linux-kernel - Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE

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Date:	Wed, 23 Feb 2011 17:16:48 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
 to kthread

On Tue, Feb 22, 2011 at 05:39:40PM -0800, Paul E. McKenney wrote:
> +/*
> + * Wake up the current CPU's kthread.  This replaces raise_softirq()
> + * in earlier versions of RCU.
> + */
> +static void invoke_rcu_kthread(void)
> +{
> +	unsigned long flags;
> +	wait_queue_head_t *q;
> +	int cpu;
> +
> +	local_irq_save(flags);
> +	cpu = smp_processor_id();
> +	if (per_cpu(rcu_cpu_kthread_task, cpu) == NULL) {
> +		local_irq_restore(flags);
> +		return;
> +	}
> +	per_cpu(rcu_cpu_has_work, cpu) = 1;
> +	q = &per_cpu(rcu_cpu_wq, cpu);

I see you make extensive use of per_cpu() accessors even for
local variables.

I tend to think it's better to use __get_cpu_var() for local
accesses and keep per_cpu() for remote accesses.

There are several reasons for that:

* __get_cpu_var() checks we are in a non-preemptible section,
per_cpu() doesn't. That may sound of a limited interest for code like the
above, but by the time code can move, and then we might lose track of some
things, etc...

* local accesses can be optimized by architectures. per_cpu() implies
finding the local cpu number, and dereference an array of cpu offsets with
that number to find the local cpu offset.
__get_cpu_var() does a direct access to __my_cpu_offset which is a nice
shortcut if the arch implements it.

* It makes code easier to review: we know that __get_cpu_var() is
for local accesses and per_cpu() for remote.

> +	wake_up(q);
> +	local_irq_restore(flags);
> +}
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