From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from wolverine01.qualcomm.com (wolverine01.qualcomm.com [199.106.114.254]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "wolverine01.qualcomm.com", Issuer "VeriSign Class 3 Secure Server CA" (not verified)) by ozlabs.org (Postfix) with ESMTPS id C34DD2C0093 for ; Tue, 8 Jan 2013 15:27:54 +1100 (EST) Date: Mon, 7 Jan 2013 20:27:50 -0800 From: Srivatsa Vaddagiri To: Russell King - ARM Linux Subject: Re: [PATCH 1/2] cpuhotplug/nohz: Remove offline cpus from nohz-idle state Message-ID: <20130108042750.GA18214@quicinc.com> References: <1357268318-7993-1-git-send-email-vatsa@codeaurora.org> <20130105103627.GU2631@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 In-Reply-To: <20130105103627.GU2631@n2100.arm.linux.org.uk> Cc: linux-mips@linux-mips.org, linux-sh@vger.kernel.org, mhocko@suse.cz, "H. Peter Anvin" , sparclinux@vger.kernel.org, linux-s390@vger.kernel.org, x86@kernel.org, Ingo Molnar , "Paul E. McKenney" , Mike Frysinger , linux-arm-msm@vger.kernel.org, Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Stephen Boyd , linux-kernel@vger.kernel.org, Ralf Baechle , Paul Mundt , srivatsa.bhat@linux.vnet.ibm.com, Martin Schwidefsky , uclinux-dist-devel@blackfin.uclinux.org, linuxppc-dev@lists.ozlabs.org, "David S. Miller" Reply-To: Srivatsa Vaddagiri List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , * Russell King - ARM Linux [2013-01-05 10:36:27]: > On Thu, Jan 03, 2013 at 06:58:38PM -0800, Srivatsa Vaddagiri wrote: > > I also think that the > > wait_for_completion() based wait in ARM's __cpu_die() can be replaced with a > > busy-loop based one, as the wait there in general should be terminated within > > few cycles. > > Why open-code this stuff when we have infrastructure already in the kernel > for waiting for stuff to happen? I chose to use the standard infrastructure > because its better tested, and avoids having to think about whether we need > CPU barriers and such like to ensure that updates are seen in a timely > manner. I was primarily thinking of calling as few generic functions as possible on a dead cpu. I recall several "am I running on a dead cpu?" checks (cpu_is_offline(this_cpu) that were put in generic routines during early versions of cpu hotplug [1] to educate code running on dead cpu, the need for which went away though with introduction of atomic/stop-machine variant. The need to add a RCU_NONIDLE() wrapper around ARM's cpu_die() [2] is perhaps a more recent example of educating code running on dead cpu. As quickly we die as possible after idle thread of dying cpu gains control, the better! 1. http://lwn.net/Articles/69040/ 2. http://lists.infradead.org/pipermail/linux-arm-kernel/2012-July/107971.html - vatsa -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation