Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759247Ab2EXXw5 (ORCPT ); Thu, 24 May 2012 19:52:57 -0400 Received: from terminus.zytor.com ([198.137.202.10]:32974 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752419Ab2EXXwz (ORCPT ); Thu, 24 May 2012 19:52:55 -0400 Date: Thu, 24 May 2012 16:52:41 -0700 From: tip-bot for Mike Galbraith Message-ID: Cc: linux-kernel@vger.kernel.org, mgalbraith@suse.de, hpa@zytor.com, mingo@kernel.org, tglx@linutronix.de Reply-To: mingo@kernel.org, hpa@zytor.com, mgalbraith@suse.de, linux-kernel@vger.kernel.org, tglx@linutronix.de In-Reply-To: <1336472458.21924.78.camel@marge.simpson.net> References: <1336472458.21924.78.camel@marge.simpson.net> To: linux-tip-commits@vger.kernel.org Subject: [tip:timers/core] tick: Add tick skew boot option Git-Commit-ID: 5307c9556bc17e3cd26d4e94fc3b2565921834de X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.6 (terminus.zytor.com [127.0.0.1]); Thu, 24 May 2012 16:52:46 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4277 Lines: 115 Commit-ID: 5307c9556bc17e3cd26d4e94fc3b2565921834de Gitweb: http://git.kernel.org/tip/5307c9556bc17e3cd26d4e94fc3b2565921834de Author: Mike Galbraith AuthorDate: Tue, 8 May 2012 12:20:58 +0200 Committer: Thomas Gleixner CommitDate: Fri, 25 May 2012 01:44:50 +0200 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 af5ab277ded04bd9bc6b048c5a2f0e7d70ef0867: "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: Mike Galbraith Link: http://lkml.kernel.org/r/1336472458.21924.78.camel@marge.simpson.net Signed-off-by: Thomas Gleixner --- Documentation/kernel-parameters.txt | 9 +++++++++ kernel/time/tick-sched.c | 18 ++++++++++++++++++ 2 files changed, 27 insertions(+), 0 deletions(-) diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index b69cfdc12..ea38cd1 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -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 diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 6a3a5b9..4eddbb5 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -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); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/