linux-kernel - Re: [PATCH v2] cpu/hotplug: Set st->cpu earlier

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Date:   Wed, 23 Mar 2022 12:21:48 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Price <steven.price@....com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Baokun Li <libaokun1@...wei.com>,
        Dongli Zhang <dongli.zhang@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Yuan ZhaoXiong <yuanzhaoxiong@...du.com>,
        YueHaibing <yuehaibing@...wei.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v2] cpu/hotplug: Set st->cpu earlier

On Wed, Mar 23 2022 at 10:10, Steven Price wrote:
> On 22/03/2022 22:58, Thomas Gleixner wrote:
>> Indeed. But the description is not the only problem here:
>> 
>> It's completely uncomprehensible from the code in _cpu_up() _WHY_ this
>> 
>>      st->cpu = cpu;
>>      
>> assignment has to be there.
>> 
>> It's non-sensical if you really think about it, right?
>
> I entirely agree, and I did ask in my v1 posting[1] if anyone could
> point me to a better place to do the assignment. Vincent suggested
> moving it earlier in _cpu_up() which is this v2.
>
> But it still seems out-of-place to me. I've just had a go at simply
> removing the 'cpu' member and it doesn't look too bad. I'll post that
> patch as a follow up. I'm open to other suggestions for the best way to
> fix this.

Yes, we can do that. The alternative solution is to initialize the
states once upfront. Something like the uncompiled below.

Thanks,

        tglx
---
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -714,15 +714,6 @@ static int cpuhp_up_callbacks(unsigned i
 /*
  * The cpu hotplug threads manage the bringup and teardown of the cpus
  */
-static void cpuhp_create(unsigned int cpu)
-{
-	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
-
-	init_completion(&st->done_up);
-	init_completion(&st->done_down);
-	st->cpu = cpu;
-}
-
 static int cpuhp_should_run(unsigned int cpu)
 {
 	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
@@ -882,15 +873,28 @@ static int cpuhp_kick_ap_work(unsigned i
 
 static struct smp_hotplug_thread cpuhp_threads = {
 	.store			= &cpuhp_state.thread,
-	.create			= &cpuhp_create,
 	.thread_should_run	= cpuhp_should_run,
 	.thread_fn		= cpuhp_thread_fun,
 	.thread_comm		= "cpuhp/%u",
 	.selfparking		= true,
 };
 
+static __init void cpuhp_init_state(void)
+{
+	struct cpuhp_cpu_state *st;
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		st = per_cpu_ptr(&cpuhp_state, cpu);
+		init_completion(&st->done_up);
+		init_completion(&st->done_down);
+		st->cpu = cpu;
+	}
+}
+
 void __init cpuhp_threads_init(void)
 {
+	cpuhp_init_state();
 	BUG_ON(smpboot_register_percpu_thread(&cpuhp_threads));
 	kthread_unpark(this_cpu_read(cpuhp_state.thread));
 }