Re: [PATCH v7] sched/clock: Avoid false sharing for sched_clock_irqtime

[Shrikanth Hegde]

On 1/28/26 12:48 PM, K Prateek Nayak wrote:
> On 1/28/2026 11:56 AM, Shrikanth Hegde wrote:
> > On 1/28/26 8:35 AM, K Prateek Nayak wrote:
> > > On 1/28/2026 7:49 AM, Guo, Wangyang wrote:
> > > > Yes, when clock mark unstable through tsc_.*mark_unstable() with non-native_sched_clock, clear_sched_clock_stable won't be called, thus sched_clock_irqtime still keep enabled.
> > > >
> > > > Maybe the dedicated workqueue for sched_clock_irqtime is still needed considering this case.
> > >
> > > In that case, shouldn't tsc_init() only enable irqtime when
> > > using_native_sched_clock()? How can tsc_init() make a call on irqtime if
> > > TSC isn't being used as the sched_clock() ultimately?
> > >
> > > For kvmclock, if PVCLOCK_TSC_STABLE_BIT is not set, it'll call
> > > clear_sched_clock_stable() at kvm_sched_clock_init() but none of the
> > > other clocksources do so we can assume once we override the sched_clock()
> > > it is up to the sched_clock() provider to deal with the clock stability.
> >
> > I think this would depend if mark_tsc_unstable happens after system boot,
> > specially while running kvm guest?
>
> I don't see anything on the guest side that would mark the kvmclock as
> unstable if host's TSC turns unstable post init and since kvmclock
> doesn't set CLOCK_SOURCE_MUST_VERIFY, I doubt if a watchdog runs to
> verify it in the guest.
>
> I have the following in the guest:
>
>      $ sudo dmesg | grep -i clock
>      [    0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
>      [    0.000000] kvm-clock: using sched offset of 423259259 cycles

This means pv_sched_clock is kvm_sched_clock_read from now. and
irqtime is enabled in the guest. right?

>      [    0.000002] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
>      [    0.071675] clocksource: refined-jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645519600211568 ns
>      [    0.378467] clocksource: hpet: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604467 ns
>      [    0.388678] clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x398cb1e4d56, max_idle_ns: 881590790753 ns
>      [    0.679262] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645041785100000 ns
>      [    0.903121] PTP clock support registered
>      [    0.927243] clocksource: Switched to clocksource kvm-clock
>      [    0.944986] clocksource: acpi_pm: mask: 0xffffff max_cycles: 0xffffff, max_idle_ns: 2085701024 ns
>      [    0.993198] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x398cb1e4d56, max_idle_ns: 881590790753 ns
>      [    1.123796] rtc_cmos 00:05: setting system clock to 2026-01-28T07:03:45 UTC (1769583825)
>      [    1.155755] sched_clock: Marking stable (940009972, 212965288)->(1171254846, -18279586)
>      [    1.712598] clk: Disabling unused clocks
>
> Then I mark TSC unstable on the host
>
>      tsc: Marking TSC unstable due to Faking unreliable TSC!
>      TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.
>      clocksource: Checking clocksource tsc synchronization from CPU 93 to CPUs 0,2,26,75,101,114,118,195.
>      sched_clock: Marking unstable (945948313746, 69389667)<-(947618130068, -1600430832)
>      clocksource:         CPU 93 check durations 3436ns - 25277ns for clocksource tsc.
>      clocksource: Switched to clocksource hpet

so now, using_native_sched_clock should fail in guest? If so, with the patch,
irqtime won't be disabled no?

> And nothing happens in the guest.
>
>      cat /sys/devices/system/clocksource/clocksource0/current_clocksource
>      kvm-clock
>
>
> If I launch the guest after marking host TSC unstable, I see:
>
>      Unstable clock detected, switching default tracing clock to "global"
>
> and I don't get any "sched_clock: Marking stable" messages.

Maybe kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT) won't be set.