linux-kernel - Re: [PATCH v4 5/6] timerfd: Add support for deferrable timers

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Message-ID: <530D5715.1050901@mit.edu>
Date:	Tue, 25 Feb 2014 18:53:09 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Alexey Perevalov <a.perevalov@...sung.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	john.stultz@...aro.org
CC:	Anton Vorontsov <anton@...msg.org>, kyungmin.park@...sung.com,
	cw00.choi@...sung.com, akpm@...ux-foundation.org,
	Anton Vorontsov <anton.vorontsov@...aro.org>
Subject: Re: [PATCH v4 5/6] timerfd: Add support for deferrable timers

On 02/20/2014 08:23 AM, Alexey Perevalov wrote:
> From: Anton Vorontsov <anton@...msg.org>
> 
> This patch implements a userland-side API for generic deferrable timers,
> per linux/timer.h:
> 
>  * A deferrable timer will work normally when the system is busy, but
>  * will not cause a CPU to come out of idle just to service it; instead,
>  * the timer will be serviced when the CPU eventually wakes up with a
>  * subsequent non-deferrable timer.
> 
> These timers are crucial for power saving, i.e. periodic tasks that want
> to work in background when the system is under use, but don't want to
> cause wakeups themselves.

Please don't.  This API sucks for all kinds of reasons:

 - Why is it a new kind of clock?
 - How deferrable is deferrable?
 - It adds new core code, which serves no purpose (the problem is
already solved).

On the other hand, if you added a fancier version of timerfd_settime
that could explicitly set the slack value (or, equivalently, the
earliest and latest allowable times), that could be quite useful.

It's often bugged me that timer slack is per-process.

--Andy

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