From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756986AbcKLL1z (ORCPT ); Sat, 12 Nov 2016 06:27:55 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:37050 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753115AbcKLL1y (ORCPT ); Sat, 12 Nov 2016 06:27:54 -0500 Date: Sat, 12 Nov 2016 12:25:07 +0100 (CET) From: Thomas Gleixner To: Saravana Kannan cc: Joonwoo Park , John Stultz , Eric Dumazet , Frederic Weisbecker , Linus Torvalds , "Paul E. McKenney" , Peter Zijlstra , linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org Subject: Re: [PATCH] timers: Fix timer inaccuracy In-Reply-To: <58267675.9030006@codeaurora.org> Message-ID: References: <1478684203-11966-1-git-send-email-joonwoop@codeaurora.org> <53b2d275-23f2-9ee2-7bde-e441659cf885@codeaurora.org> <58267675.9030006@codeaurora.org> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 11 Nov 2016, Saravana Kannan wrote: > On 11/10/2016 02:07 AM, Thomas Gleixner wrote: > > Deferrable timers shouldn't have been invented in the first place and yes, > > they are not going to happen on hrtimers, quite the contrary, I'm working > > on eliminating them completely. > > If you do that, how exactly do you propose drivers do periodic polling while > the Linux isn't idling, but stop polling when Linux is idle? devfreq is a > classic example. devfreq is used in a lot of mobile devices. Across different > vendors, devfreq is used for scaling system memory, flash storage, GPU, etc. > You are going to kill power if you remove deferrable timers without having an > alternate mechanism to solve this requirement. > > For example, when you are browsing on your phone and reading something on > screen (but not interacting with the device), the CPUs/clusters/caches go idle > for long periods (several seconds) of time. If you remove deferrable timers, > you are going to force a CPU wake up every 10ms or 50ms or whatever it's > configured for. I know how all that works, but there are other ways to deal with those timers. We had complaints in the past because those timers can be stuck forever on a fully idle cpu and that's not a good thing either. The whole concept is ill defined or not defined at all, lacks any form of sane semantics and was shoved into the kernel w/i much thought. In hindsight I should have rejected that deferrable mess 5+ years ago. > > When you can come up with a real regression caused by the rework and not > > just some handwaving theories then we can revisit that, but until then it's > > going to stay as is. > > If the polling interval isn't accurate, the regression isn't so much about the > timer being inaccurate, but more so in the fact that it'll take that much > longer to react and increase the device frequency. Frame rendering time is > 16ms. If you react after 20ms instead of 10ms, that's way past a frame > rendering time. System memory frequency matters a lot for frame rendering too. If you need precise timers use hrtimers and not a tick based mechanism, it's that simple. Thanks, tglx