Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752628AbbDCRVy (ORCPT ); Fri, 3 Apr 2015 13:21:54 -0400 Received: from mail-wi0-f170.google.com ([209.85.212.170]:37834 "EHLO mail-wi0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752112AbbDCRVw (ORCPT ); Fri, 3 Apr 2015 13:21:52 -0400 Date: Fri, 3 Apr 2015 19:21:49 +0200 From: Frederic Weisbecker To: Rik van Riel Cc: Peter Zijlstra , 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 Message-ID: <20150403172148.GB13598@lerouge> 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> <551D9458.9070502@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <551D9458.9070502@redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1524 Lines: 36 On Thu, Apr 02, 2015 at 03:11:20PM -0400, Rik van Riel wrote: > 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. Indeed, we could add another check for a global flag but that's going to bloat the syscall fastpath. -- 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/