Michael Kelley <mikelley@xxxxxxxxxxxxx> writes: > From: Tianyu Lan <lantianyu1986@xxxxxxxxx> Sent: Tuesday, July 30, 2019 6:41 AM >> >> On Mon, Jul 29, 2019 at 8:13 PM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote: >> > >> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes: >> > >> > > On Mon, Jul 29, 2019 at 12:59:26PM +0200, Vitaly Kuznetsov wrote: >> > >> lantianyu1986@xxxxxxxxx writes: >> > >> >> > >> > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx> >> > >> > >> > >> > 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). >> > >> >> Yes, that will be slow if Hyper-V doesn't expose hv tsc page and >> kernel uses MSR based >> clocksource. Each MSR read will trigger one VM-EXIT. This also happens on other >> hypervisors (e,g, KVM doesn't expose KVM clock). Hypervisor should >> take this into >> account and determine which clocksource should be exposed or not. >> > > We've confirmed with the Hyper-V team that the TSC page is always available > on Hyper-V 2016 and later, and on Hyper-V 2012 R2 when the physical > hardware presents an InvariantTSC. Currently we check that TSC page is valid on every read and it seems this is redundant, right? It is either available on boot or not. I can only imagine migrating a VM to a non-InvariantTSC host when Hyper-V will likely disable the page (and we can get reenlightenment notification then). > But the Linux Kconfig's are set up so > the TSC page is not used for 32-bit guests -- all clock reads are synthetic MSR > reads. For 32-bit, this set of changes will add more overhead because the > sched clock reads will now be MSR reads. > > I would be inclined to fix the problem, even with the perf hit on 32-bit Linux. > I don’t have any data on 32-bit Linux being used in a Hyper-V guest, but it's not > supported in Azure so usage is pretty small. The alternative would be to continue > to use the raw TSC value on 32-bit, even with the risk of a discontinuity in case of > live migration or similar scenarios. The issue needs fixing, I agree, however using MSR based clocksource as sched clock may give us too big of a performance hit (not sure who cares about 32 bit guest performance nowadays but still). What stops us from enabling TSC page for 32 bit guests if it is available? -- Vitaly
