From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756576AbcASRD3 (ORCPT ); Tue, 19 Jan 2016 12:03:29 -0500 Received: from mail-wm0-f68.google.com ([74.125.82.68]:35122 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754881AbcASRDV (ORCPT ); Tue, 19 Jan 2016 12:03:21 -0500 Date: Tue, 19 Jan 2016 18:03:19 +0100 From: Frederic Weisbecker To: Peter Zijlstra Cc: LKML , Byungchul Park , Chris Metcalf , Thomas Gleixner , Luiz Capitulino , Christoph Lameter , "Paul E . McKenney" , Mike Galbraith , Rik van Riel Subject: Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task enqueue/dequeue Message-ID: <20160119170317.GC5317@lerouge> References: <1452700891-21807-1-git-send-email-fweisbec@gmail.com> <1452700891-21807-5-git-send-email-fweisbec@gmail.com> <20160119131708.GF6344@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160119131708.GF6344@twins.programming.kicks-ass.net> 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 Tue, Jan 19, 2016 at 02:17:08PM +0100, Peter Zijlstra wrote: > On Wed, Jan 13, 2016 at 05:01:31PM +0100, Frederic Weisbecker wrote: > > The full nohz CPU load is currently accounted on tick restart only. > > But there are a few issues with this model: > > > > _ On tick restart, if cpu_load[0] doesn't contain the load of the actual > > tickless load that just ran, we are going to account a wrong value. > > And it is very likely to be so given that cpu_load[0] doesn't have > > an opportunity to be updated between tick stop and tick restart. > > > > _ If the runqueue had updates that didn't trigger a tick restart, we > > are going to miss those CPU load changes. > > > > A solution to fix this is to update the CPU load everytime we enqueue > > or dequeue a task in the fair runqueue and more than a jiffy occured > > since the last update. > > Would not a much better solution be to do this remotely instead of from > one of the hottest functions in the scheduler? The problem with doing this remotely is that we can miss past cpu loads if there was several enqueue/dequeue operations happening while tickless. For example if CPU 1 runs sched entity A with a load of 5 (purely theoric) for 5 sec then it sleeps, entity B runs with a load of 1 and then CPU 2 updates the load of CPU 1 remotely. cpu_load[0] will be accurate because it's the current load of CPU 1 (which is the load of entity B), but the other indexes won't take the decayed load of entity A into account. Now we can indeed remove the queue time local update and only rely on remote updates when needed if we can live with a light precision on target_load() and source_load().