From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with ESMTP id 834AC6B016A for ; Thu, 25 Aug 2011 18:56:48 -0400 (EDT) Subject: Re: [PATCH] memcg: remove unneeded preempt_disable From: Peter Zijlstra Date: Fri, 26 Aug 2011 00:56:27 +0200 In-Reply-To: References: <1313650253-21794-1-git-send-email-gthelen@google.com> <20110818144025.8e122a67.akpm@linux-foundation.org> <1314284272.27911.32.camel@twins> <1314289208.3268.4.camel@mulgrave> <986ca4ed-6810-426f-b32f-5c8687e3a10b@email.android.com> <1e295500-5d1f-45dd-aa5b-3d2da2cf1a62@email.android.com> <1314299115.26922.2.camel@twins> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Message-ID: <1314312987.26922.13.camel@twins> Mime-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: James Bottomley , Andrew Morton , Greg Thelen , linux-kernel@vger.kernel.org, linux-mm@kvack.org, KAMEZAWA Hiroyuki , Balbir Singh , Daisuke Nishimura , linux-arch@vger.kernel.org On Thu, 2011-08-25 at 14:19 -0500, Christoph Lameter wrote: > On Thu, 25 Aug 2011, Peter Zijlstra wrote: >=20 > > Also, I thought this_cpu thing's were at best locally atomic. If you > > make them full blown atomic ops then even __this_cpu ops will have to b= e > > full atomic ops, otherwise: > > > > > > CPU0 CPU(1) > > > > this_cpu_inc(&foo); preempt_disable(); > > __this_cpu_inc(&foo); > > preempt_enable(); > > > > might step on each other's toes. >=20 > They would both have their own instance of "foo". per cpu atomicity is > only one requirement of this_cpu_ops. The other is the ability to relocat= e > accesses relative to the current per cpu area. Ah, but not if the this_cpu_inc() thing ends up being more than a single instruction, then you have preemption/migration windows. Only when LL/SC can deal with SC having a different EA from the LL and supports a big enough offset could this possibly work. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org