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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 840DAC433FE for ; Sat, 29 Oct 2022 09:15:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229756AbiJ2JPX (ORCPT ); Sat, 29 Oct 2022 05:15:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55092 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229542AbiJ2JPU (ORCPT ); Sat, 29 Oct 2022 05:15:20 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 310784661C for ; Sat, 29 Oct 2022 02:15:18 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.57]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4Mztt31T62zmVWx; Sat, 29 Oct 2022 17:10:19 +0800 (CST) Received: from [10.174.151.185] (10.174.151.185) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Sat, 29 Oct 2022 17:15:15 +0800 Subject: Re: Regression on vcpu_is_preempted() To: Peter Zijlstra CC: "mingo@redhat.com" , , , , , Steven Rostedt , , , , , linux-kernel References: <89856431-e68b-ebe9-90cb-e46ed8065659@huawei.com> From: Miaohe Lin Message-ID: Date: Sat, 29 Oct 2022 17:15:15 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.151.185] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/10/29 16:58, Peter Zijlstra wrote: > On Fri, Oct 28, 2022 at 04:48:21PM +0800, Miaohe Lin wrote: >> When scheduler tries to select a CPU to run the gc thread, >> available_idle_cpu() will check whether vcpu_is_preempted(). It >> will choose other vcpu to run gc threads when the current vcpu is >> preempted. But the preempted vcpu has no other work to do except >> continuing to do gc. In our guest, there are more vcpus than java gc >> threads. So there could always be some available vcpus when >> scheduler tries to select a idle vcpu (runing on host). This leads >> to lots of cpu migrations and results in regression. >> >> I'm not really familiar with this mechanism. Is this a problem that >> needs to be fixed or improved? Or is this just expected behavior? >> Any response would be really appreciated! > > This is pretty much expected behaviour. When a vCPU is preempted the > guest cannot know it's state or latency. Typically in the overcomitted > case another vCPU will be running on the CPU and getting our vCPU thread > back will take a considerable amount of time. I see. Many thanks for your kindly reply and explanation. :) > > If you know you're not over-committed, perhaps you should configure your > VM differently. Do you have any suggestion about how should I configure my VM when it's not over-committed? Thanks, Miaohe Lin