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

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Message-ID: <20190107234839.GA15269@nautica>
Date:   Tue, 8 Jan 2019 00:48:39 +0100
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
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

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