Hi Peter, Rik,

Running sysbench measurements in a 16CPU/30GB KVM guest on a 20CPU/40GB 
s390x host, we noticed a throughput degradation (anywhere between 13% 
and 40%, depending on test) when moving the host from kernel 4.12 to 
4.13.  The rest of the host and the entire guest remain unchanged; it is 
only the host kernel that changes.  Bisecting the host kernel blames 
commit 3fed382b46ba ("sched/numa: Implement NUMA node level wake_affine()").

Reverting 3fed382b46ba and 815abf5af45f ("sched/fair: Remove 
effective_load()") from a clean 4.13.0 build erases the throughput 
degradation and returns us to what we see in 4.12.0.

A little poking around points us to a fix/improvement to this, commit 
90001d67be2f ("sched/fair: Fix wake_affine() for !NUMA_BALANCING"), 
which went in the 4.14 merge window and an unmerged fix [1] that 
corrects a small error in that patch.  Hopeful, since we were running 
!NUMA_BALANCING, I applied these two patches to a clean 4.13.0 tree but 
continue to see the performance degradation.  Pulling current master or 
linux-next shows no improvement lurking in the shadows.

Running perf stat on the host during the guest sysbench run shows a 
significant increase in cpu-migrations over the 4.12.0 run.  Abbreviated 
examples follow:

# 4.12.0
# perf stat -p 11473 -- sleep 5
       62305.199305      task-clock (msec)         #   12.458 CPUs
            368,607      context-switches
              4,084      cpu-migrations
                416      page-faults

# 4.13.0
# perf stat -p 11444 -- sleep 5
       35892.653243      task-clock (msec)         #    7.176 CPUs
            249,251      context-switches
             56,850      cpu-migrations
                804      page-faults

# 4.13.0-revert-3fed382b46ba-and-815abf5af45f
# perf stat -p 11441 -- sleep 5
       62321.767146      task-clock (msec)         #   12.459 CPUs
            387,661      context-switches
              5,687      cpu-migrations
              1,652      page-faults

# 4.13.0-apply-90001d67be2f
# perf stat -p 11438 -- sleep 5
       48654.988291      task-clock (msec)         #    9.729 CPUs
            363,150      context-switches
             43,778      cpu-migrations
                641      page-faults

I'm not sure what doc to supply here and am unfamiliar with this code or 
its recent changes, but I'd be happy to pull/try whatever is needed to 
help debug things.  Looking forward to hearing what I can do.

Thanks,
Eric

[1] https://lkml.org/lkml/2017/9/6/196