linux-kernel - Re: [PATCH v7] sched/clock: Avoid false sharing for sched_clock

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Message-ID: <2a7521d9-e4d8-4dc6-8e5a-122796f0d1ab@linux.ibm.com>
Date: Wed, 28 Jan 2026 13:02:59 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: K Prateek Nayak <kprateek.nayak@....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 1/28/26 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.