linux-kernel - Re: [PATCH 0/3] timers: Allocate per-cpu tvec

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Date:	Mon, 30 Mar 2015 10:18:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] timers: Allocate per-cpu tvec_base's statically

On Mon, Mar 30, 2015 at 10:47:16AM +0530, Viresh Kumar wrote:
> Hi Ingo/Thomas,
> 
> Here are few cleanups around timer-core initialization. The first one is
> suggested by Peter Zijlstra [1] and the other two make few more changes in order
> to simplify it.

I did the below on top; I'll sit on these patches for a wee while in the
hope of the blackfin maintainer explaining his reasons for wrecking
____cacheline_aligned.

---
Subject: timer: Further simplify SMP and HOTPLUG logic
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon Mar 30 10:09:19 CEST 2015

Remove one CONFIG_HOTPLUG_CPU #ifdef in trade for introducing one
CONFIG_SMP #ifdef.

The CONFIG_SMP ifdef avoids declaring the per-cpu __tvec_bases storage
on UP systems since they already have boot_tvec_bases.

Also (re)add a runtime check on the base alignment -- for the paranoid
amongst us :-)

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/time/timer.c |   28 +++++++++++++---------------
 1 file changed, 13 insertions(+), 15 deletions(-)

--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1580,12 +1580,10 @@ static void migrate_timers(int cpu)
 	spin_unlock_irq(&new_base->lock);
 	put_cpu_var(tvec_bases);
 }
-#endif /* CONFIG_HOTPLUG_CPU */
 
 static int timer_cpu_notify(struct notifier_block *self,
 				unsigned long action, void *hcpu)
 {
-#ifdef CONFIG_HOTPLUG_CPU
 	switch (action) {
 	case CPU_DEAD:
 	case CPU_DEAD_FROZEN:
@@ -1594,18 +1592,17 @@ static int timer_cpu_notify(struct notif
 	default:
 		break;
 	}
-#endif
+
 	return NOTIFY_OK;
 }
-
-static struct notifier_block timers_nb = {
-	.notifier_call	= timer_cpu_notify,
-};
+#endif /* CONFIG_HOTPLUG_CPU */
 
 static void __init init_timer_cpu(struct tvec_base *base, int cpu)
 {
 	int j;
 
+	BUG_ON(base != tbase_get_base(base));
+
 	base->cpu = cpu;
 	per_cpu(tvec_bases, cpu) = base;
 	spin_lock_init(&base->lock);
@@ -1625,18 +1622,19 @@ static void __init init_timer_cpu(struct
 
 static void __init init_timer_cpus(void)
 {
-	/* Used for all CPUs, except boot CPU (which uses boot_tvec_bases) */
-	static DEFINE_PER_CPU(struct tvec_base, __tvec_bases);
-	struct tvec_base *base;
 	int local_cpu = smp_processor_id();
+	struct tvec_base *base;
 	int cpu;
 
 	for_each_possible_cpu(cpu) {
-		if (likely(cpu != local_cpu))
-			base = per_cpu_ptr(&__tvec_bases, cpu);
-		else
+		if (cpu == local_cpu)
 			base = &boot_tvec_bases;
-
+#ifdef CONFIG_SMP
+		else {
+			static DEFINE_PER_CPU(struct tvec_base, __tvec_bases);
+			base = per_cpu_ptr(&__tvec_bases, cpu);
+		}
+#endif
 		init_timer_cpu(base, cpu);
 	}
 }
@@ -1648,7 +1646,7 @@ void __init init_timers(void)
 
 	init_timer_cpus();
 	init_timer_stats();
-	register_cpu_notifier(&timers_nb);
+	cpu_notifier(timer_cpu_notify, 0);
 	open_softirq(TIMER_SOFTIRQ, run_timer_softirq);
 }
 
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