linux-kernel - Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu hotplug threads

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Date:	Wed, 13 Jun 2012 11:57:48 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu
 hotplug threads

On Wed, Jun 13, 2012 at 11:00:54AM -0000, Thomas Gleixner wrote:
> Provide a generic interface for setting up and tearing down percpu
> threads.
> 
> On registration the threads for already online cpus are created and
> started. On deregistration (modules) the threads are stoppped.
> 
> During hotplug operations the threads are created, started, parked and
> unparked. The datastructure for registration provides a pointer to
> percpu storage space and optional setup, cleanup, park, unpark
> functions. These functions are called when the thread state changes.
> 
> Thread functions should look like the following:
> 
> int thread_fn(void *cookie)
> {
> 	while (!smpboot_thread_check_parking(cookie)) {
> 		do_stuff();
> 	}
> 	return 0;
> }
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

[ . . . ]

> Index: tip/kernel/cpu.c
> ===================================================================
> --- tip.orig/kernel/cpu.c
> +++ tip/kernel/cpu.c
> @@ -280,6 +280,7 @@ static int __ref _cpu_down(unsigned int 
>  				__func__, cpu);
>  		goto out_release;
>  	}
> +	smpboot_park_threads(cpu);
> 
>  	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
>  	if (err) {
> @@ -354,6 +355,10 @@ static int __cpuinit _cpu_up(unsigned in
>  		goto out;
>  	}
> 
> +	ret = smpboot_create_threads(cpu);
> +	if (ret)
> +		goto out;
> +
>  	ret = __cpu_notify(CPU_UP_PREPARE | mod, hcpu, -1, &nr_calls);
>  	if (ret) {
>  		nr_calls--;
> @@ -370,6 +375,7 @@ static int __cpuinit _cpu_up(unsigned in
> 
>  	/* Now call notifier in preparation. */
>  	cpu_notify(CPU_ONLINE | mod, hcpu);
> +	smpboot_unpark_threads(cpu);

OK, RCU must use the lower-level interfaces, given that one of
then CPU_ONLINE notifiers might invoke synchronize_rcu().

							Thanx, Paul

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