From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758419AbaELCWF (ORCPT ); Sun, 11 May 2014 22:22:05 -0400 Received: from mga11.intel.com ([192.55.52.93]:46323 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758401AbaELCV6 (ORCPT ); Sun, 11 May 2014 22:21:58 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.97,1032,1389772800"; d="scan'208";a="530200531" From: Yuyang Du To: mingo@redhat.com, peterz@infradead.org, rafael.j.wysocki@intel.com, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org Cc: arjan.van.de.ven@intel.com, len.brown@intel.com, alan.cox@intel.com, mark.gross@intel.com, morten.rasmussen@arm.com, vincent.guittot@linaro.org, rajeev.d.muralidhar@intel.com, vishwesh.m.rudramuni@intel.com, nicole.chalhoub@intel.com, ajaya.durg@intel.com, harinarayanan.seshadri@intel.com, jacob.jun.pan@linux.intel.com, fengguang.wu@intel.com, yuyang.du@intel.com Subject: =?UTF-8?q?=5BRFC=20PATCH=2000/12=20v2=5D=20A=20new=20CPU=20load=20metric=20for=20power-efficient=20scheduler=3A=20CPU=20ConCurrency?= Date: Mon, 12 May 2014 02:16:49 +0800 Message-Id: <1399832221-8314-1-git-send-email-yuyang.du@intel.com> X-Mailer: git-send-email 1.7.9.5 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Ingo, PeterZ, Rafael, and others, The current scheduler’s load balancing is completely work-conserving. In some workload, generally low CPU utilization but immersed with CPU bursts of transient tasks, migrating task to engage all available CPUs for work-conserving can lead to significant overhead: cache locality loss, idle/active HW state transitional latency and power, shallower idle state, etc, which are both power and performance inefficient especially for today’s low power processors in mobile. This RFC introduces a sense of idleness-conserving into work-conserving (by all means, we really don’t want to be overwhelming in only one way). But to what extent the idleness-conserving should be, bearing in mind that we don’t want to sacrifice performance? We first need a load/idleness indicator to that end. Thanks to CFS’s “model an ideal, precise multi-tasking CPU”, tasks can be seen as concurrently running (the tasks in the runqueue). So it is natural to use task concurrency as load indicator. Having said that, we do two things: 1) Divide continuous time into periods of time, and average task concurrency in period, for tolerating the transient bursts: a = sum(concurrency * time) / period 2) Exponentially decay past periods, and synthesize them all, for hysteresis to load drops or resilience to load rises (let f be decaying factor, and a_x the xth period average since period 0): s = a_n + f^1 * a_n-1 + f^2 * a_n-2 +, ..., + f^(n-1) * a_1 + f^n * a_0 We name this load indicator as CPU ConCurrency (CC): task concurrency determines how many CPUs are needed to be running concurrently. Another two ways of how to interpret CC: 1) the current work-conserving load balance also uses CC, but instantaneous CC. 2) CC vs. CPU utilization. CC is runqueue-length-weighted CPU utilization. If we change: "a = sum(concurrency * time) / period" to "a' = sum(1 * time) / period". Then a' is just about the CPU utilization. And the way we weight runqueue-length is the simplest one (excluding the exponential decays, and you may have other ways). To track CC, we intercept the scheduler in 1) enqueue, 2) dequeue, 3) scheduler tick, and 4) enter/exit idle. After CC, in the consolidation part, we do 1) attach the CPU topology to be adaptive beyond our experimental platforms, and 2) intercept the current load balance for load and load balancing containment. Currently, CC is per CPU. To consolidate, the formula is based on a heuristic. Suppose we have 2 CPUs, their task concurrency over time is ('-' means no task, 'x' having tasks): 1) CPU0: ---xxxx---------- (CC[0]) CPU1: ---------xxxx---- (CC[1]) 2) CPU0: ---xxxx---------- (CC[0]) CPU1: ---xxxx---------- (CC[1]) If we consolidate CPU0 and CPU1, the consolidated CC will be: CC' = CC[0] + CC[1] for case 1 and CC'' = (CC[0] + CC[1]) * 2 for case 2. For the cases in between case 1 and 2 in terms of how xxx overlaps, the CC should be between CC' and CC''. So, we uniformly use this condition for consolidation (suppose we consolidate m CPUs to n CPUs, m > n): (CC[0] + CC[1] + ... + CC[m-2] + CC[m-1]) * (n + log(m-n)) >=