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

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Date:   Fri, 28 Jun 2019 18:55:13 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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, Jun 27, 2019 at 11:10:08PM +0200, Thomas Gleixner wrote:
> 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/

Fixed.

> 
> > + *
> > + * 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.

Frankly speaking I still think add_timer_on() is preferred here
because mod_timer() users will really need to be careful to make sure
they'll pin the timers correctly all the time, and I assume that's why
we've tried to find all the TIMER_PINNED users and tried to make sure
there's nothing wrong on using them during previous discussion (and
more than half of them do use add_timer_on() which seems to be good).
In all cases, I'll take your suggestion to drop the last sentence.

Thanks for reviewing this document patch.  I'll repost.

-- 
Peter Xu