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_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 3B45FC7618B for ; Mon, 29 Jul 2019 12:13:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 144DD206B8 for ; Mon, 29 Jul 2019 12:13:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727171AbfG2MNV (ORCPT ); Mon, 29 Jul 2019 08:13:21 -0400 Received: from mail-wr1-f65.google.com ([209.85.221.65]:44458 "EHLO mail-wr1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726833AbfG2MNU (ORCPT ); Mon, 29 Jul 2019 08:13:20 -0400 Received: by mail-wr1-f65.google.com with SMTP id p17so61564546wrf.11 for ; Mon, 29 Jul 2019 05:13:18 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:in-reply-to:references:date :message-id:mime-version; bh=OAFiZVjNkFUKfTygRlCRfwGcUdmnJeWZ6/SJ1tkJ51o=; b=UoZ8nFmiYwo+fe67NqY0zX+EqSZ3TAjR1Pzpuhn4M32XFlmSp3zWgATaxVufHX0L8R PrqGeP5lxGoH73ZvCkrnX4PdpRX3qBf+SmcMFy/+Zhh2JkKEypzbBwPZHhCRicqI9pXE n96Gm25gDUpYArfoL1n61xX7sv5VtSQ5MC9PwS0HIghug5lmOLM1qj5XhwWNeVWtXZza Fu6rhD7X6uIGKRxLMct9hpSvYdQa/dsX3WdDp6YPcm47UcRs1xGkjz3Yn4QaCOZz/W8v Eejit5kjWvt3cp0e/Rt9GQzkAxrutv/UHQ46vnMHi0yHG5pl+rPzvnbJYXHGf+XOpCxX QM/g== X-Gm-Message-State: APjAAAUmEMOvYyJ6dVBvv2Yh/J8lRj5XpzYx56v17jmcvA2GsMQEwVAh ObLnoE/Jo5R/E4qmwRHbjdblXA== X-Google-Smtp-Source: APXvYqxYLwdceXu+TMC5pTK1KmiI8eJ05r+J2TyubiU3AKCGtptL9+JT+SU512H9+wia+z4GtJd3Gg== X-Received: by 2002:adf:f450:: with SMTP id f16mr88352291wrp.335.1564402398265; Mon, 29 Jul 2019 05:13:18 -0700 (PDT) Received: from vitty.brq.redhat.com (nat-pool-brq-t.redhat.com. [213.175.37.10]) by smtp.gmail.com with ESMTPSA id y24sm45263300wmi.10.2019.07.29.05.13.16 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Mon, 29 Jul 2019 05:13:17 -0700 (PDT) From: Vitaly Kuznetsov To: Peter Zijlstra 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 In-Reply-To: <20190729110927.GC31398@hirez.programming.kicks-ass.net> References: <20190729075243.22745-1-Tianyu.Lan@microsoft.com> <87zhkxksxd.fsf@vitty.brq.redhat.com> <20190729110927.GC31398@hirez.programming.kicks-ass.net> Date: Mon, 29 Jul 2019 14:13:16 +0200 Message-ID: <87wog1kpib.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra writes: > 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. Currently, we mark raw TSC as unstable when running on Hyper-V (see 88c9281a9fba6), 'TSC page' (which is TSC * scale + offset) is being used instead. The problem is that 'TSC page' can be disabled by the hypervisor and in that case the only remaining clocksource is MSR-based (slow). -- Vitaly From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vitaly Kuznetsov Subject: Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function Date: Mon, 29 Jul 2019 14:13:16 +0200 Message-ID: <87wog1kpib.fsf@vitty.brq.redhat.com> References: <20190729075243.22745-1-Tianyu.Lan@microsoft.com> <87zhkxksxd.fsf@vitty.brq.redhat.com> <20190729110927.GC31398@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <20190729110927.GC31398@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra 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.comashal@kernel.org List-Id: linux-arch.vger.kernel.org Peter Zijlstra writes: > 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. Currently, we mark raw TSC as unstable when running on Hyper-V (see 88c9281a9fba6), 'TSC page' (which is TSC * scale + offset) is being used instead. The problem is that 'TSC page' can be disabled by the hypervisor and in that case the only remaining clocksource is MSR-based (slow). -- Vitaly