linux-kernel - Re: [PATCH v15 23/26] sched: early boot clock

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Date:   Mon, 7 Jan 2019 20:04:41 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     Salvatore Bonaccorso <carnil@...ian.org>,
        Steven Sistare <steven.sistare@...cle.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, John Stultz <john.stultz@...aro.org>,
        sboyd@...eaurora.org, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>, mingo@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>, hpa@...or.com,
        douly.fnst@...fujitsu.com, Peter Zijlstra <peterz@...radead.org>,
        prarit@...hat.com, feng.tang@...el.com,
        Petr Mladek <pmladek@...e.com>, gnomes@...rguk.ukuu.org.uk,
        linux-s390@...r.kernel.org, boris.ostrovsky@...cle.com,
        jgross@...e.com, pbonzini@...hat.com,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        qemu-devel@...gnu.org
Subject: Re: [PATCH v15 23/26] sched: early boot clock

I did exactly the same sequence on Kaby Lake CPU and could not
reproduce it. What is your host CPU?

Thank you,
Pasha

On Mon, Jan 7, 2019 at 6:48 PM Dominique Martinet
<asmadeus@...ewreck.org> wrote:
>
> Pavel Tatashin wrote on Mon, Jan 07, 2019:
> > I could not reproduce the problem. Did you suspend to memory between
> > wake ups? Does this time jump happen every time, even if your laptop
> > sleeps for a minute?
>
> I'm not sure I understand "suspend to memory between the wake ups".
> The full sequence is:
>  - start a VM (just in case, I let it boot till the end)
>  - suspend to memory (aka systemctl suspend) the host
>  - after resuming the host, soft reboot the VM (login through
> serial/ssh/whatever and reboot or in the qemu console 'system_reset')
>
> I've just slept exactly one minute and reproduced again with the fedora
> stock kernel now (4.19.13-300.fc29.x86_64) in the VM.
>
> Interestingly I'm not getting the same offset between multiple reboots
> now despite not suspending again; but if I don't suspend I cannot seem
> to get it to give an offset at all (only tried for a few minutes; this
> might not be true) ; OTOH I pushed my luck further and even with a five
> seconds sleep I'm getting a noticeable offset on first VM reboot after
> resume:
>
> [    0.000000] Hypervisor detected: KVM
> [    0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
> [  179.362163] kvm-clock: cpu 0, msr 13c01001, primary cpu clock
> [  179.362163] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
>
> Honestly not sure what more information I could give, I'll try on some
> other hardware than my laptop (if I can get a server to resume after
> suspend through ipmi or wake on lan); but I don't have anything I could
> install ubuntu on to try their qemu's version... although I really don't
> want to believe that's the difference...
>
> Thanks,
> --
> Dominique