From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933860AbbJANHm (ORCPT ); Thu, 1 Oct 2015 09:07:42 -0400 Received: from mail-wi0-f172.google.com ([209.85.212.172]:37620 "EHLO mail-wi0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933787AbbJANHD (ORCPT ); Thu, 1 Oct 2015 09:07:03 -0400 Date: Thu, 1 Oct 2015 15:07:01 +0200 From: Frederic Weisbecker To: Andy Lutomirski Cc: Chris Metcalf , Gilad Ben Yossef , Steven Rostedt , Ingo Molnar , Peter Zijlstra , Andrew Morton , Rik van Riel , Tejun Heo , Thomas Gleixner , "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 Message-ID: <20151001130700.GE3432@lerouge> References: <1443453446-7827-1-git-send-email-cmetcalf@ezchip.com> <1443453446-7827-7-git-send-email-cmetcalf@ezchip.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. See update_cpu_load_active() for exemple. > > One possible issue: __vdso_clock_gettime with any of the COARSE clocks > as well as __vdso_time will break if the timekeeping code doesn't run > somewhere with reasonable frequency on some core. Hopefully this > always works. > > --Andy