From mboxrd@z Thu Jan 1 00:00:00 1970 From: Morten Rasmussen Subject: [3/11] issue 3: No understanding of potential cpu capacity Date: Fri, 20 Dec 2013 16:45:43 +0000 Message-ID: <1387557951-21750-4-git-send-email-morten.rasmussen@arm.com> References: <1387557951-21750-1-git-send-email-morten.rasmussen@arm.com> Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable Return-path: Received: from service87.mimecast.com ([91.220.42.44]:59264 "EHLO service87.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754573Ab3LTQph (ORCPT ); Fri, 20 Dec 2013 11:45:37 -0500 In-Reply-To: <1387557951-21750-1-git-send-email-morten.rasmussen@arm.com> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: peterz@infradead.org, mingo@kernel.org Cc: rjw@rjwysocki.net, markgross@thegnar.org, vincent.guittot@linaro.org, catalin.marinas@arm.com, morten.rasmussen@arm.com, linux-pm@vger.kernel.org To minimize energy it may sometimes be better to put waking tasks on partially loaded cpus instead of powering up more cpus (particularly if it implies powering up a new cluster/group of cpus with associated caches). To make that call, information about the potential spare cycles on the busy cpus is required. Currently, the CFS scheduler has no knowledge about frequency scaling. Frequency scaling governors generally try to match the frequency to the load, which means that the idle time has no absolute meaning. The potential spare cpu capacity may be much higher than indicated by the idle time if the cpu is running at a low P-state. The energy trade-off may justify putting another task on a loaded cpu even if it causes a change to a higher P-state to handle the extra load. Related issues are frequency (and cpu micro architecture) invariant task load and power topology information, which are both needed to enable the scheduler for energy-aware task placement. This is covered in more detail in issue 5. The potential cpu capacity cannot be assumed to be constant as thermal management may restrict the usage of high performance P-states dynamically.