linux-kernel - Re: [PATCH v5 16/18] timer: Implement the hierarchical pull model

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Date:   Tue, 21 Mar 2023 12:17:00 +0100
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>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v5 16/18] timer: Implement the hierarchical pull model

On Wed, Mar 01, 2023 at 03:17:42PM +0100, Anna-Maria Behnsen wrote:
> +static u64 tmigr_handle_remote_cpu(unsigned int cpu, u64 now,
> +				   unsigned long jif)
> +{
> +	struct timer_events tevt;
> +	struct tmigr_walk data;
> +	struct tmigr_cpu *tmc;
> +	u64 next = KTIME_MAX;
> +	unsigned long flags;
> +
> +	tmc = per_cpu_ptr(&tmigr_cpu, cpu);
> +
> +	raw_spin_lock_irqsave(&tmc->lock, flags);
> +	/*
> +	 * Remote CPU is offline or no longer idle or other cpu handles cpu
> +	 * timers already or next event was already expired - return!
> +	 */
> +	if (!tmc->online || tmc->remote || tmc->cpuevt.ignore ||
> +	    now < tmc->cpuevt.nextevt.expires) {
> +		raw_spin_unlock_irqrestore(&tmc->lock, flags);
> +		return next;
> +	}
> +
> +	tmc->remote = 1;
> +
> +	/* Drop the lock to allow the remote CPU to exit idle */
> +	raw_spin_unlock_irqrestore(&tmc->lock, flags);
> +
> +	if (cpu != smp_processor_id())
> +		timer_expire_remote(cpu);
> +
> +	/* next event of cpu */
> +	fetch_next_timer_interrupt_remote(jif, now, &tevt, cpu);

If the target CPU gets an idle interrupt right after the above call and enqueues
a new timer (which becomes the new earliest), tmigr_cpu_deactivate() ->
tmigr_new_timer() is going to ignore it due to tmc->remote = 1, right?

Or am I missing something else that would make that timer correctly handled?

Thanks.