Commit 265f22a9 authored by Frederic Weisbecker's avatar Frederic Weisbecker
Browse files

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: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarFrederic 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>
parent 73c30828

include/linux/sched.h

+1 −0
Original line number Diff line number Diff line
@@ -1862,6 +1862,7 @@ static inline void wake_up_nohz_cpu(int cpu) { } 


#ifdef CONFIG_NO_HZ_FULL

extern bool sched_can_stop_tick(void);

extern u64 scheduler_tick_max_deferment(void);

#else

static inline bool sched_can_stop_tick(void) { return false; }

#endif

kernel/sched/core.c

+30 −0
Original line number Diff line number Diff line
@@ -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);

kernel/sched/idle_task.c

+1 −0
Original line number Diff line number Diff line
@@ -17,6 +17,7 @@ select_task_rq_idle(struct task_struct *p, int sd_flag, int flags) 
static void pre_schedule_idle(struct rq *rq, struct task_struct *prev)

{

	idle_exit_fair(rq);

	rq_last_tick_reset(rq);

}



static void post_schedule_idle(struct rq *rq)

kernel/sched/sched.h

+10 −0
Original line number Diff line number Diff line
@@ -409,6 +409,9 @@ struct rq { 
#ifdef CONFIG_NO_HZ_COMMON

	u64 nohz_stamp;

	unsigned long nohz_flags;

#endif

#ifdef CONFIG_NO_HZ_FULL

	unsigned long last_sched_tick;

#endif

	int skip_clock_update;
@@ -1090,6 +1093,13 @@ static inline void dec_nr_running(struct rq *rq) 
	rq->nr_running--;

}



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);

kernel/time/tick-sched.c

+7 −0
Original line number Diff line number Diff line
@@ -600,6 +600,13 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts, 
			time_delta = KTIME_MAX;

		}



#ifdef CONFIG_NO_HZ_FULL

		if (!ts->inidle) {

			time_delta = min(time_delta,

					 scheduler_tick_max_deferment());

		}

#endif



		/*

		 * calculate the expiry time for the next timer wheel

		 * timer. delta_jiffies >= NEXT_TIMER_MAX_DELTA signals

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