From owner-freebsd-hackers@freebsd.org  Fri Feb 24 21:26:31 2017
Return-Path: <owner-freebsd-hackers@freebsd.org>
Delivered-To: freebsd-hackers@mailman.ysv.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 by mailman.ysv.freebsd.org (Postfix) with ESMTP id 4280DCEBDB2
 for <freebsd-hackers@mailman.ysv.freebsd.org>;
 Fri, 24 Feb 2017 21:26:31 +0000 (UTC)
 (envelope-from vangyzen@FreeBSD.org)
Received: from smtp.vangyzen.net (hotblack.vangyzen.net
 [IPv6:2607:fc50:1000:7400:216:3eff:fe72:314f])
 by mx1.freebsd.org (Postfix) with ESMTP id 2D327954;
 Fri, 24 Feb 2017 21:26:31 +0000 (UTC)
 (envelope-from vangyzen@FreeBSD.org)
Received: from ford.home.vangyzen.net (unknown [76.164.15.242])
 by smtp.vangyzen.net (Postfix) with ESMTPSA id 7DD2D56483;
 Fri, 24 Feb 2017 15:26:30 -0600 (CST)
Subject: Re: Absolute timeouts and clock adjustments
To: Ian Lepore <ian@freebsd.org>,
 Sebastian Huber <sebastian.huber@embedded-brains.de>,
 FreeBSD <freebsd-hackers@freebsd.org>
References: <58AD5802.30908@embedded-brains.de>
 <1ff4d78a-a157-53c5-af7e-b516bc1b6187@FreeBSD.org>
 <1487778365.73144.144.camel@freebsd.org>
From: Eric van Gyzen <vangyzen@FreeBSD.org>
Message-ID: <bb91620e-8fbf-db73-82ca-704ea01a182d@FreeBSD.org>
Date: Fri, 24 Feb 2017 15:26:28 -0600
User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:45.0) Gecko/20100101
 Thunderbird/45.6.0
MIME-Version: 1.0
In-Reply-To: <1487778365.73144.144.camel@freebsd.org>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.23
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
 <freebsd-hackers.freebsd.org>
List-Unsubscribe: <https://lists.freebsd.org/mailman/options/freebsd-hackers>, 
 <mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers/>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <https://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
 <mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 24 Feb 2017 21:26:31 -0000

> Using CLOCK_MONOTONIC doesn't avoid the issue, it just explicitly asks
> for the current behavior.  If the behavior you need is to wake up when
> CLOCK_REALTIME exceeds some value, you're still screwed.
>
> It would be easy enough to fix the current behavior by adding something
> like "if (timo > hz) timo = hz;" to the existing loop so that
> CLOCK_REALTIME gets re-checked once a second.

That would work, but I don't really like introducing spurious wakeups just to 
handle a very rare event.

How about this approach?

	https://reviews.freebsd.org/D9791

It's also not ideal, and it's obviously much larger than your suggestion, but 
it's very cheap in the common paths.

Eric