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From: Thomas Gleixner <tglx@linutronix.de>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Anna-Maria Gleixner <anna-maria@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH 2/9] timer: Add comments about calc_index() ceiling work
Date: Tue, 14 Jul 2020 11:13:26 +0200	[thread overview]
Message-ID: <87tuyav4qx.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200707013253.26770-3-frederic@kernel.org>

Frederic Weisbecker <frederic@kernel.org> writes:
>  static inline unsigned calc_index(unsigned expires, unsigned lvl)
>  {
> +	/*
> +	 * Time may have past since the clock last reached an index of
> +	 * this @lvl. And that time, below LVL_GRAN(@lvl), is going to
> +	 * be substracted from the delta until we reach @expires. To
> +	 * fix that we must add one level granularity unit to make sure
> +	 * we rather expire late than early. Prefer ceil over floor.

This comment confuses the hell out of me.

        /*
         * The timer wheel has to guarantee that a timer does not fire
         * early. Early expiry can happen due to:
         * - Timer is armed at the edge of a tick
         * - Truncation of the expiry time in the outer wheel levels
         *
         * Round up with level granularity to prevent this.
         */

Hmm?

Thanks,

        tglx

  reply	other threads:[~2020-07-14  9:13 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-07  1:32 [PATCH 0/9] timer: Reduce timers softirq v2 Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 1/9] timer: Move trigger_dyntick_cpu() to enqueue_timer() Frederic Weisbecker
2020-07-09 12:17   ` Anna-Maria Behnsen
2020-07-15 13:20     ` Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 2/9] timer: Add comments about calc_index() ceiling work Frederic Weisbecker
2020-07-14  9:13   ` Thomas Gleixner [this message]
2020-07-17 12:55     ` Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 3/9] timer: Optimize _next_timer_interrupt() level iteration Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 4/9] timers: Always keep track of next expiry Frederic Weisbecker
2020-07-14  8:49   ` Anna-Maria Behnsen
2020-07-17 12:51     ` Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 5/9] timer: Reuse next expiry cache after nohz exit Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 6/9] timer: Expand clk forward logic beyond nohz Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 7/9] timer: Spare timer softirq until next expiry Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 8/9] timer: Remove must_forward_clk Frederic Weisbecker
2020-07-07  1:32 ` [PATCH 9/9] timer: Lower base clock forwarding threshold Frederic Weisbecker
2020-07-09  7:02 ` [PATCH 0/9] timer: Reduce timers softirq v2 Juri Lelli

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