,
void *hcpu) void *hcpu)
{ {
unsigned int cpu = (unsigned long)hcpu; unsigned int cpu = (unsigned long)hcpu;
switch (action & ~CPU_TASKS_FROZEN) { switch (action & ~CPU_TASKS_FROZEN) {
case CPU_DOWN_PREPARE: case CPU_DOWN_PREPARE:
/* /*
* If we handle the timekeeping duty for full dynticks CPUs, * The boot CPU handles housekeeping duty (unbound timers,
* we can't safely shutdown that CPU. * 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) if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
return NOTIFY_BAD; return NOTIFY_BAD;
@ -370,6 +371,12 @@ void __init tick_nohz_init(void)
cpu_notifier(tick_nohz_cpu_down_callback, 0); cpu_notifier(tick_nohz_cpu_down_callback, 0);
pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n", pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n",
cpumask_pr_args(tick_nohz_full_mask)); 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 #endif