linux-kernel - Re: [PATCH v7 19/21] timer: Implement the hierarchical pull model

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 6 Jun 2023 22:10:43 +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>,
        Frederic Weisbecker <fweisbec@...il.com>,
        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>
Subject: Re: [PATCH v7 19/21] timer: Implement the hierarchical pull model

Le Wed, May 24, 2023 at 09:06:27AM +0200, Anna-Maria Behnsen a écrit :
> +u64 tmigr_cpu_deactivate(u64 nextexp)
> +{
> +	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
> +	u64 ret;
> +
> +	if (!is_tmigr_enabled() || !tmc->tmgroup || !tmc->online)
> +		return nextexp;
> +
> +	raw_spin_lock(&tmc->lock);
> +
> +	/*
> +	 * CPU is already deactivated in timer migration
> +	 * hierarchy. tick_nohz_get_sleep_length() calls
> +	 * tick_nohz_next_event() and thereby timer idle path is
> +	 * executed once more. tmc->wakeup holds the first timer, when
> +	 * timer migration hierarchy is completely idle and remote
> +	 * expiry was done.

This can also hold the first timer in the hierarchy on nodes
having no true migrator, but still with upper nodes having potentially
a migrator, right??

> +	 * If there is no new next expiry value
> +	 * handed in which should be inserted into the timer migration
> +	 * hierarchy, wakeup value is returned.
> +	 */
> +	if (tmc->idle) {
> +		ret = tmc->wakeup;
> +
> +		tmc->wakeup = KTIME_MAX;
> +
> +		if (nextexp != KTIME_MAX) {

This seem to assume that tick_nohz_next_event() on the last idle CPU in an idle
hierarchy is always called right after a timer interrupt arriving on time
(ie: right after a call to tmigr_handle_remote()), but this can actually be called
after any interrupt. Can the following happen or am I overlooking something?

do_idle() {
	// ===> <IRQ>
	tmigr_handle_remote() {
		// find some timer in the hierarchy
		// expiring in 2 jiffies
		tmc->wakeup = jiffies + 2
	}
	//<=== </IRQ>
	tick_nohz_get_sleep_length() {
		get_next_timer_interrupt() {
			tmigr_cpu_deactivate() {
				wakeup = tmc->wakeup
				tmc->wakeup = KTIME_MAX
				return wakeup // jiffies + 2
			}
		}
	}
	tick_nohz_idle_stop_tick() {
		// SLEEP 2 jiffies
	}

	// ===> <IRQ>
	// whatever IRQ that is not timer
	// or a timer IRQ firing too early
	//<=== </IRQ>
	
	tick_nohz_get_sleep_length() {
		get_next_timer_interrupt() {
			tmigr_cpu_deactivate() {
				wakeup = tmc->wakeup
				 tmc->wakeup = KTIME_MAX
				return wakeup // KTIME_MAX
			}
		}
	}
	tick_nohz_idle_stop_tick() {
		// SLEEP forever, misses the timer in 2 jiffies
	}
}

Thanks.