From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760266AbaGYMpF (ORCPT ); Fri, 25 Jul 2014 08:45:05 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:49615 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751648AbaGYMpC (ORCPT ); Fri, 25 Jul 2014 08:45:02 -0400 Date: Fri, 25 Jul 2014 14:44:59 +0200 From: Peter Zijlstra To: Kaushal Kumar Cc: Ingo Molnar , Michael Wang , Arun Bharadwaj , linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org Subject: Re: [PATCH] sched: Remove synchronize rcu/sched calls from _cpu_down Message-ID: <20140725124459.GM20603@laptop.programming.kicks-ass.net> References: <1406289368-11332-1-git-send-email-kaushalk@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1406289368-11332-1-git-send-email-kaushalk@codeaurora.org> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 25, 2014 at 05:26:08PM +0530, Kaushal Kumar wrote: > synchronize_rcu calls fail hot-unplug latency tests since > wait_rcu_gp can have very high latency, at times up to few > hundred of millisecs. So bloody what? > These calls were earlier removed for the same reason by the > commit 9ee349ad6d326df3633d43f54202427295999c47, but got added > back as part of commit c4575f83b9fe87bb57b737bc8a667f746a20320a. fatal: bad object c4575f83b9fe87bb57b737bc8a667f746a20320 > Moreover there should be no need for sync_sched() in _cpu_down > as stop_machine() provides that barrier implicitly. We should not rely on that, but also you've removed synchronize_rcu() and this is not at all provided by stop_machine(). So NAK.