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Subject: Re: [PATCH v3] xen: Fix x86 sched_clock() interface for xen
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        x86@kernel.org, boris.ostrovsky@oracle.com, sstabellini@kernel.org,
        hpa@zytor.com, mingo@redhat.com, bp@alien8.de, hans@knorrie.org,
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References: <20190114124413.3742-1-jgross@suse.com>
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From:   Juergen Gross <jgross@suse.com>
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Date:   Tue, 15 Jan 2019 12:03:48 +0100
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On 15/01/2019 11:43, Thomas Gleixner wrote:
> On Mon, 14 Jan 2019, 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
> 
> I see that it's broken, but the changelog could do with an explanation WHY
> it broke.

This seems to be rather complex.

I believe the mentioned commit just ignored Xen guests resulting in a
"stable" clock where it shouldn't, but maybe I have missed another
aspect of this commit which is to blame. I tried to fix that by
replacing using_native_sched_clock() with a hypervisor specific pvops
function.

Unfortunately this didn't work, maybe due to other uses of
using_native_sched_clock() added by later patches. In the end it was
quite clear that updating the Xen clock offset was the right thing to
do, so I ended up with this patch.


Juergen