[SRU][J][PATCH 0/2] rcu stalls with many storage key guests (LP: 1975582)
frank.heimes at canonical.com
frank.heimes at canonical.com
Fri Jun 10 12:45:28 UTC 2022
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
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