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Subject[PATCH 4/6] sched/isolation: Residual 1Hz scheduler tick offload
Date
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 | 91 +++++++++++++++++++++++++++++++++++++++++++++++-
kernel/sched/isolation.c | 4 +++
kernel/sched/sched.h | 2 ++
3 files changed, 96 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fc9fa25..5c0e8b6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3120,7 +3120,94 @@ u64 scheduler_tick_max_deferment(void)

return jiffies_to_nsecs(next - now);
}
-#endif
+
+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.
+ */
+ 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 we tick in 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);
+ }
+
+ /*
+ * Perform remote tick every second. The arbitrary frequence is
+ * large enough to avoid overload and short enough to keep sched
+ * internal stats alive.
+ */
+ 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);
+ if (!tick_work_cpu) {
+ pr_err("Can't allocate remote tick struct\n");
+ return -ENOMEM;
+ }
+
+ return 0;
+}
+
+#else
+static void sched_tick_start(int cpu) { }
+static void sched_tick_stop(int cpu) { }
+#endif /* CONFIG_NO_HZ_FULL */

#if defined(CONFIG_PREEMPT) && (defined(CONFIG_DEBUG_PREEMPT) || \
defined(CONFIG_PREEMPT_TRACER))
@@ -5781,6 +5868,7 @@ 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 +5880,7 @@ 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 8f1c1de..d782302 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
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