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SubjectRe: nohz: remove nohz_cpu_mask
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 <paulmck@linux.vnet.ibm.com>

> Signed-off-by: Alex Shi <alex.shi@intel.com>
> ---
> 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
> /*
>
>
> --
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