ACK/cmnt: [SRU][PATCH 0/2][Bionic] Fix aggressive CFS throttling

[Kleber Souza]

On 16.10.19 23:15, Khalid Elmously wrote:
> BugLink: https://bugs.launchpad.net/bugs/1832151

The bug report is outdated it seems. It says that 512ac999d275 (sched/fair: Fix bandwidth
timer clock drift condition) is the fix for the bug, but the patches submitted are follow
up fixes for it. Maybe it needs some update? It would be nice to also have it with the
SRU template.

> 
> "sched/fair: Fix bandwidth timer clock drift condition" enabled CPU bandwidth "slice expiry" behaviour in cpu-local silos (something that should have been in effect all along but was basically completely broken). The undesired side-effect of this expirty being enabled is that threads of highly-threaded non-CPU-bound applications get throttled even when the application isn't using its full quota.
> 
> This fix eliminates the problem by removing cpu-local slice expiry altogether. A small pre-requisite patch makes the fix apply nicely.
> 
> A derivative 4.15 cloud kernel was tested with this fix and approved by the cloud provider, and I've tested this fix on the master 4.15 with positive results.

Do you have some more information on how this was tested for regressions on bionic
master kernel? What are the regression potentials?

Would this fix be needed for all newer series as well?
 
> 
> More info in launchpad and salesforce.
> 
> 
> 
> Dave Chiluk (1):
>   sched/fair: Fix low cpu usage with high throttling by removing
>     expiration of cpu-local slices
> 
> Patrick Bellasi (1):
>   sched/fair: Add lsub_positive() and use it consistently
> 
>  Documentation/scheduler/sched-bwc.txt | 74 +++++++++++++++++++-----
>  kernel/sched/fair.c                   | 83 ++++++---------------------
>  kernel/sched/sched.h                  |  4 --
>  3 files changed, 79 insertions(+), 82 deletions(-)
> 

The backport looks good, fix is upstream and has been tested to fix
the issue.

I'm ACK'ing it for now, but I would like to get some more information about
regression tests and potential before applying it.

Acked-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>