Commit 27185016 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

nohz: Track last do_timer() cpu 



The previous patch which limits the sleep time to the maximum
deferment time of the time keeping clocksource has some limitations on
SMP machines: if all CPUs are idle then for all CPUs the maximum sleep
time is limited.

Solve this by keeping track of which cpu had the do_timer() duty
assigned last and limit the sleep time only for this cpu.

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
LKML-Reference: <new-submission>
Cc: Jon Hunter <jon-hunter@ti.com>
Cc: John Stultz <johnstul@us.ibm.com>
parent 98962465

include/linux/tick.h

+2 −0
Original line number Diff line number Diff line
@@ -43,6 +43,7 @@ enum tick_nohz_mode { 
 * @idle_exittime:	Time when the idle state was left

 * @idle_sleeptime:	Sum of the time slept in idle with sched tick stopped

 * @sleep_length:	Duration of the current idle sleep

 * @do_timer_lst:	CPU was the last one doing do_timer before going idle

 */

struct tick_sched {

	struct hrtimer			sched_timer;
@@ -64,6 +65,7 @@ struct tick_sched { 
	unsigned long			last_jiffies;

	unsigned long			next_jiffies;

	ktime_t				idle_expires;

	int				do_timer_last;

};



extern void __init tick_init(void);

kernel/time/tick-sched.c

+28 −24
Original line number Diff line number Diff line
@@ -263,17 +263,7 @@ void tick_nohz_stop_sched_tick(int inidle) 
		seq = read_seqbegin(&xtime_lock);

		last_update = last_jiffies_update;

		last_jiffies = jiffies;



		/*

		 * On SMP we really should only care for the CPU which

		 * has the do_timer duty assigned. All other CPUs can

		 * sleep as long as they want.

		 */

		if (cpu == tick_do_timer_cpu ||

		    tick_do_timer_cpu == TICK_DO_TIMER_NONE)

		time_delta = timekeeping_max_deferment();

		else

			time_delta = KTIME_MAX;

	} while (read_seqretry(&xtime_lock, seq));



	if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
@@ -295,6 +285,29 @@ void tick_nohz_stop_sched_tick(int inidle) 
	/* Schedule the tick, if we are at least one jiffie off */

	if ((long)delta_jiffies >= 1) {



		/*

		 * If this cpu is the one which updates jiffies, then

		 * give up the assignment and let it be taken by the

		 * cpu which runs the tick timer next, which might be

		 * this cpu as well. If we don't drop this here the

		 * jiffies might be stale and do_timer() never

		 * invoked. Keep track of the fact that it was the one

		 * which had the do_timer() duty last. If this cpu is

		 * the one which had the do_timer() duty last, we

		 * limit the sleep time to the timekeeping

		 * max_deferement value which we retrieved

		 * above. Otherwise we can sleep as long as we want.

		 */

		if (cpu == tick_do_timer_cpu) {

			tick_do_timer_cpu = TICK_DO_TIMER_NONE;

			ts->do_timer_last = 1;

		} else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {

			time_delta = KTIME_MAX;

			ts->do_timer_last = 0;

		} else if (!ts->do_timer_last) {

			time_delta = KTIME_MAX;

		}



		/*

		 * calculate the expiry time for the next timer wheel

		 * timer. delta_jiffies >= NEXT_TIMER_MAX_DELTA signals
@@ -312,21 +325,12 @@ void tick_nohz_stop_sched_tick(int inidle) 
			 */

			time_delta = min_t(u64, time_delta,

					   tick_period.tv64 * delta_jiffies);

			expires = ktime_add_ns(last_update, time_delta);

		} else {

			expires.tv64 = KTIME_MAX;

		}



		/*

		 * If this cpu is the one which updates jiffies, then

		 * give up the assignment and let it be taken by the

		 * cpu which runs the tick timer next, which might be

		 * this cpu as well. If we don't drop this here the

		 * jiffies might be stale and do_timer() never

		 * invoked.

		 */

		if (cpu == tick_do_timer_cpu)

			tick_do_timer_cpu = TICK_DO_TIMER_NONE;

		if (time_delta < KTIME_MAX)

			expires = ktime_add_ns(last_update, time_delta);

		else

			expires.tv64 = KTIME_MAX;



		if (delta_jiffies > 1)

			cpumask_set_cpu(cpu, nohz_cpu_mask);

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