From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758304Ab1GKRfV (ORCPT ); Mon, 11 Jul 2011 13:35:21 -0400 Received: from e8.ny.us.ibm.com ([32.97.182.138]:51946 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758255Ab1GKRfT (ORCPT ); Mon, 11 Jul 2011 13:35:19 -0400 Date: Mon, 11 Jul 2011 10:30:47 -0700 From: "Paul E. McKenney" To: "Alex,Shi" Cc: tglx@linutronix.de, mingo@redhat.com, "linux-kernel@vger.kernel.org" , "Fu, Michael" Subject: Re: nohz: remove nohz_cpu_mask Message-ID: <20110711173047.GL2245@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1310345225.28599.10.camel@debian> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1310345225.28599.10.camel@debian> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 11, 2011 at 08:47:05AM +0800, Alex,Shi wrote: > RCU didn't use this global variable now. Currently no user on it. Indeed. RCU's use of it turned out to be quite buggy. :-( > Since the ts->do_timer_last is not the real last periodic tick cpu in > most of time. I once want to compare the cpu_online_mask and > nohz_cpu_mask to get a real one, and than only let that cpu sleep > shorter, other cpu will try to sleep KTIME_MAX, that need a extra lock > for nohz_cpu_mask. But I checked my all platforms, from NHM-EX server to > laptops, all of them are waked up a few times per second. So, the > advantage is only in theory. > > Since no clear usage of this variable, why not remove it? That can save > a cache-line in all cpus and reduce atomic sync contention. Works for me! Reviewed-by: Paul E. McKenney > Signed-off-by: Alex Shi > --- > diff --git a/include/linux/sched.h b/include/linux/sched.h > index a837b20..6f5cfb3 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -270,7 +270,6 @@ extern void init_idle_bootup_task(struct task_struct *idle); > > extern int runqueue_is_locked(int cpu); > > -extern cpumask_var_t nohz_cpu_mask; > #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ) > extern void select_nohz_load_balancer(int stop_tick); > extern int get_nohz_timer_target(void); > diff --git a/kernel/sched.c b/kernel/sched.c > index 3f2e502..a48343c 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -5907,15 +5907,6 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu) > } > > /* > - * In a system that switches off the HZ timer nohz_cpu_mask > - * indicates which cpus entered this state. This is used > - * in the rcu update to wait only for active cpus. For system > - * which do not switch off the HZ timer nohz_cpu_mask should > - * always be CPU_BITS_NONE. > - */ > -cpumask_var_t nohz_cpu_mask; > - > -/* > * Increase the granularity value when there are more CPUs, > * because with more CPUs the 'effective latency' as visible > * to users decreases. But the relationship is not linear, > @@ -8010,8 +8001,6 @@ void __init sched_init(void) > */ > current->sched_class = &fair_sched_class; > > - /* Allocate the nohz_cpu_mask if CONFIG_CPUMASK_OFFSTACK */ > - zalloc_cpumask_var(&nohz_cpu_mask, GFP_NOWAIT); > #ifdef CONFIG_SMP > zalloc_cpumask_var(&sched_domains_tmpmask, GFP_NOWAIT); > #ifdef CONFIG_NO_HZ > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index d5097c4..eb98e55 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -139,7 +139,6 @@ static void tick_nohz_update_jiffies(ktime_t now) > struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu); > unsigned long flags; > > - cpumask_clear_cpu(cpu, nohz_cpu_mask); > ts->idle_waketime = now; > > local_irq_save(flags); > @@ -389,9 +388,6 @@ void tick_nohz_stop_sched_tick(int inidle) > else > expires.tv64 = KTIME_MAX; > > - if (delta_jiffies > 1) > - cpumask_set_cpu(cpu, nohz_cpu_mask); > - > /* Skip reprogram of event if its not changed */ > if (ts->tick_stopped && ktime_equal(expires, dev->next_event)) > goto out; > @@ -441,7 +437,6 @@ void tick_nohz_stop_sched_tick(int inidle) > * softirq. > */ > tick_do_update_jiffies64(ktime_get()); > - cpumask_clear_cpu(cpu, nohz_cpu_mask); > } > raise_softirq_irqoff(TIMER_SOFTIRQ); > out: > @@ -524,7 +519,6 @@ void tick_nohz_restart_sched_tick(void) > /* Update jiffies first */ > select_nohz_load_balancer(0); > tick_do_update_jiffies64(now); > - cpumask_clear_cpu(cpu, nohz_cpu_mask); > > #ifndef CONFIG_VIRT_CPU_ACCOUNTING > /* > > > -- > 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/