From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754122Ab3KEAxc (ORCPT ); Mon, 4 Nov 2013 19:53:32 -0500 Received: from mga14.intel.com ([143.182.124.37]:32069 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753833Ab3KEAxa (ORCPT ); Mon, 4 Nov 2013 19:53:30 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.93,535,1378882800"; d="scan'208";a="316987204" Message-ID: <52783EFA.5080901@intel.com> Date: Tue, 05 Nov 2013 08:42:34 +0800 From: Lan Tianyu User-Agent: Mozilla/5.0 (X11; Linux i686; rv:14.0) Gecko/20120714 Thunderbird/14.0 MIME-Version: 1.0 To: paulmck@linux.vnet.ibm.com CC: rjw@rjwysocki.net, viresh.kumar@linaro.org, dipankar@in.ibm.com, fweisbec@gmail.com, youquan.song@intel.com, riel@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] Tick: Introduce tick_nohz_check() to check nohz enable status References: <1383036537-30362-1-git-send-email-tianyu.lan@intel.com> <20131029095135.GH4126@linux.vnet.ibm.com> <526F8E21.1030702@intel.com> <527710C8.8020908@intel.com> <20131104095258.GI3947@linux.vnet.ibm.com> In-Reply-To: <20131104095258.GI3947@linux.vnet.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2013年11月04日 17:52, Paul E. McKenney wrote: > On Mon, Nov 04, 2013 at 11:13:12AM +0800, Lan Tianyu wrote: >> On 2013年10月29日 18:29, Lan Tianyu wrote: >>> On 10/29/2013 05:51 PM, Paul E. McKenney wrote: >>>> On Tue, Oct 29, 2013 at 04:48:56PM +0800, Lan Tianyu wrote: >>>>> In some cases, nohz enable status needs to be checked. E.G, in RCU >>>>> and cpufreq >>>>> ondemand governor. So add tick_nohz_check() to return >>>>> tick_nohz_enabled value >>>>> And use tick_nohz_check() instead of referencing tick_nohz_enabled in >>>>> the rcutree_plugin.h. >>>>> >>>>> Signed-off-by: Lan Tianyu >>>> >>>> NACK on the rcutree change unless you put the ACCESS_ONCE() in. >>>> >>>> Or is there some reason that ACCESS_ONCE() is not needed? If so, what >>>> is that reason? >>> >>> Hi Paul: >>> >>> Thanks for review. When I change this code, I find the tick_nohz_enabled >>> isn't changed dynamically. It's only changed during parsing kernel >>> params when "nohz=off/on" is set. Except this, it will not be changed. >>> So I ignored ACCESS_ONCE(). If necessary, I can add it back. >> >> Hi Paul: >> Does this reason make sense to you? Or you still prefer to add >> ACCESS_ONCE() in the new tick_nohz_check()? > > I prefer the ACCESS_ONCE(). It adds little or no overhead, and Frederic > has been heard to say that he mkight allow tick_nohz_check() to change > in the future. > Ok. I will update it. > Thanx, Paul > >>>>> --- >>>>> include/linux/tick.h | 2 ++ >>>>> kernel/rcutree_plugin.h | 4 +--- >>>>> kernel/time/tick-sched.c | 8 +++++++- >>>>> 3 files changed, 10 insertions(+), 4 deletions(-) >>>>> >>>>> diff --git a/include/linux/tick.h b/include/linux/tick.h >>>>> index 5128d33..a9c5374 100644 >>>>> --- a/include/linux/tick.h >>>>> +++ b/include/linux/tick.h >>>>> @@ -136,6 +136,7 @@ static inline int tick_nohz_tick_stopped(void) >>>>> extern void tick_nohz_idle_enter(void); >>>>> extern void tick_nohz_idle_exit(void); >>>>> extern void tick_nohz_irq_exit(void); >>>>> +extern int tick_nohz_check(void); >>>>> extern ktime_t tick_nohz_get_sleep_length(void); >>>>> extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time); >>>>> extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time); >>>>> @@ -155,6 +156,7 @@ static inline ktime_t >>>>> tick_nohz_get_sleep_length(void) >>>>> >>>>> return len; >>>>> } >>>>> +static inline int tick_nohz_check(void) { return 0; } >>>>> static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { >>>>> return -1; } >>>>> static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { >>>>> return -1; } >>>>> # endif /* !CONFIG_NO_HZ_COMMON */ >>>>> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h >>>>> index 130c97b..af167ec 100644 >>>>> --- a/kernel/rcutree_plugin.h >>>>> +++ b/kernel/rcutree_plugin.h >>>>> @@ -1627,8 +1627,6 @@ module_param(rcu_idle_gp_delay, int, 0644); >>>>> static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY; >>>>> module_param(rcu_idle_lazy_gp_delay, int, 0644); >>>>> >>>>> -extern int tick_nohz_enabled; >>>>> - >>>>> /* >>>>> * Try to advance callbacks for all flavors of RCU on the current CPU. >>>>> * Afterwards, if there are any callbacks ready for immediate >>>>> invocation, >>>>> @@ -1718,7 +1716,7 @@ static void rcu_prepare_for_idle(int cpu) >>>>> int tne; >>>>> >>>>> /* Handle nohz enablement switches conservatively. */ >>>>> - tne = ACCESS_ONCE(tick_nohz_enabled); >>>>> + tne = tick_nohz_check(); >>>>> if (tne != rdtp->tick_nohz_enabled_snap) { >>>>> if (rcu_cpu_has_callbacks(cpu, NULL)) >>>>> invoke_rcu_core(); /* force nohz to see update. */ >>>>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c >>>>> index 3612fc7..d381a22 100644 >>>>> --- a/kernel/time/tick-sched.c >>>>> +++ b/kernel/time/tick-sched.c >>>>> @@ -361,7 +361,13 @@ void __init tick_nohz_init(void) >>>>> /* >>>>> * NO HZ enabled ? >>>>> */ >>>>> -int tick_nohz_enabled __read_mostly = 1; >>>>> +static int tick_nohz_enabled __read_mostly = 1; >>>>> + >>>>> +int tick_nohz_check(void) >>>>> +{ >>>>> + return tick_nohz_enabled; >>>>> +} >>>>> +EXPORT_SYMBOL_GPL(tick_nohz_check); >>>>> >>>>> /* >>>>> * Enable / Disable tickless mode >>>>> -- >>>>> 1.8.4.rc0.1.g8f6a3e5.dirty >>>>> >>>> >>> >> >> >> -- >> Best regards >> Tianyu Lan >> > -- Best regards Tianyu Lan