linux-kernel - Re: [PATCH v6 15/21] timer: Add get next timer interrupt functionality for remote CPUs

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Message-ID: <ZFtu9l35Tg89NAiZ@localhost.localdomain>
Date:   Wed, 10 May 2023 12:16:22 +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 v6 15/21] timer: Add get next timer interrupt
 functionality for remote CPUs

Le Wed, May 10, 2023 at 09:28:11AM +0200, Anna-Maria Behnsen a écrit :
> +/**
> + * fetch_next_timer_interrupt_remote
> + * @basej:	base time jiffies
> + * @basem:	base time clock monotonic
> + * @tevt:	Pointer to the storage for the expiry values
> + * @cpu:	Remote CPU
> + *
> + * Stores the next pending local and global timer expiry values in the
> + * struct pointed to by @tevt. If a queue is empty the corresponding
> + * field is set to KTIME_MAX. If local event expires before global
> + * event, global event is set to KTIME_MAX as well.
> + *
> + * Caller needs to make sure timer base locks are held (use
> + * timer_lock_remote_bases() for this purpose). Caller must make sure
> + * interrupts are reopened, if required.
> + */
> +void fetch_next_timer_interrupt_remote(unsigned long basej, u64 basem,
> +				       struct timer_events *tevt,
> +				       unsigned int cpu)
> +{
> +	struct timer_base *base_local, *base_global;
> +
> +	/* Preset local / global events */
> +	tevt->local = tevt->global = KTIME_MAX;
> +
> +	base_local = per_cpu_ptr(&timer_bases[BASE_LOCAL], cpu);
> +	base_global = per_cpu_ptr(&timer_bases[BASE_GLOBAL], cpu);
> +
> +	lockdep_assert_held(&base_local->lock);
> +	lockdep_assert_held(&base_global->lock);
> +
> +	fetch_next_timer_interrupt(base_local, base_global, basej, basem, tevt);
> +
> +	raw_spin_unlock(&base_global->lock);
> +	raw_spin_unlock(&base_local->lock);

Oh so that makes:

LOCK(baseL)
LOCK(baseG)
LOCK(tmc)
UNLOCK(baseG)
UNLOCK(baseL)
UNLOCK(tmc)

I guess you can keep the bases locks locked until the end of
tmigr_handle_remote_cpu(). After all that's what get_next_timer_interrupt()
does. I'm not sure the above early release of bases locks will bring much
in case of contention...

Then a timer_unlock_remote_bases() would restore symmetry.

> +/**
> + * timer_lock_remote_bases - lock timer bases of cpu
> + * @cpu:	Remote CPU
> + *
> + * Locks the remote timer bases.
> + *
> + * Returns false if cpu is offline, otherwise true is returned.
> + */
> +bool timer_lock_remote_bases(unsigned int cpu)
> +{
> +	struct timer_base *base_local, *base_global;
> +
> +	/*
> +	 * Pretend that there is no timer pending if the cpu is offline.
> +	 * Possible pending timers will be migrated later to an active cpu.
> +	 */
> +	if (cpu_is_offline(cpu))
> +		return false;

This value is never checked and the caller assumes the bases are
always locked upon calling this (more on this on the big patch).

Thanks.

> +
> +	base_local = per_cpu_ptr(&timer_bases[BASE_LOCAL], cpu);
> +	base_global = per_cpu_ptr(&timer_bases[BASE_GLOBAL], cpu);
> +
> +	raw_spin_lock_irq(&base_local->lock);
> +	raw_spin_lock_nested(&base_global->lock, SINGLE_DEPTH_NESTING);
> +
> +	return true;
> +}