linux-kernel - Re: [PATCH v8 08/25] timer: Rework idle logic

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Message-ID: <878r8bphn6.ffs@tglx>
Date:   Tue, 10 Oct 2023 00:15:09 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>,
        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>,
        Frederic Weisbecker <frederic@...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>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: [PATCH v8 08/25] timer: Rework idle logic

On Wed, Oct 04 2023 at 14:34, Anna-Maria Behnsen wrote:
>  
> -	if (time_before_eq(nextevt, basej)) {
> -		expires = basem;
> -		base->is_idle = false;
> -	} else {
> -		if (base->timers_pending)
> -			expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
> -		/*
> -		 * If we expect to sleep more than a tick, mark the base idle.
> -		 * Also the tick is stopped so any added timer must forward
> -		 * the base clk itself to keep granularity small. This idle
> -		 * logic is only maintained for the BASE_STD base, deferrable
> -		 * timers may still see large granularity skew (by design).
> -		 */
> -		if ((expires - basem) > TICK_NSEC)
> -			base->is_idle = true;
> +	/*
> +	 * Base is idle if the next event is more than a tick away. Also
> +	 * the tick is stopped so any added timer must forward the base clk
> +	 * itself to keep granularity small. This idle logic is only
> +	 * maintained for the BASE_STD base, deferrable timers may still
> +	 * see large granularity skew (by design).
> +	 */
> +	base->is_idle = time_after(nextevt, basej + 1);

This is wrongly ordered. base->is_idle must be updated _after_
evaluating base->timers_pending because the below can change nextevt,
no?

> +	if (base->timers_pending) {
> +		/* If we missed a tick already, force 0 delta */
> +		if (time_before(nextevt, basej))
> +			nextevt = basej;
> +		expires = basem + (u64)(nextevt - basej) * TICK_NSEC;

Thanks,

        tglx