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

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Date:   Mon, 16 Oct 2023 10:11:35 +0200 (CEST)
From:   Anna-Maria Behnsen <anna-maria@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
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 Thu, 5 Oct 2023, Frederic Weisbecker wrote:

> 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 The related comment is outdated and was not updated when the check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!
> 

Good point! I will do this.

Thanks,

	Anna-Maria