APPLIED[F/G]: [SRU][F][G][H][PATCH 0/1] s390/vtime: fix increased steal time accounting (LP: 1921498)

[Kelsey Skunberg]

Applied to Focal and Groovy master-next. Thank you! 

-Kelsey

On 2021-03-29 18:15:46 , frank.heimes at canonical.com wrote:
> BugLink: https://bugs.launchpad.net/bugs/1921498
> 
> SRU Justification:
> 
> [Impact]
> 
> * The reported steal time on s390x is erroneously increasing and therefore broken.
> 
> * This problem got introduced with commit 152e9b8676c6e
>   ("s390/vtime: steal time exponential moving average") - upstream with v5.1
> 
> [Fix]
> 
> * d54cb7d54877d529bc1e0e1f47a3dd082f73add3 d54cb7d54877 "s390/vtime: fix increased steal time accounting"
> 
> [Test Case]
> 
> * An IBM Z or LinuxONE systems, installed with Ubuntu Server 20.04, 20.10 and 21.04 on LPAR, are needed.
> 
> * The system needs to be configured as KVM host with one or more KVM guests.
> 
> * Now put significant workload on the guest(s), so that the hypervisor starts to 'steal time'.
>   This can be best forced by having only very limited CPU resources for the hypervisor itself.
> 
> * Start monitoring the steal time, that is reported at /proc/stat,
>   but can be more easily verified with tools like vmstat or even top.
> 
> * If the steal time starts ever growing (exponentially), the situation is broken.
> 
> * In case the steal time stays relatively constant,
>   or grows only very limited after a ramp up phase of some minutes,
>   or just oscillates around a certain value, a fixed/patched kernel is in use.
>   
> [Regression Potential]
> 
> * The patch only changes the single line that calculates: account_steal_time
> 
> * In case the account_steal_time calculation is still broken after the modification,
>   the steal time is just again not reported correctly on s390x,
>   but maybe in a different way (rather than exponentially growing).
> 
> * But it will not further harm virtualization on s390x systems.
> 
> * The modification is very limited with that one line change.
> 
> * The commit got upstream accepted with 5.12-rc4.
> 
> [Other]
> 
> * Since the patch is needed for kernel higher than 5.1 and got upstream accepted with 5.12-rc4,
>   hirsute, groovy and focal are affected.
> 
> * The upstream commit can be cleanly cherry-picked from all three affected Ubuntu releases.
> 
> Gerald Schaefer (1):
>   s390/vtime: fix increased steal time accounting
> 
>  arch/s390/kernel/vtime.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> -- 
> 2.25.1
> 
> 
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