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Message-ID: <a50d1213-2390-14f1-fe78-e8058266978c@suse.com>
Date: Tue, 15 Jan 2019 12:03:48 +0100
From: Juergen Gross <jgross@...e.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
x86@...nel.org, boris.ostrovsky@...cle.com, sstabellini@...nel.org,
hpa@...or.com, mingo@...hat.com, bp@...en8.de, hans@...rrie.org,
stable@...r.kernel.org
Subject: Re: [PATCH v3] xen: Fix x86 sched_clock() interface for xen
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
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