NACK - [SRU][J][PATCH 0/2] rcu stalls with many storage key guests (LP: 1975582)

[Frank Heimes]

Please NACK, since this is not only for jammy, but also for impish and
focal (need to adjust the Subjects) on a v2

On Fri, Jun 10, 2022 at 2:46 PM <frank.heimes at canonical.com> wrote:

> BugLink: https://bugs.launchpad.net/bugs/1975582
>
> SRU Justification:
>
> [Impact]
>
>  * Ubuntu on s390x KVM environments with lots of large guests with storage
>    keys can be affected by rcu stalls.
>
>  * These rcu stalls can cause the system to crash/dump.
>
> [Fix]
>
>  * 3ae11dbcfac9 3ae11dbcfac906a8c3a480e98660a823130dc16a "s390/mm: use
> non-quiescing sske for KVM switch to keyed guest"
>
>  * 6d5946274df1 6d5946274df1fff539a7eece458a43be733d1db8 "s390/gmap:
> voluntarily schedule during key setting"
>
> [Test Plan]
>
>  * There is no trigger or direct test or re-creation of the
>    problem situation possible, but...
>
>  * and IBM z13 or LinuxONE (or never) LPAR is needed that
>    runs Ubuntu Server 20.04 LTS or 18.04 LTS with HWE kernel
>    and acts as KVM host with again several large guests running
>    on top with storage groups.
>
>  * Let such a system running for days under significant load
>    and watch the logs for rcu issues.
>
>  * Prior to the submission of this SRU patched test kernels
>    for focal 5.4 and bionic hwe-5.4 were created and tested.
>    They ran for days at a staging environemnt at IBM
>    without further issues.
>
>  * The modifications are all limited to s390x.
>
>  * A test kernel was build (see below) that ran in a test environment
>    at IBM under appropriate load for several days.
>
> [Where problems could occur]
>
>  * Due to the change for the KVM switch to keyed guest
>    from classic sske to non-quiescing sske
>    the KVM behaviour might have changed and the storage keys harmed.
>
>  * The now more generous scheduling while setting keys
>    has an impact on the guest memory management and mapping
>    which will lead to a different performance.
>
>  * This, with the introduction of __s390_enable_skey_pmd and
>    cond_resched, might increase the overhead in certain situations,
>    but eventually improves the responsiveness over time,
>    hence avoid rcu stalls.
>
> [Other Info]
>
>  * Since the patches are upstream in 5.19-rc1,
>    they will be included in the kernel that is planned for kinetic (5.19).
>
>  * Hence this is an SRU to jammy, impish and focal.
>
> Christian Borntraeger (2):
>   s390/gmap: voluntarily schedule during key setting
>   s390/mm: use non-quiescing sske for KVM switch to keyed guest
>
>  arch/s390/mm/gmap.c    | 14 ++++++++++++++
>  arch/s390/mm/pgtable.c |  2 +-
>  2 files changed, 15 insertions(+), 1 deletion(-)
>
> --
> 2.25.1
>
> --
> kernel-team mailing list
> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.ubuntu.com/archives/kernel-team/attachments/20220610/f63179d1/attachment.html>