linux-kernel - Re: [PATCH v2] timer: document TIMER

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Message-ID: <alpine.DEB.2.21.1906272304480.32342@nanos.tec.linutronix.de>
Date:   Thu, 27 Jun 2019 23:10:08 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Xu <peterx@...hat.com>
cc:     linux-kernel@...r.kernel.org,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>
Subject: Re: [PATCH v2] timer: document TIMER_PINNED

On Thu, 27 Jun 2019, Peter Xu wrote:
> + * @TIMER_PINNED: A pinned timer will not be affected by any timer
> + * placement heuristics (like, NOHZ) and will always be run on the CPU
> + * when the timer was enqueued.

s/when/on which/

> + *
> + * Note: Because enqueuing of timers can actually migrate the timer
> + * from one CPU to another, pinned timers are not guaranteed to stay
> + * on the initialy selected CPU.  They move to the CPU on which the
> + * enqueue function is invoked via mod_timer() or add_timer().  If the
> + * timer should be placed on a particular CPU, then add_timer_on() has
> + * to be used.  It is also suggested that the user should always use
> + * add_timer_on() explicitly for pinned timers.

That last sentence is not correct. add_timer_on() has limitations over
mod_timer(). As pinned prevents the timer from being queued on a remote CPU
mod timer is perfectly fine for many cases.

add_timer_on() is really about queueing a timer on a dedicated CPU, which
is often enough a remote CPU.

Thanks,

	tglx