linux-kernel - Re: [PATCH v8 05/25] timers: Clarify check in forward_timer

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Message-ID: <ZR6NVdfsb6+Hujy0@lothringen>
Date:   Thu, 5 Oct 2023 12:17:57 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Lukasz Luba <lukasz.luba@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH v8 05/25] timers: Clarify check in forward_timer_base()

On Wed, Oct 04, 2023 at 02:34:34PM +0200, Anna-Maria Behnsen wrote:
> The current check whether a forward of the timer base is required can be
> simplified by using an already existing comparison function which is easier
> to read. The related comment is outdated and was not updated when the check
> changed in commit 36cd28a4cdd0 ("timers: Lower base clock forwarding
> threshold").
> 
> Use time_before_eq() for the check and replace the comment by copying the
> comment from the same check inside get_next_timer_interrupt().
> 
> No functional change.
> 
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> ---
>  kernel/time/timer.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 5e17244a9465..31aed8353db1 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -944,11 +944,10 @@ static inline void forward_timer_base(struct timer_base *base)
>  	unsigned long jnow = READ_ONCE(jiffies);
>  
>  	/*
> -	 * No need to forward if we are close enough below jiffies.
> -	 * Also while executing timers, base->clk is 1 offset ahead
> -	 * of jiffies to avoid endless requeuing to current jiffies.
> +	 * Check whether we can forward the base. We can only do that when
> +	 * @basej is past base->clk otherwise we might rewind base->clk.

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Also can we keep the precious information in the comment and move it to
the right place? Such as:

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 63a8ce7177dd..3e70ac818034 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -2015,6 +2015,10 @@ static inline void __run_timers(struct timer_base *base)
 		 */
 		WARN_ON_ONCE(!levels && !base->next_expiry_recalc
 			     && base->timers_pending);
+		/*
+		 * While executing timers, base->clk is set 1 offset ahead of
+		 * jiffies to avoid endless requeuing to current jiffies.
+		 */
 		base->clk++;
 		base->next_expiry = __next_timer_interrupt(base);
 

Thanks!

>  	 */
> -	if ((long)(jnow - base->clk) < 1)
> +	if (time_before_eq(jnow, base->clk))
>  		return;
>  
>  	/*
> -- 
> 2.39.2
>