From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function Date: Mon, 29 Jul 2019 13:09:27 +0200 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 Return-path: Content-Disposition: inline In-Reply-To: <87zhkxksxd.fsf@vitty.brq.redhat.com> Sender: linux-kernel-owner@vger.kernel.org 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.comashal@kernel.org List-Id: linux-arch.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. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: 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 Date: Mon, 29 Jul 2019 13:09:27 +0200 From: Peter Zijlstra 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> Sender: linux-arch-owner@vger.kernel.org List-ID: 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.comashal@kernel.org Message-ID: <20190729110927.AhfhRZNAERqnJ4TQzCZ4xxghPron35JLDR3dV-txwzQ@z> 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.