linux-kernel - Re: [patch 13/20] timer: Switch to a non cascading wheel

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Message-ID: <20160613143647.GA17907@localhost.localdomain>
Date:	Mon, 13 Jun 2016 16:36:48 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de
Subject: Re: [patch 13/20] timer: Switch to a non cascading wheel

On Mon, Jun 13, 2016 at 08:41:00AM -0000, Thomas Gleixner wrote:
> +static inline struct timer_base *get_timer_base(u32 tflags)
> +{
> +	return get_timer_cpu_base(tflags, tflags & TIMER_BASEMASK);
> +}

This should rather be (tflags & TIMER_CPUMASK) to avoid using
per_cpu_ptr() with the TIMER_MIGRATING bit set in the CPU index.

The one caller in this patch is okay, since it already checks that
TIMER_MIGRATING is clear:

>  static struct timer_base *lock_timer_base(struct timer_list *timer,
> -					unsigned long *flags)
> +					  unsigned long *flags)
>  	__acquires(timer->base->lock)
>  {
>  	for (;;) {
> -		u32 tf = timer->flags;
>  		struct timer_base *base;
> +		u32 tf = timer->flags;
>  
>  		if (!(tf & TIMER_MIGRATING)) {
> -			base = per_cpu_ptr(&timer_bases, tf & TIMER_CPUMASK);
> +			base = get_timer_base(tf);

However, in patch #20, we'll have this in __mod_timer();

		/*
		 * Take the current timer_jiffies of base, but without holding
		 * the lock!
		 */
		base = get_timer_base(timer->flags);

Thanks,
Richard