** Also affects: linux (Ubuntu Lunar)
Importance: Undecided
Status: New
** Also affects: linux (Ubuntu Jammy)
Importance: Undecided
Status: New
** Changed in: linux (Ubuntu)
Status: Confirmed => Invalid
** Changed in: linux (Ubuntu Jammy)
Status: New => Fix Committed
** Changed in: linux (Ubuntu Lunar)
Status: New => Fix Committed
** Changed in: linux (Ubuntu Jammy)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Lunar)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Jammy)
Assignee: (unassigned) => Krister Johansen (kmjohansen)
** Changed in: linux (Ubuntu Lunar)
Assignee: (unassigned) => Krister Johansen (kmjohansen)
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https://bugs.launchpad.net/bugs/2033122
Title:
Request backport of xen timekeeping performance improvements
Status in linux package in Ubuntu:
Invalid
Status in linux source package in Jammy:
Fix Committed
Status in linux source package in Lunar:
Fix Committed
Bug description:
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
requests a backport of 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.
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