From owner-svn-src-head@FreeBSD.ORG Wed Oct 24 19:17:10 2012 Return-Path: Delivered-To: svn-src-head@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 67851CC8 for ; Wed, 24 Oct 2012 19:17:10 +0000 (UTC) (envelope-from andre@freebsd.org) Received: from c00l3r.networx.ch (c00l3r.networx.ch [62.48.2.2]) by mx1.freebsd.org (Postfix) with ESMTP id B0EA78FC16 for ; Wed, 24 Oct 2012 19:17:09 +0000 (UTC) Received: (qmail 35318 invoked from network); 24 Oct 2012 20:54:55 -0000 Received: from c00l3r.networx.ch (HELO [127.0.0.1]) ([62.48.2.2]) (envelope-sender ) by c00l3r.networx.ch (qmail-ldap-1.03) with SMTP for ; 24 Oct 2012 20:54:55 -0000 Message-ID: <50883EA8.1010308@freebsd.org> Date: Wed, 24 Oct 2012 21:16:56 +0200 From: Andre Oppermann User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:16.0) Gecko/20121010 Thunderbird/16.0.1 MIME-Version: 1.0 To: Jim Harris Subject: Re: svn commit: r242014 - head/sys/kern References: <201210241836.q9OIafqo073002@svn.freebsd.org> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: svn-src-head@freebsd.org, Adrian Chadd , src-committers@freebsd.org, svn-src-all@freebsd.org X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.14 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: Wed, 24 Oct 2012 19:17:10 -0000 On 24.10.2012 20:56, Jim Harris wrote: > On Wed, Oct 24, 2012 at 11:41 AM, Adrian Chadd wrote: >> On 24 October 2012 11:36, Jim Harris wrote: >> >>> Pad tdq_lock to avoid false sharing with tdq_load and tdq_cpu_idle. >> >> Ok, but.. >> >> >>> struct mtx tdq_lock; /* run queue lock. */ >>> + char pad[64 - sizeof(struct mtx)]; >> >> .. don't we have an existing compile time macro for the cache line >> size, which can be used here? > > Yes, but I didn't use it for a couple of reasons: > > 1) struct tdq itself is currently using __aligned(64), so I wanted to > keep it consistent. > 2) CACHE_LINE_SIZE is currently defined as 128 on x86, due to > NetBurst-based processors having 128-byte cache sectors a while back. > I had planned to start a separate thread on arch@ about this today on > whether this was still appropriate. See also the discussion on svn-src-all regarding global struct mtx alignment. Thank you for proving my point. ;) Let's go back and see how we can do this the sanest way. These are the options I see at the moment: 1. sprinkle __aligned(CACHE_LINE_SIZE) all over the place 2. use a macro like MTX_ALIGN that can be SMP/UP aware and in the future possibly change to a different compiler dependent align attribute 3. embed __aligned(CACHE_LINE_SIZE) into struct mtx itself so it automatically gets aligned in all cases, even when dynamically allocated. Personally I'm undecided between #2 and #3. #1 is ugly. In favor of #3 is that there possibly isn't any case where you'd actually want the mutex to share a cache line with anything else, even a data structure. -- Andre