nohz: remove nohz_cpu_mask

RCU didn't use this global variable now. Currently no user on it.

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.

Signed-off-by Alex Shi <[email protected]>

---

include/linux/sched.h +0 -1

index a837b20..6f5cfb3 100644
@@ -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);

kernel/sched.c +0 -11

index 3f2e502..a48343c 100644
@@ -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

kernel/time/tick-sched.c +0 -7

index d5097c4..eb98e55 100644
@@ -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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Quoted 1 line · Alex,Shi

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. :-(

Quoted 8 lines

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.

Quoted 2 lines

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 <[email protected]>

Quoted 13 lines · Paul E. McKenney

Quoted 1 line · Alex,Shi

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. :-(

Quoted 8 lines

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.

Quoted 7 lines

Quoted 2 lines

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 <[email protected]>

Thomas, would you like to give some comments of this?

Best regards!
Alex