From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756111AbbJAOO4 (ORCPT ); Thu, 1 Oct 2015 10:14:56 -0400 Received: from www.linutronix.de ([62.245.132.108]:36397 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752863AbbJAOOy (ORCPT ); Thu, 1 Oct 2015 10:14:54 -0400 Date: Thu, 1 Oct 2015 16:13:44 +0200 (CEST) From: Thomas Gleixner To: Frederic Weisbecker cc: Andy Lutomirski , Chris Metcalf , Gilad Ben Yossef , Steven Rostedt , Ingo Molnar , Peter Zijlstra , Andrew Morton , Rik van Riel , Tejun Heo , "Paul E. McKenney" , Christoph Lameter , Viresh Kumar , Catalin Marinas , Will Deacon , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v7 06/11] nohz: task_isolation: allow tick to be fully disabled In-Reply-To: <20151001130700.GE3432@lerouge> Message-ID: References: <1443453446-7827-1-git-send-email-cmetcalf@ezchip.com> <1443453446-7827-7-git-send-email-cmetcalf@ezchip.com> <20151001130700.GE3432@lerouge> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 1 Oct 2015, Frederic Weisbecker wrote: > On Mon, Sep 28, 2015 at 04:40:56PM -0400, Andy Lutomirski wrote: > > On Mon, Sep 28, 2015 at 11:17 AM, Chris Metcalf wrote: > > > While the current fallback to 1-second tick is still helpful for > > > maintaining completely correct kernel semantics, processes using > > > prctl(PR_SET_TASK_ISOLATION) semantics place a higher priority on > > > running completely tickless, so don't bound the time_delta for such > > > processes. In addition, due to the way such processes quiesce by > > > waiting for the timer tick to stop prior to returning to userspace, > > > without this commit it won't be possible to use the task_isolation > > > mode at all. > > > > > > Removing the 1-second cap was previously discussed (see link > > > below) and Thomas Gleixner observed that vruntime, load balancing > > > data, load accounting, and other things might be impacted. > > > Frederic Weisbecker similarly observed that allowing the tick to > > > be indefinitely deferred just meant that no one would ever fix the > > > underlying bugs. However it's at least true that the mode proposed > > > in this patch can only be enabled on a nohz_full core by a process > > > requesting task_isolation mode, which may limit how important it is > > > to maintain scheduler data correctly, for example. > > > > What goes wrong when a task enables this? Presumably either tasks > > that enable it experience problems or performance issues or it should > > always be enabled. > > We need to make the scheduler resilient to 0Hz tick. Currently it doesn't > even correctly support 1Hz or any dynticks behaviour that isn't idle. Rik has started to work on this. No idea what the status of that is. Thanks, tglx