linux-kernel - Re: [PATCH] hotplug: Optimize {get,put}_online

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Date:	Mon, 23 Sep 2013 10:50:17 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On Thu, 19 Sep 2013 16:32:41 +0200
Peter Zijlstra <peterz@...radead.org> wrote:


> +extern void __get_online_cpus(void);
> +
> +static inline void get_online_cpus(void)
> +{
> +	might_sleep();
> +
> +	preempt_disable();
> +	if (likely(!__cpuhp_writer || __cpuhp_writer == current))
> +		this_cpu_inc(__cpuhp_refcount);
> +	else
> +		__get_online_cpus();
> +	preempt_enable();
> +}


This isn't much different than srcu_read_lock(). What about doing
something like this:

static inline void get_online_cpus(void)
{
	might_sleep();

	srcu_read_lock(&cpuhp_srcu);
	if (unlikely(__cpuhp_writer || __cpuhp_writer != current)) {
		srcu_read_unlock(&cpuhp_srcu);
		__get_online_cpus();
		current->online_cpus_held++;
	}
}

static inline void put_online_cpus(void)
{
	if (unlikely(current->online_cpus_held)) {
		current->online_cpus_held--;
		__put_online_cpus();
		return;
	}

	srcu_read_unlock(&cpuhp_srcu);
}

Then have the writer simply do:

	__cpuhp_write = current;
	synchronize_srcu(&cpuhp_srcu);

	<grab the mutex here>

-- Steve
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