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

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Date:   Thu, 3 Jan 2019 16:28:56 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Salvatore Bonaccorso <carnil@...ian.org>
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        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@...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, 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

Could you please send the config file and qemu arguments that were
used to reproduce this problem.

Thank you,
Pasha

On Wed, Jan 2, 2019 at 3:20 PM Salvatore Bonaccorso <carnil@...ian.org> wrote:
>
> Hi,
>
> On Tue, Nov 06, 2018 at 06:35:36AM -0500, Steven Sistare wrote:
> > Pavel has a new email address, cc'd - steve
> >
> > On 11/6/2018 12:42 AM, Dominique Martinet wrote:
> > > (added various kvm/virtualization lists in Cc as well as qemu as I don't
> > > know who's "wrong" here)
> > >
> > > Pavel Tatashin wrote on Thu, Jul 19, 2018:
> > >> Allow sched_clock() to be used before schec_clock_init() is called.
> > >> This provides with a way to get early boot timestamps on machines with
> > >> unstable clocks.
> > >
> > > This isn't something I understand, but bisect tells me this patch
> > > (landed as 857baa87b64 ("sched/clock: Enable sched clock early")) makes
> > > a VM running with kvmclock take a step in uptime/printk timer early in
> > > boot sequence as illustrated below. The step seems to be related to the
> > > amount of time the host was suspended while qemu was running before the
> > > reboot.
> > >
> > > $ dmesg
> > > ...
> > > [    0.000000] SMBIOS 2.8 present.
> > > [    0.000000] DMI: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20180724_192412-buildhw-07.phx2.fedoraproject.org-1.fc29 04/01/2014
> > > [    0.000000] Hypervisor detected: KVM
> > > [    0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
> > > [283120.529821] kvm-clock: cpu 0, msr 321a8001, primary cpu clock
> > > [283120.529822] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
> > > [283120.529824] tsc: Detected 2592.000 MHz processor
> > > ...
> > >
> > > (The VM is x86_64 on x86_64, I can provide my .config on request but
> > > don't think it's related)
> > >
> > >
> > > It's rather annoying for me as I often reboot VMs and rely on the
> > > 'uptime' command to check if I did just reboot or not as I have the
> > > attention span of a goldfish; I'd rather not have to find something else
> > > to check if I did just reboot or not.
> > >
> > > Note that if the qemu process is restarted, there is no offset anymore.
> > >
> > > I unfortunately just did that so cannot say with confidence (putting my
> > > laptop to sleep for 30s only led to a 2s offset and I do not want to
> > > wait longer right now), but it looks like the clock is still mostly
> > > correct after reboot after disabling my VM's ntp client. Will infirm
> > > that tomorrow if I was wrong.
> > >
> > >
> > > Happy to try to help fixing this in any way, as written above the quote
> > > I'm not even actually sure who is wrong here.
>
> A user in Debian reported the same/similar issue (with 4.19.13):
>
> https://bugs.debian.org/918036
>
> Regards,
> Salvatore