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

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Message-ID: <20130925155515.GA17447@redhat.com>
Date:	Wed, 25 Sep 2013 17:55:15 +0200
From:	Oleg Nesterov <oleg@...hat.com>
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>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	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:
>
> So now we drop from a no memory barriers fast path, into a memory
> barrier 'slow' path into blocking.

Cough... can't understand the above ;) In fact I can't understand
the patch... see below. But in any case, afaics the fast path
needs mb() unless you add another synchronize_sched() into
cpu_hotplug_done().

> +static inline void get_online_cpus(void)
> +{
> +	might_sleep();
> +
> +	/* Support reader-in-reader recursion */
> +	if (current->cpuhp_ref++) {
> +		barrier();
> +		return;
> +	}
> +
> +	preempt_disable();
> +	if (likely(!__cpuhp_writer))
> +		__this_cpu_inc(__cpuhp_refcount);

mb() to ensure the reader can't miss, say, a STORE done inside
the cpu_hotplug_begin/end section.

put_online_cpus() needs mb() as well.

> +void __get_online_cpus(void)
> +{
> +	if (__cpuhp_writer == 1) {
> +		/* See __srcu_read_lock() */
> +		__this_cpu_inc(__cpuhp_refcount);
> +		smp_mb();
> +		__this_cpu_inc(cpuhp_seq);
> +		return;
> +	}

OK, cpuhp_seq should guarantee cpuhp_readers_active_check() gets
the "stable" numbers. Looks suspicious... but lets assume this
works.

However, I do not see how "__cpuhp_writer == 1" can work, please
see below.

> +	/*
> +	 * XXX list_empty_careful(&cpuhp_readers.task_list) ?
> +	 */
> +	if (atomic_dec_and_test(&cpuhp_waitcount))
> +		wake_up_all(&cpuhp_writer);

Same problem as in previous version. __get_online_cpus() succeeds
without incrementing __cpuhp_refcount. "goto start" can't help
afaics.

>  void cpu_hotplug_begin(void)
>  {
> -	cpu_hotplug.active_writer = current;
> +	unsigned int count = 0;
> +	int cpu;
>  
> -	for (;;) {
> -		mutex_lock(&cpu_hotplug.lock);
> -		if (likely(!cpu_hotplug.refcount))
> -			break;
> -		__set_current_state(TASK_UNINTERRUPTIBLE);
> -		mutex_unlock(&cpu_hotplug.lock);
> -		schedule();
> -	}
> +	lockdep_assert_held(&cpu_add_remove_lock);
> +
> +	/* allow reader-in-writer recursion */
> +	current->cpuhp_ref++;
> +
> +	/* make readers take the slow path */
> +	__cpuhp_writer = 1;
> +
> +	/* See percpu_down_write() */
> +	synchronize_sched();

Suppose there are no readers at this point,

> +
> +	/* make readers block */
> +	__cpuhp_writer = 2;
> +
> +	/* Wait for all readers to go away */
> +	wait_event(cpuhp_writer, cpuhp_readers_active_check());

So wait_event() "quickly" returns.

Now. Why the new reader should see __cpuhp_writer = 2 ? It can
still see it == 1, and take that "if (__cpuhp_writer == 1)" path
above.

Oleg.

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