linux-kernel - Re: [PATCH v2 RESEND/RFC] timer: make deferrable cpu unbound timers really not bound to a cpu

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Message-ID: <alpine.DEB.2.10.1409231808110.4604@nanos>
Date:	Tue, 23 Sep 2014 20:33:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Joonwoo Park <joonwoop@...eaurora.org>
cc:	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2 RESEND/RFC] timer: make deferrable cpu unbound timers
 really not bound to a cpu

On Mon, 15 Sep 2014, Joonwoo Park wrote:
> +#ifdef CONFIG_SMP
> +static struct tvec_base *tvec_base_deferral = &boot_tvec_bases;
> +#endif

In principle I like the idea of a deferrable wheel, but this
implementation is going to go nowhere.

First of all making it SMP only is silly. The deferrable stuff is a
pain in other places as well.

But whats way worse is:

> +static inline void __run_timers(struct tvec_base *base, bool try)
>  {
>  	struct timer_list *timer;
>  
> -	spin_lock_irq(&base->lock);
> +	if (!try)
> +		spin_lock_irq(&base->lock);
> +	else if (!spin_trylock_irq(&base->lock))
> +		return;

Yuck. All cpus fighting about a single spinlock roughly at the same
time? You just created a proper thundering herd cacheline bouncing
issue. 

No way. We have already mechanisms in place to deal with such
problems, you just have to use them.

Thanks,

	tglx
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