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

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Date:   Tue, 16 May 2023 11:37:15 +0200
From:   Sebastian Siewior <bigeasy@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Anna-Maria Behnsen <anna-maria@...utronix.de>,
        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 2023-05-16 11:24:51 [+0200], Frederic Weisbecker wrote:
> > 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.
> 
> 
>    LOCK(A)
>    LOCK(B)
>    UNLOCK(A)
>    UNLOCK(B)
> 
> 
> Cross rhymes aren't pretty in locking. They don't make the review smooth
> but yeah it's not incorrect either. And since I don't have a better way to
> propose...

tmigr_cpu::lock is required longer than timer_base::lock. You can't
acquire tmigr_cpu::lock -> timer_base::lock due to lock ordering.

So you follow the lock order, acquire what needed, drop everything else
to lower lock contention for others.

> > > Thanks,
> > > 
> > > 	Anna-Maria

Sebastian