On Tue, Jul 13, 2021 at 08:35:21AM +0530, Ani Sinha wrote:
> Marking TSC as unstable has a side effect of marking sched_clock as
> unstable when TSC is still being used as the sched_clock. This is not
> desirable. Hyper-V ultimately uses a paravirtualized clock source that
> provides a stable scheduler clock even on systems without TscInvariant
> CPU capability. Hence, mark_tsc_unstable() call should be called _after_
> scheduler clock has been changed to the paravirtualized clocksource. This
> will prevent any unwanted manipulation of the sched_clock. Only TSC will
> be correctly marked as unstable.
Correct me if I'm wrong, what you're trying to address is that
sched_clock remains marked as unstable even after Linux has switched to
a stable clock source.
I think a better approach will be to mark the sched_clock as stable when
we switch to the paravirtualized clock source.
See hyperv_timer.c:hv_setup_sched_clock.
Wei.
>
> Signed-off-by: Ani Sinha <ani@xxxxxxxxxxx>
> ---
> arch/x86/kernel/cpu/mshyperv.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 22f13343b5da..715458b7729a 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -370,8 +370,6 @@ static void __init ms_hyperv_init_platform(void)
> if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
> wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x1);
> setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> - } else {
> - mark_tsc_unstable("running on Hyper-V");
> }
>
> /*
> @@ -432,6 +430,12 @@ static void __init ms_hyperv_init_platform(void)
> /* Register Hyper-V specific clocksource */
> hv_init_clocksource();
> #endif
> + /* TSC should be marked as unstable only after Hyper-V
> + * clocksource has been initialized. This ensures that the
> + * stability of the sched_clock is not altered.
> + */
> + if (!(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))
> + mark_tsc_unstable("running on Hyper-V");
> }
>
> static bool __init ms_hyperv_x2apic_available(void)
> --
> 2.25.1
>
