Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753752AbbDBTLZ (ORCPT ); Thu, 2 Apr 2015 15:11:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50667 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752576AbbDBTLW (ORCPT ); Thu, 2 Apr 2015 15:11:22 -0400 Message-ID: <551D9458.9070502@redhat.com> Date: Thu, 02 Apr 2015 15:11:20 -0400 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: Peter Zijlstra , Frederic Weisbecker CC: LKML , Dave Jones , Thomas Gleixner , Oleg Nesterov , "Paul E . McKenney" , Ingo Molnar Subject: Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks instead of context switches References: <1427996365-12101-1-git-send-email-fweisbec@gmail.com> <1427996365-12101-3-git-send-email-fweisbec@gmail.com> <20150402190910.GF27490@worktop.programming.kicks-ass.net> In-Reply-To: <20150402190910.GF27490@worktop.programming.kicks-ass.net> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1306 Lines: 32 On 04/02/2015 03:09 PM, Peter Zijlstra wrote: > On Thu, Apr 02, 2015 at 07:39:24PM +0200, Frederic Weisbecker wrote: >> TIF_NOHZ is used by context_tracking to force syscall slow-path on every >> task in order to track userspace roundtrips. As such, it must be set on >> all running tasks. >> >> It's currently explicitly inherited through context switches. There is >> no need to do it on this fast-path though. The flag could be simply >> set once for all on all tasks, whether they are running or not. >> >> Lets do this by setting the flag to init task on early boot and let it >> propagate through fork inheritance. >> > > One must ask, what's the point of the flag if everybody must always have > it set? We already test this word full of flags in the syscall entry and exit path. Testing this same word for an additional flag is cheaper than testing a different variable. See the places in entry_{32,64}.S where do_notify_resume, syscall_trace_enter, syscall_trace_leave, etc get called. All are called as a result of testing flags in the same word. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/