From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751898AbaANStP (ORCPT ); Tue, 14 Jan 2014 13:49:15 -0500 Received: from mx1.redhat.com ([209.132.183.28]:62437 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751847AbaANSsx (ORCPT ); Tue, 14 Jan 2014 13:48:53 -0500 Date: Tue, 14 Jan 2014 19:48:28 +0100 From: Oleg Nesterov To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH tip/core/timers 1/3] timers: Reduce __run_timers() latency for empty list Message-ID: <20140114184828.GA29331@redhat.com> References: <20140114041449.GA13934@linux.vnet.ibm.com> <1389672919-14621-1-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1389672919-14621-1-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/13, Paul E. McKenney wrote: > > The __run_timers() function currently steps through the list one jiffy at > a time in order to update the timer wheel. However, if the timer wheel > is empty, no adjustment is needed other than updating ->timer_jiffies. Yes, but ->active_timers == 0 doesn't necessarily mean "empty", it only counts the non-deferrable timers? > In this case, which is likely to be common for NO_HZ_FULL kernels, the > kernel currently incurs a large latency for no good reason. This commit > therefore short-circuits this case. > > Signed-off-by: Paul E. McKenney > --- > kernel/timer.c | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) > > diff --git a/kernel/timer.c b/kernel/timer.c > index 6582b82fa966..21849275828f 100644 > --- a/kernel/timer.c > +++ b/kernel/timer.c > @@ -337,6 +337,17 @@ void set_timer_slack(struct timer_list *timer, int slack_hz) > } > EXPORT_SYMBOL_GPL(set_timer_slack); > > +static bool catchup_timer_jiffies(struct tvec_base *base) > +{ > +#ifdef CONFIG_NO_HZ_FULL > + if (!base->active_timers) { > + base->timer_jiffies = jiffies; > + return 1; > + } > +#endif /* #ifdef CONFIG_NO_HZ_FULL */ > + return 0; > +} > + > static void > __internal_add_timer(struct tvec_base *base, struct timer_list *timer) > { > @@ -1146,6 +1157,10 @@ static inline void __run_timers(struct tvec_base *base) > struct timer_list *timer; > > spin_lock_irq(&base->lock); Do we really need to take base->lock before catchup_timer_jiffies() ? ->timer_jiffies can only be changed by us, and it seems that we do not care if we race with base->active_timers++. > + if (catchup_timer_jiffies(base)) { > + spin_unlock_irq(&base->lock); > + return; This is what I can't understand... Doesn't this mean that, unless this base have a non-deferrable timer, we can never run the pending deferrable timers even if the system/cpu is "busy" ? Oleg.