Re: nohz: remove nohz_cpu_mask

[Paul E. McKenney]

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@xxxxxxxxxxxxxxxxxx>

> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
> ---
> 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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