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

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Date:	Tue, 24 Sep 2013 18:31:46 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On 09/24, Peter Zijlstra wrote:
>
> +void __get_online_cpus(void)
>  {
> -	if (cpu_hotplug.active_writer == current)
> +	/* Support reader-in-writer recursion */
> +	if (__cpuhp_writer == current)
>  		return;
> -	mutex_lock(&cpu_hotplug.lock);
>  
> -	if (WARN_ON(!cpu_hotplug.refcount))
> -		cpu_hotplug.refcount++; /* try to fix things up */
> +	atomic_inc(&cpuhp_waitcount);
>  
> -	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
> -		wake_up_process(cpu_hotplug.active_writer);
> -	mutex_unlock(&cpu_hotplug.lock);
> +	/*
> +	 * We either call schedule() in the wait, or we'll fall through
> +	 * and reschedule on the preempt_enable() in get_online_cpus().
> +	 */
> +	preempt_enable_no_resched();
> +	wait_event(cpuhp_readers, !__cpuhp_writer);
> +	preempt_disable();
> +
> +	if (atomic_dec_and_test(&cpuhp_waitcount))
> +		wake_up_all(&cpuhp_writer);

Yes, this should fix the races with the exiting writer, but still this
doesn't look right afaics.

In particular let me repeat,

>  void cpu_hotplug_begin(void)
>  {
> -	cpu_hotplug.active_writer = current;
> +	unsigned int count = 0;
> +	int cpu;
> +
> +	lockdep_assert_held(&cpu_add_remove_lock);
>  
> -	for (;;) {
> -		mutex_lock(&cpu_hotplug.lock);
> -		if (likely(!cpu_hotplug.refcount))
> -			break;
> -		__set_current_state(TASK_UNINTERRUPTIBLE);
> -		mutex_unlock(&cpu_hotplug.lock);
> -		schedule();
> +	__cpuhp_writer = current;
> +
> +	/* 
> +	 * After this everybody will observe writer and take the slow path.
> +	 */
> +	synchronize_sched();

synchronize_sched() is slow. The new readers will likely notice
__cpuhp_writer != NULL much earlier and they will be blocked in
__get_online_cpus() while the writer sleeps before it actually
enters the critical section.

Or I completely misunderstood this all?

Oleg.

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