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=-0.8 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 D9708C0044C for ; Wed, 31 Oct 2018 15:07:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 98EEB205F4 for ; Wed, 31 Oct 2018 15:07:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 98EEB205F4 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729506AbeKAAFs convert rfc822-to-8bit (ORCPT ); Wed, 31 Oct 2018 20:05:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38114 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729445AbeKAAFs (ORCPT ); Wed, 31 Oct 2018 20:05:48 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 63FD181DFE; Wed, 31 Oct 2018 15:07:24 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-8.bos.redhat.com [10.18.17.8]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8B66F6107F; Wed, 31 Oct 2018 15:07:22 +0000 (UTC) Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall To: Peter Zijlstra , Yi Sun Cc: Juergen Gross , linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de, chao.p.peng@intel.com, chao.gao@intel.com, isaku.yamahata@intel.com, michael.h.kelley@microsoft.com, tianyu.lan@microsoft.com, "K. Y. Srinivasan" , Haiyang Zhang , Stephen Hemminger , "mingo@redhat.com" , Will Deacon References: <1539954835-34035-3-git-send-email-yi.y.sun@linux.intel.com> <20181022015342.GK11769@yi.y.sun> <2e0d62cb-b706-a5b4-65f7-982a913fb32b@suse.com> <20181022171516.GH3117@worktop.programming.kicks-ass.net> <20181023025740.GL11769@yi.y.sun> <20181023085127.GG3109@worktop.c.hoisthospitality.com> <20181023093328.GA15378@yi.y.sun> <20181031015417.GC15378@yi.y.sun> <20181031141030.GB13219@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: Date: Wed, 31 Oct 2018 11:07:22 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: <20181031141030.GB13219@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Wed, 31 Oct 2018 15:07:24 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/31/2018 10:10 AM, Peter Zijlstra wrote: > On Wed, Oct 31, 2018 at 09:54:17AM +0800, Yi Sun wrote: >> On 18-10-23 17:33:28, Yi Sun wrote: >>> On 18-10-23 10:51:27, Peter Zijlstra wrote: >>>> Can you try and explain why vcpu_is_preempted() doesn't work for you? >>> I thought HvSpinWaitInfo is used to notify hypervisor the spin number >>> which is different with vcpu_is_preempted. So I did not consider >>> vcpu_is_preempted. >>> >>> But HvSpinWaitInfo is a quite simple function and could be combined >>> with vcpu_is_preempted together. So I think it is OK to use >>> vcpu_is_preempted to make codes clean. I will have a try. >> After checking codes, there is one issue to call vcpu_is_preempted. >> There are two spin loops in qspinlock_paravirt.h. One loop in >> 'pv_wait_node' calls vcpu_is_preempted. But another loop in >> 'pv_wait_head_or_lock' does not call vcpu_is_preempted. It also does >> not call any other ops of 'pv_lock_ops' in the loop. So I am afraid >> we have to add one more ops in 'pv_lock_ops' to do this. > Why? Would not something like the below cure that? Waiman, can you have > a look at this; I always forget how that paravirt crud works. There are two major reasons why the vcpu_is_preempt() test isn't done at pv_wait_head_or_lock(). First of all, we may not have a valid prev pointer after all if it is the first one to enter the queue while the lock is busy. Secondly, because of lock stealing, the cpu number pointed by a valid prev pointer may not be the actual cpu that is currently holding the lock. Another minor reason is that we want to minimize the lock transfer latency and so don't want to sleep too early while waiting at the queue head. Cheers, Longman