From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758001AbcATT36 (ORCPT ); Wed, 20 Jan 2016 14:29:58 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:60108 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754529AbcATT3z (ORCPT ); Wed, 20 Jan 2016 14:29:55 -0500 Date: Wed, 20 Jan 2016 20:29:52 +0100 From: Peter Zijlstra To: Nicolas Pitre Cc: Daniel Lezcano , tglx@linutronix.de, rafael@kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, vincent.guittot@linaro.org Subject: Re: [RFC V2 2/2] sched: idle: IRQ based next prediction for idle period Message-ID: <20160120192952.GU6357@twins.programming.kicks-ass.net> References: <1453305636-22156-1-git-send-email-daniel.lezcano@linaro.org> <1453305636-22156-3-git-send-email-daniel.lezcano@linaro.org> <20160120190205.GR6357@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 20, 2016 at 02:17:57PM -0500, Nicolas Pitre wrote: > > It is also a friggin pointless /1000. The cpuidle code also loves to do > > this, and its silly, u64 add/sub are _way_ cheaper than u64 / 1000. > > For the purpose of this code, nanoseconds simply provides too many bits > for what we care. Computing the variance implies squared values. > > *However* we can simply do diff = (timestamp - w->timestamp) >> 10 > instead. No need to have an exact microsecs base. Right, you could also reduce bits at the variance computation, but yes.