Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Steven Rostedt]

On Tue, 24 Sep 2013 14:38:21 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> +#define cpuhp_writer_wait(cond)						\
> +do {									\
> +	for (;;) {							\
> +		set_current_state(TASK_UNINTERRUPTIBLE);		\
> +		if (cond)						\
> +			break;						\
> +		schedule();						\
> +	}								\
> +	__set_current_state(TASK_RUNNING);				\
> +} while (0)
> +
> +void __get_online_cpus(void)

The above really needs a comment about how it is used. Otherwise, I can
envision someone calling this as "oh I can use this when I'm in a
preempt disable section", and the comment below for the
preempt_enable_no_resched() will no longer be true.

-- Steve


>  {
> -	if (cpu_hotplug.active_writer == current)
> +	if (cpuhp_writer_task == 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);
> +
> +	/*
> +	 * 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_wq, !__cpuhp_writer);
> +	preempt_disable();
> +
> +	/*
> +	 * It would be possible for cpu_hotplug_done() to complete before
> +	 * the atomic_inc() above; in which case there is no writer waiting
> +	 * and doing a wakeup would be BAD (tm).
> +	 *
> +	 * If however we still observe cpuhp_writer_task here we know
> +	 * cpu_hotplug_done() is currently stuck waiting for cpuhp_waitcount.
> +	 */
> +	if (atomic_dec_and_test(&cpuhp_waitcount) && cpuhp_writer_task)
> +		cpuhp_writer_wake();
> +}
> +EXPORT_SYMBOL_GPL(__get_online_cpus);
>  
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