linux-kernel - Re: [PATCH v6 19/21] timer: Implement the hierarchical pull model

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Date:   Mon, 15 May 2023 14:41:33 +0200
From:   Sebastian Siewior <bigeasy@...utronix.de>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        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>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Lukasz Luba <lukasz.luba@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>
Subject: Re: [PATCH v6 19/21] timer: Implement the hierarchical pull model

On 2023-05-15 12:50:25 [+0200], Anna-Maria Behnsen wrote:
> The part will be replaced by:
> 
> ---8<----
> 
> 	local_irq_disable();
> 	timer_lock_remote_bases(cpu);

Do you s/raw_spin_lock_irq/raw_spin_lock/ in timer_lock_remote_bases()?
Otherwise you disable interrupts twice. Not wrong but not needed.

> 	raw_spin_lock(&tmc->lock);
> 
> 	if (!tmc->online || !tmc->idle) {
> 		timer_unlock_remote_bases(cpu);
> 		goto unlock;
> 	} else {
> 	        fetch_next_timer_interrupt_remote(jif, now, &tevt, cpu);
> 	}
> 
> 	timer_unlock_remote_bases(cpu);

The locking, unlocking and fetch_next_timer_interrupt_remote() is only
used here. So it if it wouldn't be for tmc-> then you could do
everything timer.c in once place…

> 	-> do the preparation and the walk
> 
> unlock:
> 	raw_spin_unlock_irq(&tmc->lock);
> 
> ---8<----
> 
> As menitoned in the reply last week to Frederics objections regarding the
> locking asymmetry, I would like to keep it to make the locking region of
> timer base locks as small as possible and to prevent holding up to five
> locks during the walk.

This looks okay. I wouldn't suggest to hold the timer_base::lock or
tmigr_cpu::lock longer than needed. Both can be acquired independently
cross CPU and have explicit locking order. So it is fine.

> Thanks,
> 
> 	Anna-Maria

Sebastian