linux-kernel - Re: [RFC patch 5/5] infiniband: ehca: Use hotplug thread infrastructure

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Date:	Mon, 18 Jun 2012 16:00:16 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [RFC patch 5/5] infiniband: ehca: Use hotplug thread infrastructure

On Wed, 13 Jun 2012 11:00:56 -0000, Thomas Gleixner <tglx@...utronix.de> wrote:
> @@ -662,10 +663,15 @@ static inline int find_next_online_cpu(s
>  		ehca_dmp(cpu_online_mask, cpumask_size(), "");
>  
>  	spin_lock_irqsave(&pool->last_cpu_lock, flags);
> -	cpu = cpumask_next(pool->last_cpu, cpu_online_mask);
> -	if (cpu >= nr_cpu_ids)
> -		cpu = cpumask_first(cpu_online_mask);
> -	pool->last_cpu = cpu;
> +	while (1) {
> +		cpu = cpumask_next(pool->last_cpu, cpu_online_mask);
> +		if (cpu >= nr_cpu_ids)
> +			cpu = cpumask_first(cpu_online_mask);
> +		pool->last_cpu = cpu;
> +		/* Might be on the way out */
> +		if (per_cpu_ptr(pool->cpu_comp_tasks, cpu)->active)
> +			break;
> +	}

Heh, isn't this what we used to call a "do while" loop? :)

Your infrastructure is a really weird mix.  On the one hand, it's a set
of callbacks: setup, cleanup, park, unpark.  Cool.

On the other hand, instead of a 'run' callback, you've got a thread_fn,
which has to loop and call smpboot_thread_check_parking().

If you just had the thread_fn, it'd be trivial to follow program flow.
If you just had the callbacks, it'd still be pretty easy, though it
seems like a little too much help.

As it is, we have Paul doing setup stuff inside his thread_fn:

     +	trace_rcu_utilization("Start CPU kthread@...ark");
     +	sp.sched_priority = RCU_KTHREAD_PRIO;
     +	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);

I'm just not sure this complexity wins us anything.  Why not
just let people "register_percpu_kthread" and make the thread_fn
identical to normal kthread fns:

        while (!kthread_should_stop()) {
                if (kthread_should_park()) {
                        kthread_parkme();
                        continue;
                }

Maybe implement a 'bool kthread_stop_or_park()' helper.

I'll whip up a patch on top of yours if you don't think it's crazy...

Cheers,
Rusty.
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