From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EF91DC43381 for ; Thu, 21 Feb 2019 16:19:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BECF920836 for ; Thu, 21 Feb 2019 16:19:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726659AbfBUQTv (ORCPT ); Thu, 21 Feb 2019 11:19:51 -0500 Received: from foss.arm.com ([217.140.101.70]:47406 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725932AbfBUQTv (ORCPT ); Thu, 21 Feb 2019 11:19:51 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A7D71A78; Thu, 21 Feb 2019 08:19:50 -0800 (PST) Received: from [10.1.194.37] (e113632-lin.cambridge.arm.com [10.1.194.37]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8B4F83F690; Thu, 21 Feb 2019 08:19:48 -0800 (PST) Subject: Re: [RFC][PATCH 15/16] sched: Trivial forced-newidle balancer To: Peter Zijlstra , mingo@kernel.org, tglx@linutronix.de, pjt@google.com, tim.c.chen@linux.intel.com, torvalds@linux-foundation.org Cc: linux-kernel@vger.kernel.org, subhra.mazumdar@oracle.com, fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com References: <20190218165620.383905466@infradead.org> <20190218173514.796920915@infradead.org> From: Valentin Schneider Message-ID: Date: Thu, 21 Feb 2019 16:19:46 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190218173514.796920915@infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 18/02/2019 16:56, Peter Zijlstra wrote: [...] > +static bool try_steal_cookie(int this, int that) > +{ > + struct rq *dst = cpu_rq(this), *src = cpu_rq(that); > + struct task_struct *p; > + unsigned long cookie; > + bool success = false; > + > + local_irq_disable(); > + double_rq_lock(dst, src); > + > + cookie = dst->core->core_cookie; > + if (!cookie) > + goto unlock; > + > + if (dst->curr != dst->idle) > + goto unlock; > + > + p = sched_core_find(src, cookie); > + if (p == src->idle) > + goto unlock; > + > + do { > + if (p == src->core_pick || p == src->curr) > + goto next; > + > + if (!cpumask_test_cpu(this, &p->cpus_allowed)) > + goto next; > + > + if (p->core_occupation > dst->idle->core_occupation) > + goto next; > + IIUC, we're trying to find/steal tasks matching the core_cookie from other rqs because dst has been cookie-forced-idle. If the p we find isn't running, what's the meaning of core_occupation? I would have expected it to be 0, but we don't seem to be clearing it when resetting the state in pick_next_task(). If it is running, we prevent the stealing if the core it's on is running more matching tasks than the core of the pulling rq. It feels to me as if that's a balancing tweak to try to cram as many matching tasks as possible in a single core, so to me this reads as "don't steal my tasks if I'm running more than you are, but I will steal tasks from you if I'm given the chance". Is that correct? [...]