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Subject: Re: [PATCH v3] xen: Fix x86 sched_clock() interface for xen
To:     Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org, x86@kernel.org
Cc:     boris.ostrovsky@oracle.com, sstabellini@kernel.org, hpa@zytor.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        stable@vger.kernel.org
References: <20190114124413.3742-1-jgross@suse.com>
From:   Hans van Kranenburg <hans@knorrie.org>
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Message-ID: <ebfc24e3-21a0-6cc6-ae68-5c13c1e66b9a@knorrie.org>
Date:   Wed, 16 Jan 2019 01:24:09 +0100
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Hi,

On 1/14/19 1:44 PM, Juergen Gross wrote:
> Commit f94c8d11699759 ("sched/clock, x86/tsc: Rework the x86 'unstable'
> sched_clock() interface") broke Xen guest time handling across
> migration:
> 
> [  187.249951] Freezing user space processes ... (elapsed 0.001 seconds) done.
> [  187.251137] OOM killer disabled.
> [  187.251137] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
> [  187.252299] suspending xenstore...
> [  187.266987] xen:grant_table: Grant tables using version 1 layout
> [18446743811.706476] OOM killer enabled.
> [18446743811.706478] Restarting tasks ... done.
> [18446743811.720505] Setting capacity to 16777216
> 
> Fix that by setting xen_sched_clock_offset at resume time to ensure a
> monotonic clock value.
> 
> [...]

With v3 of the patch, I see the time jump in one log line happen, but
only when using PVH.

[   49.486453] Freezing user space processes ... (elapsed 0.002 seconds)
done.
[   49.488743] OOM killer disabled.
[   49.488764] Freezing remaining freezable tasks ... (elapsed 0.001
seconds) done.
[   49.491117] suspending xenstore...
[2000731.388722] xen:events: Xen HVM callback vector for event delivery
is enabled
[   49.491750] xen:grant_table: Grant tables using version 1 layout
[   49.810722] OOM killer enabled.
[   49.810744] Restarting tasks ... done.
[   49.856263] Setting capacity to 6291456
[   50.006002] Setting capacity to 10485760

If I start as PV, it never seems to happen.

Up to you to decide how important this is. :)

FYI this is with v3 on top of the Debian stretch-backports 4.19 kernel,
which I'm starting to use now to reboot things with.

-# uname -a
Linux appnode-kylie 4.19.0-0.bpo.1-cloud-amd64 #1 SMP Debian
4.19.12-1~bpo9+1+mendix1 (2019-01-15) x86_64 GNU/Linux

https://salsa.debian.org/knorrie-guest/linux/commits/mendix/stretch-backports

Hans