From owner-svn-src-head@freebsd.org Mon Sep 5 21:50:32 2016 Return-Path: Delivered-To: svn-src-head@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 0C558B96005; Mon, 5 Sep 2016 21:50:32 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from mail.baldwin.cx (bigwig.baldwin.cx [IPv6:2001:470:1f11:75::1]) (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id DF2E45E7; Mon, 5 Sep 2016 21:50:31 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from ralph.baldwin.cx (c-73-231-226-104.hsd1.ca.comcast.net [73.231.226.104]) by mail.baldwin.cx (Postfix) with ESMTPSA id A9C0210AF43; Mon, 5 Sep 2016 17:50:30 -0400 (EDT) From: John Baldwin To: Mark Johnston Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r305368 - head/sys/kern Date: Mon, 05 Sep 2016 11:42:05 -0700 Message-ID: <28893594.Z98fxgFVzW@ralph.baldwin.cx> User-Agent: KMail/4.14.10 (FreeBSD/11.0-PRERELEASE; KDE/4.14.10; amd64; ; ) In-Reply-To: <20160905180917.GC70066@wkstn-mjohnston.west.isilon.com> References: <201609040029.u840Tn5D019584@repo.freebsd.org> <2134943.hUK9aHv09Y@ralph.baldwin.cx> <20160905180917.GC70066@wkstn-mjohnston.west.isilon.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.4.3 (mail.baldwin.cx); Mon, 05 Sep 2016 17:50:30 -0400 (EDT) X-Virus-Scanned: clamav-milter 0.99.2 at mail.baldwin.cx X-Virus-Status: Clean X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 05 Sep 2016 21:50:32 -0000 On Monday, September 05, 2016 11:09:17 AM Mark Johnston wrote: > On Mon, Sep 05, 2016 at 10:30:24AM -0700, John Baldwin wrote: > > On Sunday, September 04, 2016 12:29:49 AM Mark Johnston wrote: > > > Author: markj > > > Date: Sun Sep 4 00:29:48 2016 > > > New Revision: 305368 > > > URL: https://svnweb.freebsd.org/changeset/base/305368 > > > > > > Log: > > > Micro-optimize sleepq_signal(). > > > > > > Lift a comparison out of the loop that finds the highest-priority thread > > > on the queue. > > > > > > MFC after: 1 week > > > > Could this safely use TAILQ_FOREACH_FROM? > > Are you suggesting something like this? > > besttd = TAILQ_FIRST(&sq->sq_blocked[queue]); > td = TAILQ_NEXT(besttd, td_slpq); > TAILQ_FOREACH_FROM(td, &sq->sq_blocked[queue], td_slpq) { > ... > > I think that would work, and it avoids visiting the first element > unnecessarily when the queue contains more than one element. If the > queue contains one element, we'd visit it because of > TAILQ_FOREACH_FROM's surprising behaviour of iterating over the entire > queue when the listelem is NULL. I was hoping it was something equivalent to: for (td = TAILQ_NEXT(besttd); td != NULL; td = TAILQ_NEXT(td)) { ... } I guess what we actually want is something like TAILQ_FOREACH_AFTER(): TAILQ_FOREACH_AFTER(td, besttd, &sq->sq_blocked[queue], td_slpq) that assumed bestttd was not NULL and so avoided the whole "scan the whole thing". For now you could perhaps open-code the above loop though? -- John Baldwin