Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933694Ab2EWPXu (ORCPT ); Wed, 23 May 2012 11:23:50 -0400 Received: from cantor2.suse.de ([195.135.220.15]:50893 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932597Ab2EWPXr (ORCPT ); Wed, 23 May 2012 11:23:47 -0400 Message-ID: <1337786613.18720.5.camel@marge.simpson.net> Subject: [PATCH v3] clockevents: Per cpu tick skew boot option From: Mike Galbraith To: paulmck@linux.vnet.ibm.com Cc: Thomas Gleixner , LKML Date: Wed, 23 May 2012 17:23:33 +0200 In-Reply-To: <20120510181607.GA14329@linux.vnet.ibm.com> References: <1336309127.7351.13.camel@marge.simpson.net> <1336447221.7364.2.camel@marge.simpson.net> <1336472458.21924.78.camel@marge.simpson.net> <20120510181607.GA14329@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4598 Lines: 135 On Thu, 2012-05-10 at 11:16 -0700, Paul E. McKenney wrote: > > --- a/Documentation/kernel-parameters.txt > > +++ b/Documentation/kernel-parameters.txt > > @@ -2426,6 +2426,15 @@ bytes respectively. Such letter suffixes > > > > 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. > > Suggest instead: > > contention on systems with large CONFIG_RCU_FANOUT > values. > > > + Format: { "0" | "1" } > > + 0 -- disable. (may be 1 via CONFIG_CMDLINE="skew_tick=1" > > Suggest simply: > > 0 -- disable (default for typical kernel builds). > > With these changes: > > Acked-by: Paul E. McKenney Hunted down round-tuit. clockevents: Per cpu tick skew boot option 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. End quote Problems: - Contrary to the above, all systems do encounter contention on xtime_lock and RCU structure locks when the tick is synchronized. - Large systems and moderate sized RT systems suffer intolerable jitter with the tick synchronized. - Fully utilized systems reap no power saving benefit, but do suffer from synchronized tick 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. Measured latency on 48 core box was >330us. Revert, amd restore skew, it dropped back to ~70us. We absorbed a 400% latency increase to combat induced contention. Let the user decide whether power consumption or jitter is the more important consideration for their machines. Signed-off-by: Mike Galbraith Acked-by: Paul E. McKenney --- Documentation/kernel-parameters.txt | 9 +++++++++ kernel/time/tick-sched.c | 19 +++++++++++++++++++ 2 files changed, 28 insertions(+) --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -2426,6 +2426,15 @@ bytes respectively. Such letter suffixes 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 systems with large CONFIG_RCU_FANOUT values. + Format: { "0" | "1" } + 0 -- disable (default for typical kernel builds). + 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 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -814,6 +814,8 @@ static enum hrtimer_restart tick_sched_t 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 +920,12 @@ int tick_check_oneshot_change(int allow_ 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/