ACK/Cmnt: [SRU][J][I][F][PATCH v2 0/2] rcu stalls with many storage key guests (LP: 1975582)

Frank Heimes frank.heimes at canonical.com
Wed Jun 15 07:57:04 UTC 2022


No, it is not super important for Impish (but for F and J) - knowing that I
reaches its end of life soon.
I just did the SRU this way (incl. Impish) since it's recommended by the
SRU process to patch without gaps (to avoid any potential regressions on
updates).
If Impish will reach it's EOL meanwhile, that fine ...


On Wed, Jun 15, 2022 at 9:50 AM Stefan Bader <stefan.bader at canonical.com>
wrote:

> On 10.06.22 14:55, 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.
> >
> > v2: since this SRU is not only for J, but also for I and F
> >
> > 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(-)
> >
>
> For Impish, there is a chance that this will not make it. There is only
> one
> cycle until EOL, so if this important it would be good if you explicitly
> mentioned this (as a reply here).
>
> Acked-by: Stefan Bader <stefan.bader at canonical.com>
>
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