tick: Add tick skew boot option (5307c955) · Commits · 戴 / test

Let the user decide whether power consumption or jitter is the more important consideration for their machines. Quoting removal commit : "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. - 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:

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

Thomas Gleixner <tglx@linutronix.de>

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);

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