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 12:59:26 +0200 Message-ID: <87zhkxksxd.fsf@vitty.brq.redhat.com> References: <20190729075243.22745-1-Tianyu.Lan@microsoft.com> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <20190729075243.22745-1-Tianyu.Lan@microsoft.com> Sender: linux-kernel-owner@vger.kernel.org To: lantianyu1986@gmail.com Cc: 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 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(). -- Vitaly From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-f67.google.com ([209.85.128.67]:53284 "EHLO mail-wm1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387530AbfG2K7a (ORCPT ); Mon, 29 Jul 2019 06:59:30 -0400 Received: by mail-wm1-f67.google.com with SMTP id x15so53422491wmj.3 for ; Mon, 29 Jul 2019 03:59:28 -0700 (PDT) From: Vitaly Kuznetsov Subject: Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function In-Reply-To: <20190729075243.22745-1-Tianyu.Lan@microsoft.com> References: <20190729075243.22745-1-Tianyu.Lan@microsoft.com> Date: Mon, 29 Jul 2019 12:59:26 +0200 Message-ID: <87zhkxksxd.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-arch-owner@vger.kernel.org List-ID: To: lantianyu1986@gmail.com Cc: 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 Message-ID: <20190729105926.9SlbxShh6mhyLBg42THAPPD5JqLEVJ9cr-H7YsxtCBs@z> 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(). -- Vitaly