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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 DBF2FC433FF for ; Mon, 29 Jul 2019 11:09:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ADD63206E0 for ; Mon, 29 Jul 2019 11:09:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="SLLPaiCl" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387545AbfG2LJm (ORCPT ); Mon, 29 Jul 2019 07:09:42 -0400 Received: from merlin.infradead.org ([205.233.59.134]:45412 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387531AbfG2LJm (ORCPT ); Mon, 29 Jul 2019 07:09:42 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=gi7iIytxwOSusKBpP3uzUn+8/N2d51dx02iEWqcYI5M=; b=SLLPaiClwMC18Nq1QTBwvP30k YyJhcamnVKRI7yAJ+XWG/FpRaXuKbQXd31bOOgYF9qgZzKE0pW8xN1pjej99z5cllncmQUtC61Vfi E2dO1D8gPpdw3I9sJy8dvjgDjv3rfsa2heKpx2h/zwDhV5MS2GVsrSUTbfHjoeS74Qu3PGduZFpev NVEBtNYI63seQnLGwQtsRhK1k55GYqZOwUOu84PXMhMLDZIWaRsZd5+QSbyrTBuk1Z9p6+1XiVL6L 3+maRa07fQD7aoWF4e1PA4I3qI9pGDHXxTZWecArSMtWttaRPgab5KibFYiSGZlUTnJR8W9mR4B+r s8xJBHnaw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1hs3Wz-0000wX-UW; Mon, 29 Jul 2019 11:09:30 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 6EA6B20B51713; Mon, 29 Jul 2019 13:09:27 +0200 (CEST) Date: Mon, 29 Jul 2019 13:09:27 +0200 From: Peter Zijlstra To: Vitaly Kuznetsov Cc: lantianyu1986@gmail.com, Tianyu Lan , linux-arch@vger.kernel.org, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org, kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com, sashal@kernel.org, daniel.lezcano@linaro.org, arnd@arndb.de, michael.h.kelley@microsoft.com, ashal@kernel.org Subject: Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function Message-ID: <20190729110927.GC31398@hirez.programming.kicks-ass.net> References: <20190729075243.22745-1-Tianyu.Lan@microsoft.com> <87zhkxksxd.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87zhkxksxd.fsf@vitty.brq.redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-hyperv-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org On Mon, Jul 29, 2019 at 12:59:26PM +0200, Vitaly Kuznetsov wrote: > lantianyu1986@gmail.com writes: > > > From: Tianyu Lan > > > > Hyper-V guests use the default native_sched_clock() in pv_ops.time.sched_clock > > on x86. But native_sched_clock() directly uses the raw TSC value, which > > can be discontinuous in a Hyper-V VM. Add the generic hv_setup_sched_clock() > > to set the sched clock function appropriately. On x86, this sets > > pv_ops.time.sched_clock to read the Hyper-V reference TSC value that is > > scaled and adjusted to be continuous. > > Hypervisor can, in theory, disable TSC page and then we're forced to use > MSR-based clocksource but using it as sched_clock() can be very slow, > I'm afraid. > > On the other hand, what we have now is probably worse: TSC can, > actually, jump backwards (e.g. on migration) and we're breaking the > requirements for sched_clock(). That (obviously) also breaks the requirements for using TSC as clocksource. IOW, it breaks the entire purpose of having TSC in the first place.