[PATCH 0/6] isolation: 1Hz residual tick offloading v6

[PATCH 0/6] isolation: 1Hz residual tick offloading v6

[Frederic Weisbecker]

The version addresses the reviews from Ingo on v5:

* Start changelog titles with verbs.
* Improve/reword several comments.
* Improve/reword documentation.
* Various cosmetic changes.
* Use BUG_ON() on boot allocation failure.

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
	sched/0hz-v6

HEAD: 7011ba38353ad8ec398ade7533c73828d27462ad

Thanks,
	Frederic
---

Frederic Weisbecker (6):
      sched: Rename init_rq_hrtick to hrtick_rq_init
      nohz: Allow to check if remote CPU tick is stopped
      sched/isolation: Isolate workqueues when "nohz_full=" is set
      sched/isolation: Offload residual 1Hz scheduler tick
      sched/nohz: Remove the 1 Hz tick code
      sched/isolation: Update nohz documentation to explain tick offload


 Documentation/admin-guide/kernel-parameters.txt |  11 +++
 include/linux/sched/isolation.h                 |   1 +
 include/linux/sched/nohz.h                      |   4 -
 include/linux/tick.h                            |   2 +
 kernel/sched/core.c                             | 117 ++++++++++++++++++------
 kernel/sched/idle_task.c                        |   1 -
 kernel/sched/isolation.c                        |   7 +-
 kernel/sched/sched.h                            |  13 +--
 kernel/time/tick-sched.c                        |  13 +--
 kernel/workqueue.c                              |   3 +-
 10 files changed, 122 insertions(+), 50 deletions(-)

[PATCH 1/6] sched: Rename init_rq_hrtick to hrtick_rq_init

[Frederic Weisbecker]

Do that rename in order to normalize the hrtick namespace.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bf724c1..f09ec5c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -333,7 +333,7 @@ void hrtick_start(struct rq *rq, u64 delay)
 }
 #endif /* CONFIG_SMP */
 
-static void init_rq_hrtick(struct rq *rq)
+static void hrtick_rq_init(struct rq *rq)
 {
 #ifdef CONFIG_SMP
 	rq->hrtick_csd_pending = 0;
@@ -351,7 +351,7 @@ static inline void hrtick_clear(struct rq *rq)
 {
 }
 
-static inline void init_rq_hrtick(struct rq *rq)
+static inline void hrtick_rq_init(struct rq *rq)
 {
 }
 #endif	/* CONFIG_SCHED_HRTICK */
@@ -6023,7 +6023,7 @@ void __init sched_init(void)
 		rq->last_sched_tick = 0;
 #endif
 #endif /* CONFIG_SMP */
-		init_rq_hrtick(rq);
+		hrtick_rq_init(rq);
 		atomic_set(&rq->nr_iowait, 0);
 	}
 
-- 
2.7.4

[PATCH 2/6] nohz: Allow to check if remote CPU tick is stopped

[Frederic Weisbecker]

This check is racy but provides a good heuristic to determine whether
a CPU may need a remote tick or not.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 include/linux/tick.h     | 2 ++
 kernel/time/tick-sched.c | 7 +++++++
 2 files changed, 9 insertions(+)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 7cc3592..944c829 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -114,6 +114,7 @@ enum tick_dep_bits {
 #ifdef CONFIG_NO_HZ_COMMON
 extern bool tick_nohz_enabled;
 extern int tick_nohz_tick_stopped(void);
+extern int tick_nohz_tick_stopped_cpu(int cpu);
 extern void tick_nohz_idle_enter(void);
 extern void tick_nohz_idle_exit(void);
 extern void tick_nohz_irq_exit(void);
@@ -125,6 +126,7 @@ extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
 #else /* !CONFIG_NO_HZ_COMMON */
 #define tick_nohz_enabled (0)
 static inline int tick_nohz_tick_stopped(void) { return 0; }
+static inline int tick_nohz_tick_stopped_cpu(int cpu) { return 0; }
 static inline void tick_nohz_idle_enter(void) { }
 static inline void tick_nohz_idle_exit(void) { }
 
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 29a5733..b517485 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -486,6 +486,13 @@ int tick_nohz_tick_stopped(void)
 	return __this_cpu_read(tick_cpu_sched.tick_stopped);
 }
 
+int tick_nohz_tick_stopped_cpu(int cpu)
+{
+	struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);
+
+	return ts->tick_stopped;
+}
+
 /**
  * tick_nohz_update_jiffies - update jiffies when idle was interrupted
  *
-- 
2.7.4

[PATCH 3/6] sched/isolation: Isolate workqueues when "nohz_full=" is set

[Frederic Weisbecker]

As we prepare for offloading the residual 1hz scheduler ticks to
workqueue, let's affine those to housekeepers so that they don't
interrupt the CPUs that don't want to be disturbed.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 include/linux/sched/isolation.h | 1 +
 kernel/sched/isolation.c        | 3 ++-
 kernel/workqueue.c              | 3 ++-
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
index d849431..4a6582c 100644
--- a/include/linux/sched/isolation.h
+++ b/include/linux/sched/isolation.h
@@ -12,6 +12,7 @@ enum hk_flags {
 	HK_FLAG_SCHED		= (1 << 3),
 	HK_FLAG_TICK		= (1 << 4),
 	HK_FLAG_DOMAIN		= (1 << 5),
+	HK_FLAG_WQ		= (1 << 6),
 };
 
 #ifdef CONFIG_CPU_ISOLATION
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index b71b436..a2500c4 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -3,6 +3,7 @@
  *  any CPU: unbound workqueues, timers, kthreads and any offloadable work.
  *
  * Copyright (C) 2017 Red Hat, Inc., Frederic Weisbecker
+ * Copyright (C) 2017-2018 SUSE, Frederic Weisbecker
  *
  */
 
@@ -119,7 +120,7 @@ static int __init housekeeping_nohz_full_setup(char *str)
 {
 	unsigned int flags;
 
-	flags = HK_FLAG_TICK | HK_FLAG_TIMER | HK_FLAG_RCU | HK_FLAG_MISC;
+	flags = HK_FLAG_TICK | HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU | HK_FLAG_MISC;
 
 	return housekeeping_setup(str, flags);
 }
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 017044c..593dbe7 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5565,12 +5565,13 @@ static void __init wq_numa_init(void)
 int __init workqueue_init_early(void)
 {
 	int std_nice[NR_STD_WORKER_POOLS] = { 0, HIGHPRI_NICE_LEVEL };
+	int hk_flags = HK_FLAG_DOMAIN | HK_FLAG_WQ;
 	int i, cpu;
 
 	WARN_ON(__alignof__(struct pool_workqueue) < __alignof__(long long));
 
 	BUG_ON(!alloc_cpumask_var(&wq_unbound_cpumask, GFP_KERNEL));
-	cpumask_copy(wq_unbound_cpumask, housekeeping_cpumask(HK_FLAG_DOMAIN));
+	cpumask_copy(wq_unbound_cpumask, housekeeping_cpumask(hk_flags));
 
 	pwq_cache = KMEM_CACHE(pool_workqueue, SLAB_PANIC);
 
-- 
2.7.4

[PATCH 4/6] sched/isolation: Offload residual 1Hz scheduler tick

[Frederic Weisbecker]

When a CPU runs in full dynticks mode, a 1Hz tick remains in order to
keep the scheduler stats alive. However this residual tick is a burden
for bare metal tasks that can't stand any interruption at all, or want
to minimize them.

The usual boot parameters "nohz_full=" or "isolcpus=nohz" will now
outsource these scheduler ticks to the global workqueue so that a
housekeeping CPU handles those remotely. The sched_class::task_tick()
implementations have been audited and look safe to be called remotely
as the target runqueue and its current task are passed in parameter
and don't seem to be accessed locally.

Note that in the case of using isolcpus, it's still up to the user to
affine the global workqueues to the housekeeping CPUs through
/sys/devices/virtual/workqueue/cpumask or domains isolation
"isolcpus=nohz,domain".

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c      | 92 ++++++++++++++++++++++++++++++++++++++++++++++++
 kernel/sched/isolation.c |  4 +++
 kernel/sched/sched.h     |  2 ++
 3 files changed, 98 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f09ec5c..86eefc4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3120,6 +3120,96 @@ u64 scheduler_tick_max_deferment(void)
 
 	return jiffies_to_nsecs(next - now);
 }
+
+struct tick_work {
+	int			cpu;
+	struct delayed_work	work;
+};
+
+static struct tick_work __percpu *tick_work_cpu;
+
+static void sched_tick_remote(struct work_struct *work)
+{
+	struct delayed_work *dwork = to_delayed_work(work);
+	struct tick_work *twork = container_of(dwork, struct tick_work, work);
+	int cpu = twork->cpu;
+	struct rq *rq = cpu_rq(cpu);
+	struct rq_flags rf;
+
+	/*
+	 * Handle the tick only if it appears the remote CPU is running in full
+	 * dynticks mode. The check is racy by nature, but missing a tick or
+	 * having one too much is no big deal because the scheduler tick updates
+	 * statistics and checks timeslices in a time-independent way, regardless
+	 * of when exactly it is running.
+	 */
+	if (!idle_cpu(cpu) && tick_nohz_tick_stopped_cpu(cpu)) {
+		struct task_struct *curr;
+		u64 delta;
+
+		rq_lock_irq(rq, &rf);
+		update_rq_clock(rq);
+		curr = rq->curr;
+		delta = rq_clock_task(rq) - curr->se.exec_start;
+
+		/*
+		 * Make sure the next tick runs within a reasonable
+		 * amount of time.
+		 */
+		WARN_ON_ONCE(delta > (u64)NSEC_PER_SEC * 3);
+		curr->sched_class->task_tick(rq, curr, 0);
+		rq_unlock_irq(rq, &rf);
+	}
+
+	/*
+	 * Run the remote tick once per second (1Hz). This arbitrary
+	 * frequency is large enough to avoid overload but short enough
+	 * to keep scheduler internal stats reasonably up to date.
+	 */
+	queue_delayed_work(system_unbound_wq, dwork, HZ);
+}
+
+static void sched_tick_start(int cpu)
+{
+	struct tick_work *twork;
+
+	if (housekeeping_cpu(cpu, HK_FLAG_TICK))
+		return;
+
+	WARN_ON_ONCE(!tick_work_cpu);
+
+	twork = per_cpu_ptr(tick_work_cpu, cpu);
+	twork->cpu = cpu;
+	INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
+	queue_delayed_work(system_unbound_wq, &twork->work, HZ);
+}
+
+#ifdef CONFIG_HOTPLUG_CPU
+static void sched_tick_stop(int cpu)
+{
+	struct tick_work *twork;
+
+	if (housekeeping_cpu(cpu, HK_FLAG_TICK))
+		return;
+
+	WARN_ON_ONCE(!tick_work_cpu);
+
+	twork = per_cpu_ptr(tick_work_cpu, cpu);
+	cancel_delayed_work_sync(&twork->work);
+}
+#endif /* CONFIG_HOTPLUG_CPU */
+
+int __init sched_tick_offload_init(void)
+{
+	tick_work_cpu = alloc_percpu(struct tick_work);
+	BUG_ON(!tick_work_cpu);
+
+	return 0;
+}
+
+#else /* !CONFIG_NO_HZ_FULL */
+static inline void sched_tick_start(int cpu) { }
+static inline void sched_tick_stop(int cpu) { }
 #endif
 
 #if defined(CONFIG_PREEMPT) && (defined(CONFIG_DEBUG_PREEMPT) || \
@@ -5781,6 +5871,7 @@ int sched_cpu_starting(unsigned int cpu)
 {
 	set_cpu_rq_start_time(cpu);
 	sched_rq_cpu_starting(cpu);
+	sched_tick_start(cpu);
 	return 0;
 }
 
@@ -5792,6 +5883,7 @@ int sched_cpu_dying(unsigned int cpu)
 
 	/* Handle pending wakeups and then migrate everything off */
 	sched_ttwu_pending();
+	sched_tick_stop(cpu);
 
 	rq_lock_irqsave(rq, &rf);
 	if (rq->rd) {
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index a2500c4..39f340d 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -13,6 +13,7 @@
 #include <linux/kernel.h>
 #include <linux/static_key.h>
 #include <linux/ctype.h>
+#include "sched.h"
 
 DEFINE_STATIC_KEY_FALSE(housekeeping_overriden);
 EXPORT_SYMBOL_GPL(housekeeping_overriden);
@@ -61,6 +62,9 @@ void __init housekeeping_init(void)
 
 	static_branch_enable(&housekeeping_overriden);
 
+	if (housekeeping_flags & HK_FLAG_TICK)
+		sched_tick_offload_init();
+
 	/* We need at least one CPU to handle housekeeping work */
 	WARN_ON_ONCE(cpumask_empty(housekeeping_mask));
 }
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index fb5fc458..c1c7c78 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1574,6 +1574,7 @@ extern void post_init_entity_util_avg(struct sched_entity *se);
 
 #ifdef CONFIG_NO_HZ_FULL
 extern bool sched_can_stop_tick(struct rq *rq);
+extern int __init sched_tick_offload_init(void);
 
 /*
  * Tick may be needed by tasks in the runqueue depending on their policy and
@@ -1598,6 +1599,7 @@ static inline void sched_update_tick_dependency(struct rq *rq)
 		tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
 }
 #else
+static inline int sched_tick_offload_init(void) { return 0; }
 static inline void sched_update_tick_dependency(struct rq *rq) { }
 #endif
 
-- 
2.7.4

[PATCH 5/6] sched/nohz: Remove the 1 Hz tick code

[Frederic Weisbecker]

Now that the 1Hz tick is offloaded to workqueues, we can safely remove
the residual code that used to handle it locally.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 include/linux/sched/nohz.h |  4 ----
 kernel/sched/core.c        | 29 -----------------------------
 kernel/sched/idle_task.c   |  1 -
 kernel/sched/sched.h       | 11 +----------
 kernel/time/tick-sched.c   |  6 ------
 5 files changed, 1 insertion(+), 50 deletions(-)

diff --git a/include/linux/sched/nohz.h b/include/linux/sched/nohz.h
index 3d3a97d..0942172 100644
--- a/include/linux/sched/nohz.h
+++ b/include/linux/sched/nohz.h
@@ -37,8 +37,4 @@ extern void wake_up_nohz_cpu(int cpu);
 static inline void wake_up_nohz_cpu(int cpu) { }
 #endif
 
-#ifdef CONFIG_NO_HZ_FULL
-extern u64 scheduler_tick_max_deferment(void);
-#endif
-
 #endif /* _LINUX_SCHED_NOHZ_H */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 86eefc4..099c9cb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3091,35 +3091,9 @@ void scheduler_tick(void)
 	rq->idle_balance = idle_cpu(cpu);
 	trigger_load_balance(rq);
 #endif
-	rq_last_tick_reset(rq);
 }
 
 #ifdef CONFIG_NO_HZ_FULL
-/**
- * scheduler_tick_max_deferment
- *
- * Keep at least one tick per second when a single
- * active task is running because the scheduler doesn't
- * yet completely support full dynticks environment.
- *
- * This makes sure that uptime, CFS vruntime, load
- * balancing, etc... continue to move forward, even
- * with a very low granularity.
- *
- * Return: Maximum deferment in nanoseconds.
- */
-u64 scheduler_tick_max_deferment(void)
-{
-	struct rq *rq = this_rq();
-	unsigned long next, now = READ_ONCE(jiffies);
-
-	next = rq->last_sched_tick + HZ;
-
-	if (time_before_eq(next, now))
-		return 0;
-
-	return jiffies_to_nsecs(next - now);
-}
 
 struct tick_work {
 	int			cpu;
@@ -6111,9 +6085,6 @@ void __init sched_init(void)
 		rq->last_load_update_tick = jiffies;
 		rq->nohz_flags = 0;
 #endif
-#ifdef CONFIG_NO_HZ_FULL
-		rq->last_sched_tick = 0;
-#endif
 #endif /* CONFIG_SMP */
 		hrtick_rq_init(rq);
 		atomic_set(&rq->nr_iowait, 0);
diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
index d518664..a64fc92 100644
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -48,7 +48,6 @@ dequeue_task_idle(struct rq *rq, struct task_struct *p, int flags)
 
 static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
 {
-	rq_last_tick_reset(rq);
 }
 
 static void task_tick_idle(struct rq *rq, struct task_struct *curr, int queued)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c1c7c78..dc6c8b5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -727,9 +727,7 @@ struct rq {
 #endif /* CONFIG_SMP */
 	unsigned long nohz_flags;
 #endif /* CONFIG_NO_HZ_COMMON */
-#ifdef CONFIG_NO_HZ_FULL
-	unsigned long last_sched_tick;
-#endif
+
 	/* capture load from *all* tasks on this cpu: */
 	struct load_weight load;
 	unsigned long nr_load_updates;
@@ -1626,13 +1624,6 @@ static inline void sub_nr_running(struct rq *rq, unsigned count)
 	sched_update_tick_dependency(rq);
 }
 
-static inline void rq_last_tick_reset(struct rq *rq)
-{
-#ifdef CONFIG_NO_HZ_FULL
-	rq->last_sched_tick = jiffies;
-#endif
-}
-
 extern void update_rq_clock(struct rq *rq);
 
 extern void activate_task(struct rq *rq, struct task_struct *p, int flags);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b517485..0c615a0 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -748,12 +748,6 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 		delta = KTIME_MAX;
 	}
 
-#ifdef CONFIG_NO_HZ_FULL
-	/* Limit the tick delta to the maximum scheduler deferment */
-	if (!ts->inidle)
-		delta = min(delta, scheduler_tick_max_deferment());
-#endif
-
 	/* Calculate the next expiry time */
 	if (delta < (KTIME_MAX - basemono))
 		expires = basemono + delta;
-- 
2.7.4

[PATCH 6/6] sched/isolation: Update nohz documentation to explain tick offload

[Frederic Weisbecker]

Update the documentation to reflect the 1Hz tick offload changes.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1d1d53f..50b9837 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1766,6 +1766,17 @@
 
 			nohz
 			  Disable the tick when a single task runs.
+
+			  A residual 1Hz tick is offloaded to workqueues, which you
+			  need to affine to housekeeping through the global
+			  workqueue's affinity configured via the
+			  /sys/devices/virtual/workqueue/cpumask sysfs file, or
+			  by using the 'domain' flag described below.
+
+			  NOTE: by default the global workqueue runs on all CPUs,
+			  so to protect individual CPUs the 'cpumask' file has to
+			  be configured manually after bootup.
+
 			domain
 			  Isolate from the general SMP balancing and scheduling
 			  algorithms. Note that performing domain isolation this way
-- 
2.7.4

Re: [PATCH 0/6] isolation: 1Hz residual tick offloading v6

[Peter Zijlstra]

On Thu, Feb 15, 2018 at 01:48:08AM +0100, Frederic Weisbecker wrote:
> The version addresses the reviews from Ingo on v5:
> 
> * Start changelog titles with verbs.
> * Improve/reword several comments.
> * Improve/reword documentation.
> * Various cosmetic changes.
> * Use BUG_ON() on boot allocation failure.
> 

Thanks!

Re: [PATCH 1/6] sched: Rename init_rq_hrtick to hrtick_rq_init

[Thomas Gleixner]

On Thu, 15 Feb 2018, Frederic Weisbecker wrote:

> Do that rename in order to normalize the hrtick namespace.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Re: [PATCH 2/6] nohz: Allow to check if remote CPU tick is stopped

[Thomas Gleixner]

On Thu, 15 Feb 2018, Frederic Weisbecker wrote:

> This check is racy but provides a good heuristic to determine whether
> a CPU may need a remote tick or not.
> 

> +int tick_nohz_tick_stopped_cpu(int cpu)

s/int/bool/ but I'm not religious about that.

> +{
> +	struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);
> +
> +	return ts->tick_stopped;
> +}

Other than that nit:

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Re: [PATCH 3/6] sched/isolation: Isolate workqueues when "nohz_full=" is set

[Thomas Gleixner]

On Thu, 15 Feb 2018, Frederic Weisbecker wrote:

> As we prepare for offloading the residual 1hz scheduler ticks to
> workqueue, let's affine those to housekeepers so that they don't
> interrupt the CPUs that don't want to be disturbed.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Re: [PATCH 4/6] sched/isolation: Offload residual 1Hz scheduler tick

[Thomas Gleixner]

On Thu, 15 Feb 2018, Frederic Weisbecker wrote:

> When a CPU runs in full dynticks mode, a 1Hz tick remains in order to
> keep the scheduler stats alive. However this residual tick is a burden
> for bare metal tasks that can't stand any interruption at all, or want
> to minimize them.
> 
> The usual boot parameters "nohz_full=" or "isolcpus=nohz" will now
> outsource these scheduler ticks to the global workqueue so that a
> housekeeping CPU handles those remotely. The sched_class::task_tick()
> implementations have been audited and look safe to be called remotely
> as the target runqueue and its current task are passed in parameter
> and don't seem to be accessed locally.

That scares me a bit. Not for the current state of affairs, but we want to
ensure that this still works in 2 years from now

So at least you want to add a comment to task_tick() which explains the
constraints which come with the remote tick.

Other than that this looks good!

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Re: [PATCH 5/6] sched/nohz: Remove the 1 Hz tick code

[Thomas Gleixner]

On Thu, 15 Feb 2018, Frederic Weisbecker wrote:

> Now that the 1Hz tick is offloaded to workqueues, we can safely remove
> the residual code that used to handle it locally.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
 

Re: [PATCH 6/6] sched/isolation: Update nohz documentation to explain tick offload

[Thomas Gleixner]

On Thu, 15 Feb 2018, Frederic Weisbecker wrote:

> Update the documentation to reflect the 1Hz tick offload changes.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Chris Metcalf <cmetcalf@mellanox.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Luiz Capitulino <lcapitulino@redhat.com>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Wanpeng Li <kernellwp@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 1d1d53f..50b9837 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1766,6 +1766,17 @@
>  
>  			nohz
>  			  Disable the tick when a single task runs.
> +
> +			  A residual 1Hz tick is offloaded to workqueues, which you
> +			  need to affine to housekeeping through the global
> +			  workqueue's affinity configured via the
> +			  /sys/devices/virtual/workqueue/cpumask sysfs file, or
> +			  by using the 'domain' flag described below.
> +
> +			  NOTE: by default the global workqueue runs on all CPUs,
> +			  so to protect individual CPUs the 'cpumask' file has to
> +			  be configured manually after bootup.

Hmm. Why don't you set that to the housekeeping cpus by default?

Thanks,

	tglx

Re: [PATCH 4/6] sched/isolation: Offload residual 1Hz scheduler tick

[Frederic Weisbecker]

On Sat, Feb 17, 2018 at 11:50:52AM +0100, Thomas Gleixner wrote:
> On Thu, 15 Feb 2018, Frederic Weisbecker wrote:
> 
> > When a CPU runs in full dynticks mode, a 1Hz tick remains in order to
> > keep the scheduler stats alive. However this residual tick is a burden
> > for bare metal tasks that can't stand any interruption at all, or want
> > to minimize them.
> > 
> > The usual boot parameters "nohz_full=" or "isolcpus=nohz" will now
> > outsource these scheduler ticks to the global workqueue so that a
> > housekeeping CPU handles those remotely. The sched_class::task_tick()
> > implementations have been audited and look safe to be called remotely
> > as the target runqueue and its current task are passed in parameter
> > and don't seem to be accessed locally.
> 
> That scares me a bit. Not for the current state of affairs, but we want to
> ensure that this still works in 2 years from now
> 
> So at least you want to add a comment to task_tick() which explains the
> constraints which come with the remote tick.

Good point, I'm adding that.

> 
> Other than that this looks good!
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Thanks!

Re: [PATCH 6/6] sched/isolation: Update nohz documentation to explain tick offload

[Frederic Weisbecker]

On Sat, Feb 17, 2018 at 11:53:22AM +0100, Thomas Gleixner wrote:
> On Thu, 15 Feb 2018, Frederic Weisbecker wrote:
> 
> > Update the documentation to reflect the 1Hz tick offload changes.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Chris Metcalf <cmetcalf@mellanox.com>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Luiz Capitulino <lcapitulino@redhat.com>
> > Cc: Mike Galbraith <efault@gmx.de>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Wanpeng Li <kernellwp@gmail.com>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 1d1d53f..50b9837 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -1766,6 +1766,17 @@
> >  
> >  			nohz
> >  			  Disable the tick when a single task runs.
> > +
> > +			  A residual 1Hz tick is offloaded to workqueues, which you
> > +			  need to affine to housekeeping through the global
> > +			  workqueue's affinity configured via the
> > +			  /sys/devices/virtual/workqueue/cpumask sysfs file, or
> > +			  by using the 'domain' flag described below.
> > +
> > +			  NOTE: by default the global workqueue runs on all CPUs,
> > +			  so to protect individual CPUs the 'cpumask' file has to
> > +			  be configured manually after bootup.
> 
> Hmm. Why don't you set that to the housekeeping cpus by default?

"nohz_full=" does affine to housekeeping by default but "isolcpus=nohz" doesn't
as it is designed for more finegrained settings.

In fact nohz_full= is what people want to use right now as isolcpus= is still
under construction. I'm waiting to be able to make its effects mutable through
cpusets before extending it further. But in the end I would like to have things
such as:

     isolcpus=nohz,async

This would affine the tick and everything that is async (workqueues, unbound timers,
unbound kthreads, etc...) to housekeeping.

And an even simpler toggle to fully isolate a CPU:

     isolcpus=all

This is to avoid that nohz alone implies side effects. For example if we affine
workqueues right now and later the 1hz offloaded tick isn't necessary anymore,
we'll stick with affined workqueues as a side effect without reason, and reverting
that would end up into regression.

Thanks.

Re: [PATCH 1/6] sched: Rename init_rq_hrtick to hrtick_rq_init

[Ingo Molnar]

* Frederic Weisbecker <frederic@kernel.org> wrote:

> Do that rename in order to normalize the hrtick namespace.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Chris Metcalf <cmetcalf@mellanox.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Luiz Capitulino <lcapitulino@redhat.com>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Wanpeng Li <kernellwp@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> ---
>  kernel/sched/core.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 36f113a..fc9fa25 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -333,7 +333,7 @@ void hrtick_start(struct rq *rq, u64 delay)
>  }
>  #endif /* CONFIG_SMP */
>  
> -static void init_rq_hrtick(struct rq *rq)
> +static void hrtick_rq_init(struct rq *rq)

On a related note, I think we should also do:

s/start_hrtick_dl
 /sched_dl_hrtick_start

or such. (In a separate patch)

Thanks,

	Ingo

[PATCH 1/6] sched: Rename init_rq_hrtick to hrtick_rq_init

[Frederic Weisbecker]

Do that rename in order to normalize the hrtick namespace.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 36f113a..fc9fa25 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -333,7 +333,7 @@ void hrtick_start(struct rq *rq, u64 delay)
 }
 #endif /* CONFIG_SMP */
 
-static void init_rq_hrtick(struct rq *rq)
+static void hrtick_rq_init(struct rq *rq)
 {
 #ifdef CONFIG_SMP
 	rq->hrtick_csd_pending = 0;
@@ -351,7 +351,7 @@ static inline void hrtick_clear(struct rq *rq)
 {
 }
 
-static inline void init_rq_hrtick(struct rq *rq)
+static inline void hrtick_rq_init(struct rq *rq)
 {
 }
 #endif	/* CONFIG_SCHED_HRTICK */
@@ -6023,7 +6023,7 @@ void __init sched_init(void)
 		rq->last_sched_tick = 0;
 #endif
 #endif /* CONFIG_SMP */
-		init_rq_hrtick(rq);
+		hrtick_rq_init(rq);
 		atomic_set(&rq->nr_iowait, 0);
 	}
 
-- 
2.7.4

[PATCH 1/6] sched: Rename init_rq_hrtick to hrtick_rq_init

[Frederic Weisbecker]

Do that rename in order to normalize the hrtick namespace.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 644fa2e..d72d0e9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -333,7 +333,7 @@ void hrtick_start(struct rq *rq, u64 delay)
 }
 #endif /* CONFIG_SMP */
 
-static void init_rq_hrtick(struct rq *rq)
+static void hrtick_rq_init(struct rq *rq)
 {
 #ifdef CONFIG_SMP
 	rq->hrtick_csd_pending = 0;
@@ -351,7 +351,7 @@ static inline void hrtick_clear(struct rq *rq)
 {
 }
 
-static inline void init_rq_hrtick(struct rq *rq)
+static inline void hrtick_rq_init(struct rq *rq)
 {
 }
 #endif	/* CONFIG_SCHED_HRTICK */
@@ -5955,7 +5955,7 @@ void __init sched_init(void)
 		rq->last_sched_tick = 0;
 #endif
 #endif /* CONFIG_SMP */
-		init_rq_hrtick(rq);
+		hrtick_rq_init(rq);
 		atomic_set(&rq->nr_iowait, 0);
 	}
 
-- 
2.7.4