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Date: Mon, 13 Jun 2016 17:14:58 +0200
From: Richard Cochran <richardcochran@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Eric Dumazet <edumazet@google.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Chris Mason <clm@fb.com>,
        Arjan van de Ven <arjan@infradead.org>, rt@linutronix.de
Subject: Re: [patch 18/20] timer: Forward wheel clock whenever possible
Message-ID: <20160613151458.GA18490@localhost.localdomain>
References: <20160613070440.950649741@linutronix.de>
 <20160613075929.693947132@linutronix.de>
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Thomas,

On Mon, Jun 13, 2016 at 08:41:04AM -0000, Thomas Gleixner wrote:
> @@ -498,23 +500,27 @@ static void internal_add_timer(struct ti
>  	__internal_add_timer(base, timer);
>  
>  	/*
> -	 * Check whether the other CPU is in dynticks mode and needs
> -	 * to be triggered to reevaluate the timer wheel.  We are
> -	 * protected against the other CPU fiddling with the timer by
> -	 * holding the timer base lock. This also makes sure that a
> -	 * CPU on the way to stop its tick can not evaluate the timer
> -	 * wheel.
> -	 *
> -	 * Spare the IPI for deferrable timers on idle targets though.
> -	 * The next busy ticks will take care of it. Except full dynticks
> -	 * require special care against races with idle_cpu(), lets deal
> -	 * with that later.
> -	 */
> -	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && base->nohz_active) {
> -		if (!(timer->flags & TIMER_DEFERRABLE) ||
> -		    tick_nohz_full_cpu(base->cpu))
> -			wake_up_nohz_cpu(base->cpu);
> -	}
> +	 * We might have to IPI the remote CPU if the base is idle and the
> +	 * timer is not deferrable. If the other cpu is on the way to idle
> +	 * then it can't set base->is_idle as we hold base lock.
> +	 */
> +	if (!IS_ENABLED(CONFIG_NO_HZ_COMMON) || !base->is_idle ||
> +	    (timer->flags & TIMER_DEFERRABLE))
> +		return;

The tests for is_idle and TIMER_DEFERRABLE are actually checking the
same thing.  I was looking at the usage of base.is_idle, and it is
only ever set for BASE_STD.

So the TIMER_DEFERRABLE test is redundant, but maybe this is on
purpose?  Anyhow, it did leave me scratching my head.

Thanks,
Richard