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

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Date:   Sat, 26 Jan 2019 16:11:37 +0000
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Dominique Martinet <asmadeus@...ewreck.org>,
        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

On 19-01-25 21:04:10, Jon DeVree wrote:
> 
> * KVM_FEATURE_CLOCKSOURCE_STABLE_BIT is always set by the hypervisor
>   starting with Linux v2.6.35 - 371bcf646d17 ("KVM: x86: Tell the guest
>   we'll warn it about tsc stability")
> * PVCLOCK_TSC_STABLE_BIT is set starting in Linux v3.8 but only if the
>   clocksource is the TSC - d828199e8444 ("KVM: x86: implement
>   PVCLOCK_TSC_STABLE_BIT pvclock flag")
> 
> I changed the clocksource of a hypervisor that wasn't having issues from
> TSC to HPET and when I started up a guest VM the bug suddenly appeared.
> I shut down the guest, set the hypervisor's clocksource back to TSC,
> started up the guest and the bug went away again.
> 
> You don't actually have to reboot the guest before the bug is visible
> either, just letting the guest sit at the GRUB menu for a minute or two
> before loading Linux is enough to make the bug plainly visible in the
> printk timestamps.
> 
> I don't know enough to actually fix the bug, but hopefully this is
> enough to allow everyone else to reproduce it and come up with a fix.

Thank you very much for your analysis, I am now able to reproduce the
problem by setting clocksource on my machine to hpet. I will soon submit a
patch with a fix.

Pasha

> 
> -- 
> Jon
> X(7): A program for managing terminal windows. See also screen(1) and tmux(1).