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Date: Tue, 25 Feb 2014 13:33:59 +0100
From: Henrik Austad <henrik@...tad.us>
To: LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...uxtronix.de>
Cc: Henrik Austad <haustad@...co.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Frederic Weisbecker <fweisbec@...il.com>,
John Stultz <john.stultz@...aro.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH 4/6] Force a specific CPU to handle all do_timer() events.
From: Henrik Austad <haustad@...co.com>
By default, this is disabled (set to -1) and must be explicitly set in order
to have an effect. The CPU must be online.
If the specified CPU is part of the NO_HZ_FULL set, it is removed from
the set. When forced tick is either disabled or moved to another CPU, it
will try to re-enable NO_HZ_FULL on previous CPU.
If the CPU is removed (hotpluggable CPUs), forced tick will be disabled.
CC: Thomas Gleixner <tglx@...utronix.de>
CC: Peter Zijlstra <peterz@...radead.org>
CC: Frederic Weisbecker <fweisbec@...il.com>
CC: John Stultz <john.stultz@...aro.org>
CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Henrik Austad <haustad@...co.com>
---
kernel/time/tick-common.c | 103 +++++++++++++++++++++++++++++++++++++++++++
kernel/time/tick-internal.h | 4 ++
kernel/time/tick-sched.c | 14 +++++-
3 files changed, 120 insertions(+), 1 deletion(-)
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 1729b4b..d4e660a 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -51,6 +51,19 @@ ktime_t tick_period;
int tick_do_timer_cpu __read_mostly = TICK_DO_TIMER_BOOT;
/*
+ * tick_do_timer_cpu_forced is YATCIV responsible for controlling the
+ * forced-timer logic. It has 2 modes
+ *
+ * - 'Off': -1 Default mode, timer ticks are being processed as normal.
+ *
+ * - 'On' : Stores the CPUid of the CPU currently assigned to handle all
+ * do_timer() events. A CPU given this responsible will not be
+ * allowed to enter NO_HZ_IDLE or NO_HZ_FULL mode.
+ */
+int tick_do_timer_cpu_forced __read_mostly = -1;
+
+
+/*
* Debugging: see timer_list.c
*/
struct tick_device *tick_get_device(int cpu)
@@ -342,6 +355,12 @@ void tick_handover_do_timer(int *cpup)
if (*cpup == tick_do_timer_cpu) {
int cpu = cpumask_first(cpu_online_mask);
+ /* forced tick on this */
+ if (tick_do_timer_cpu_forced == tick_do_timer_cpu) {
+ tick_set_forced_cpu(-1);
+ pr_info("Disabled forced timer-tick due to dying CPU\n");
+ }
+
tick_do_timer_cpu = (cpu < nr_cpu_ids) ? cpu :
TICK_DO_TIMER_NONE;
}
@@ -360,6 +379,90 @@ int tick_expose_cpu(void)
}
/*
+ * Set a forced value for tick_do_timer_cpu
+ *
+ * When not set, forced_tick_do_timer_cpu is -1, otherwise it is
+ * identical to tick_do_timer_cpu.
+ *
+ * When the core is set, this core is not able to drop into NOHZ idle
+ * and NOHZ full mode.
+ */
+int tick_set_forced_cpu(int new_cpu)
+{
+#ifdef CONFIG_NO_HZ_FULL
+ static int nohz_cpu_pre_forced = -1;
+#endif
+ int ret = 0;
+
+ DEFINE_MUTEX(forced_cpu_mutex);
+
+ mutex_lock(&forced_cpu_mutex);
+ if (new_cpu == tick_do_timer_cpu_forced)
+ goto out;
+
+ if (new_cpu == -1) {
+#ifdef CONFIG_NO_HZ_FULL
+ if (nohz_cpu_pre_forced != -1) {
+ cpumask_set_cpu(nohz_cpu_pre_forced, tick_nohz_full_mask);
+ nohz_cpu_pre_forced = -1;
+ }
+#endif
+
+ /* let the timer-machinery pick up the fallout */
+ tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+ tick_do_timer_cpu_forced = -1;
+ goto out;
+ }
+
+ /*
+ * Make sure new_cpu is actually a valid CPU.
+ */
+ if (!cpu_online(new_cpu)) {
+ ret = -EINVAL;
+ pr_warn("tick_set_forced_cpu(): Suggested CPU not available\n");
+ goto out;
+ }
+
+#ifdef CONFIG_NO_HZ_FULL
+ /*
+ * no_hz_full require some extra care
+ */
+ if (nohz_cpu_pre_forced != -1) {
+ cpumask_set_cpu(nohz_cpu_pre_forced, tick_nohz_full_mask);
+ nohz_cpu_pre_forced = -1;
+ }
+ if (tick_nohz_full_cpu(new_cpu)) {
+
+ nohz_cpu_pre_forced = new_cpu;
+ cpumask_clear_cpu(new_cpu, tick_nohz_full_mask);
+ }
+#endif
+ tick_do_timer_cpu_forced = new_cpu;
+ tick_do_timer_cpu = new_cpu;
+
+out:
+ mutex_unlock(&forced_cpu_mutex);
+ return ret;
+}
+int tick_get_forced_cpu(void)
+{
+ return tick_do_timer_cpu_forced;
+}
+
+bool forced_timer_can_stop_tick(void)
+{
+ /*
+ * can be racy if we get an update of tick_do_timer_cpu_foced
+ * right in the middle of this call
+ *
+ * Currently only called from tick-sched::can_stop_full_tick() and can_stop_idle_tick()
+ *
+ * If disabled, tick_do_timer_cpu_forced is -1, which will never be a CPUid.
+ */
+ return !(tick_do_timer_cpu_forced == raw_smp_processor_id());
+}
+
+/*
* Shutdown an event device on a given cpu:
*
* This is called on a life CPU, when a CPU is dead. So we cannot
diff --git a/kernel/time/tick-internal.h b/kernel/time/tick-internal.h
index 5051dbd..9eed487 100644
--- a/kernel/time/tick-internal.h
+++ b/kernel/time/tick-internal.h
@@ -155,5 +155,9 @@ static inline int tick_device_is_functional(struct clock_event_device *dev)
#endif
extern int tick_expose_cpu(void);
+int tick_set_forced_cpu(int new_cpu);
+int tick_get_forced_cpu(void);
+bool forced_timer_can_stop_tick(void);
+
extern void do_timer(unsigned long ticks);
extern void update_wall_time(void);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 9f8af69..95e6d7d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -172,6 +172,11 @@ static bool can_stop_full_tick(void)
return false;
}
+ if (!forced_timer_can_stop_tick()) {
+ trace_tick_stop(0, "forced timer-tick running on CPU\n");
+ return false;
+ }
+
/* sched_clock_tick() needs us? */
#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
/*
@@ -704,11 +709,18 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
* invoked.
*/
if (unlikely(!cpu_online(cpu))) {
- if (cpu == tick_do_timer_cpu)
+ if (cpu == tick_do_timer_cpu) {
tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+ }
return false;
}
+ /*
+ * if forced do_timer is set, we cannot stop the tick on this CPU.
+ */
+ if (unlikely(!forced_timer_can_stop_tick()))
+ return false;
+
if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) {
ts->sleep_length = (ktime_t) { .tv64 = NSEC_PER_SEC/HZ };
return false;
--
1.7.9.5
--
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