Message-ID: <c22ed156-b9d7-487c-93e7-685577e7f12c@amd.com>
Date: Wed, 28 Jan 2026 13:20:44 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, "Guo, Wangyang"
	<wangyang.guo@...el.com>
CC: <linux-kernel@...r.kernel.org>, Benjamin Lei <benjamin.lei@...el.com>,
	"Tim Chen" <tim.c.chen@...ux.intel.com>, Tianyou Li <tianyou.li@...el.com>,
	"Ingo Molnar" <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri
 Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v7] sched/clock: Avoid false sharing for
 sched_clock_irqtime

On 1/28/2026 1:02 PM, Shrikanth Hegde wrote:
> 
> 
> 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?

So within the guest today ...

    $ sudo dmesg | grep -i "clock\|tsc"
    [    0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
    [    0.000000] kvm-clock: using sched offset of 504626078 cycles

    # kvm_sched_clock_init() happens here so it can potentially do
    # clear_sched_clock_stable() here if !PVCLOCK_TSC_STABLE_BIT.

    [    0.000002] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
    [    0.000004] tsc: Detected 1996.251 MHz processor

    # We enable irqtime here once TSC frequency has been determined
    # without considering using_native_sched_clock()


After that TSC is never selected so we don't care if it is stable
or not since it is not the clocksource - the guest continues on
with unstable sched_clock() but also irqtime enabled since TSC
was calibrated successfully.

> 
>>      [    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?

Ideally yes, but the guest continues using kvmclock without any hitch.
I think the x86 KVM layer has something to ensure stability but I'm
not 100% sure.

Since I don't see "tsc: Marking TSC unstable ..." or "sched_clock:
Marking unstable ..." in the guest, we don't hit the mark_tsc_unstable()
path within the guest which would disable irqtime today so essentially
host's TSC turning changing doesn't seem to affect the guest.

> 
>> 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.

True but that is only for new guests launches past marking TSC unstable
on the host.

-- 
Thanks and Regards,
Prateek

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