sched: Keep at least 1 tick per second for active dynticks tasks

The scheduler doesn't yet fully support environments
with a single task running without a periodic tick.

In order to ensure we still maintain the duties of scheduler_tick(),
keep at least 1 tick per second.

This makes sure that we keep the progression of various scheduler
accounting and background maintainance even with a very low granularity.
Examples include cpu load, sched average, CFS entity vruntime,
avenrun and events such as load balancing, amongst other details
handled in sched_class::task_tick().

This limitation will be removed in the future once we get
these individual items to work in full dynticks CPUs.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 30 insertions, 0 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e94842d4400c..3bdf986a091a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2736,8 +2736,35 @@ void scheduler_tick(void)
         rq->idle_balance = idle_cpu(cpu);
         trigger_load_balance(rq, cpu);
 #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.
+ */
+u64 scheduler_tick_max_deferment(void)
+{
+        struct rq *rq = this_rq();
+        unsigned long next, now = ACCESS_ONCE(jiffies);
+
+        next = rq->last_sched_tick + HZ;
+
+        if (time_before_eq(next, now))
+                return 0;
+
+        return jiffies_to_usecs(next - now) * NSEC_PER_USEC;
+}
+#endif
+
 notrace unsigned long get_parent_ip(unsigned long addr)
 {
         if (in_lock_functions(addr)) {
@@ -6993,6 +7020,9 @@ void __init sched_init(void)
 #ifdef CONFIG_NO_HZ_COMMON
                 rq->nohz_flags = 0;
 #endif
+#ifdef CONFIG_NO_HZ_FULL
+                rq->last_sched_tick = 0;
+#endif
 #endif
                 init_rq_hrtick(rq);
                 atomic_set(&rq->nr_iowait, 0);