linux-kernel - Re: [PATCH v9 19/32] timers: add_timer_on(): Make sure TIMER

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Date:   Wed, 06 Dec 2023 11:46:35 +0100
From:   Anna-Maria Behnsen <anna-maria@...utronix.de>
To:     Sebastian Siewior <bigeasy@...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 <frederic@...nel.org>,
        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>,
        Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH v9 19/32] timers: add_timer_on(): Make sure TIMER_PINNED
 flag is set

Sebastian Siewior <bigeasy@...utronix.de> writes:

> On 2023-12-06 10:57:57 [+0100], Anna-Maria Behnsen wrote:
>> Sebastian Siewior <bigeasy@...utronix.de> writes:
>> 
>> > On 2023-12-01 10:26:41 [+0100], Anna-Maria Behnsen wrote:
>> >> When adding a timer to the timer wheel using add_timer_on(), it is an
>> >> implicitly pinned timer. With the timer pull at expiry time model in place,
>> >> TIMER_PINNED flag is required to make sure timers end up in proper base.
>> >> 
>> >> Add TIMER_PINNED flag unconditionally when add_timer_on() is executed.
>> >
>> > This is odd. I have some vague memory that this was already the case.
>> > Otherwise all add_timer_on() users without TIMER_PINNED may get it wrong
>> > due to optimisation.
>> 
>> Which optimisation are you talking about? Are you talking about the
>> heuristic for finding the best CPU in get_target_base()? This heuristic
>> is not used for add_timer_on().
>
> Yes, true. And nobody probably mixes add_timer_on() and mod_timer().

Workqueue mixes add_timer_on() and add_timer(). But therefore the
add_timer_global() function is introduced in patch 17 which drops the
TIMER_PINNED flag. In patch 18 the add_timer() in workqueue code is
replaced by the new function.

Thanks,

        Anna-Maria