APPLIED: [SRU][J/L][PATCH 0/1] Xen timekeeping performance improvement

[Roxana Nicolescu]

On 26/08/2023 02:57, Krister Johansen wrote:
> Dear Ubuntu Kernel Team:
>
> This patch modifies which default clocksource is selected on Xen.
> Users, especially those on EC2, are encouraged to select tsc as their
> default clocksource.  However, this requires manual tuning of the
> operating system.  Kvm can determine if it safe to use the tsc, and will
> default to that instead of its pvclock when appropriate.  This patch
> does the same for Xen instances.
>
> If appropriate, it's fine if this is applied to only the linux-aws
> branches.
>
> Not all Xen EC2 instances advertise explicit nomigrate support, however,
> on those that do we'll select tsc by default.  On the subset of hosts
> where this is advertised, users will safely default to the more
> performant clocksource.
>
> [Impact]
> Xen instances default to the xen clocksource which has been documented
> to be slower.  This is required for instances where the tsc is not safe
> to use, or the guest is subject to migration.  On some platforms the
> performance impact can be high, and users are encouraged to select the
> tsc when appropriate.  Instead of leaving up to users to figure this out
> by reading a variety of different documents, pick the fast clocksource
> when it can be determined to be safe to do so.
>
> [Backport]
> Clean cherry pick.  No conflicts applying to 5.15 or 6.2.
>
> [Test]
> Booted EC2 xen instances with and without this patch and validated that
> on those that properly advertised the required criteria via cpuid, that
> the clocksource defaulted to tsc instead of xen.
>
> [Potential Regression]
> Potential is low, since only absurd configurations could lead to a
> problem.  If this is considered risky, it can be applied to only
> linux-aws where the documented guidance is for users to enable tsc as
> the clocksource on Xen.
>
> Krister Johansen (1):
>    x86/xen/time: prefer tsc as clocksource when it is invariant
>
>   arch/x86/xen/time.c | 38 +++++++++++++++++++++++++++++++++++++-
>   1 file changed, 37 insertions(+), 1 deletion(-)
>
Applied to jammy,lunar:master-next with the proper commit sha. Thanks!

Roxana