104 lines
3.7 KiB
Diff
104 lines
3.7 KiB
Diff
From 59d39f2e52b8956a0d66e69ccd0aaeb80041c22d Mon Sep 17 00:00:00 2001
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From: Mike Galbraith <mgalbraith@suse.de>
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Date: Tue, 8 May 2012 12:20:58 +0200
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Subject: [004/254] tick: Add tick skew boot option
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Let the user decide whether power consumption or jitter is the
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more important consideration for their machines.
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Quoting removal commit af5ab277ded04bd9bc6b048c5a2f0e7d70ef0867:
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"Historically, Linux has tried to make the regular timer tick on the
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various CPUs not happen at the same time, to avoid contention on
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xtime_lock.
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Nowadays, with the tickless kernel, this contention no longer happens
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since time keeping and updating are done differently. In addition,
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this skew is actually hurting power consumption in a measurable way on
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many-core systems."
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Problems:
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- Contrary to the above, systems do encounter contention on both
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xtime_lock and RCU structure locks when the tick is synchronized.
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- Moderate sized RT systems suffer intolerable jitter due to the tick
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being synchronized.
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- SGI reports the same for their large systems.
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- Fully utilized systems reap no power saving benefit from skew removal,
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but do suffer from resulting induced lock contention.
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- 0209f649 rcu: limit rcu_node leaf-level fanout
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This patch was born to combat lock contention which testing showed
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to have been _induced by_ skew removal. Skew the tick, contention
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disappeared virtually completely.
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[ tglx: build fix ]
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Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
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Link: http://lkml.kernel.org/r/1336472458.21924.78.camel@marge.simpson.net
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Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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---
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Documentation/kernel-parameters.txt | 9 +++++++++
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kernel/time/tick-sched.c | 18 ++++++++++++++++++
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2 files changed, 27 insertions(+)
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diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
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index c1601e5..1e0150e 100644
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--- a/Documentation/kernel-parameters.txt
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+++ b/Documentation/kernel-parameters.txt
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@@ -2426,6 +2426,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
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sched_debug [KNL] Enables verbose scheduler debug messages.
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+ skew_tick= [KNL] Offset the periodic timer tick per cpu to mitigate
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+ xtime_lock contention on larger systems, and/or RCU lock
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+ contention on all systems with CONFIG_MAXSMP set.
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+ Format: { "0" | "1" }
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+ 0 -- disable. (may be 1 via CONFIG_CMDLINE="skew_tick=1"
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+ 1 -- enable.
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+ Note: increases power consumption, thus should only be
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+ enabled if running jitter sensitive (HPC/RT) workloads.
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+
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security= [SECURITY] Choose a security module to enable at boot.
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If this boot parameter is not specified, only the first
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security module asking for security registration will be
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diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
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index 6a3a5b9..efd3866 100644
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--- a/kernel/time/tick-sched.c
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+++ b/kernel/time/tick-sched.c
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@@ -814,6 +814,16 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
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return HRTIMER_RESTART;
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}
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+static int sched_skew_tick;
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+
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+static int __init skew_tick(char *str)
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+{
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+ get_option(&str, &sched_skew_tick);
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+
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+ return 0;
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+}
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+early_param("skew_tick", skew_tick);
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+
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/**
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* tick_setup_sched_timer - setup the tick emulation timer
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*/
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@@ -831,6 +841,14 @@ void tick_setup_sched_timer(void)
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/* Get the next period (per cpu) */
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hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());
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+ /* Offset the tick to avert xtime_lock contention. */
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+ if (sched_skew_tick) {
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+ u64 offset = ktime_to_ns(tick_period) >> 1;
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+ do_div(offset, num_possible_cpus());
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+ offset *= smp_processor_id();
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+ hrtimer_add_expires_ns(&ts->sched_timer, offset);
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+ }
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+
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for (;;) {
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hrtimer_forward(&ts->sched_timer, now, tick_period);
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hrtimer_start_expires(&ts->sched_timer,
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