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Commit 5307c955 authored by Mike Galbraith's avatar Mike Galbraith Committed by Thomas Gleixner
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tick: Add tick skew boot option 



Let the user decide whether power consumption or jitter is the
more important consideration for their machines.

Quoting removal commit af5ab277:

"Historically, Linux has tried to make the regular timer tick on the
 various CPUs not happen at the same time, to avoid contention on
 xtime_lock.
    
 Nowadays, with the tickless kernel, this contention no longer happens
 since time keeping and updating are done differently. In addition,
 this skew is actually hurting power consumption in a measurable way on
 many-core systems."

Problems:

- Contrary to the above, systems do encounter contention on both
  xtime_lock and RCU structure locks when the tick is synchronized.
  
- Moderate sized RT systems suffer intolerable jitter due to the tick
  being synchronized.

- SGI reports the same for their large systems.

- Fully utilized systems reap no power saving benefit from skew removal,
  but do suffer from resulting induced lock contention.

- 0209f649 rcu: limit rcu_node leaf-level fanout
  This patch was born to combat lock contention which testing showed
  to have been _induced by_ skew removal.  Skew the tick, contention
  disappeared virtually completely.

Signed-off-by: default avatarMike Galbraith <mgalbraith@suse.de>
Link: http://lkml.kernel.org/r/1336472458.21924.78.camel@marge.simpson.net


Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent ce004178

Documentation/kernel-parameters.txt

+9 −0
Original line number Diff line number Diff line
@@ -2532,6 +2532,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted. 


	sched_debug	[KNL] Enables verbose scheduler debug messages.



	skew_tick=	[KNL] Offset the periodic timer tick per cpu to mitigate

			xtime_lock contention on larger systems, and/or RCU lock

			contention on all systems with CONFIG_MAXSMP set.

			Format: { "0" | "1" }

			0 -- disable. (may be 1 via CONFIG_CMDLINE="skew_tick=1"

			1 -- enable.

			Note: increases power consumption, thus should only be

			enabled if running jitter sensitive (HPC/RT) workloads.



	security=	[SECURITY] Choose a security module to enable at boot.

			If this boot parameter is not specified, only the first

			security module asking for security registration will be

kernel/time/tick-sched.c

+18 −0
Original line number Diff line number Diff line
@@ -814,6 +814,8 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer) 
	return HRTIMER_RESTART;

}



static int sched_skew_tick;



/**

 * tick_setup_sched_timer - setup the tick emulation timer

 */
@@ -831,6 +833,14 @@ void tick_setup_sched_timer(void) 
	/* Get the next period (per cpu) */

	hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());



	/* Offset the tick to avert xtime_lock contention. */

	if (sched_skew_tick) {

		u64 offset = ktime_to_ns(tick_period) >> 1;

		do_div(offset, num_possible_cpus());

		offset *= smp_processor_id();

		hrtimer_add_expires_ns(&ts->sched_timer, offset);

	}



	for (;;) {

		hrtimer_forward(&ts->sched_timer, now, tick_period);

		hrtimer_start_expires(&ts->sched_timer,
@@ -910,3 +920,11 @@ int tick_check_oneshot_change(int allow_nohz) 
	tick_nohz_switch_to_nohz();

	return 0;

}



static int __init skew_tick(char *str)

{

	get_option(&str, &sched_skew_tick);



	return 0;

}

early_param("skew_tick", skew_tick);