Re: [RFC PATCH 0/6] sched/fair: Introduce the SMT_BUDDY_ASSIST feature

[Kayra Cizmeci]

On Tue, Oct 20, 2026 at 04:37:40PM +0800, Li kunyu wrote:

> Several benchmarks show performance regressions:
> 
> 1. tbench 32      8325.17    6772.693     -18.65%(degeneration):
> Each tbench sample was run three times. Initially, some measurement
> error was observed. Moreover, this patch series essentially utilized all
> idle CPUs, whereas without the feature enabled, 5% to 10% of idle CPUs may
> remain idle. This led to more CPUs experiencing cache interference and
> other effects. The results are benign and within an acceptable range.

According to what the results are 'acceptable'? If there were some error
why even talk about them?

> 
> 2. schbench(99th) 8       13.8    15.0       -8.70%(degeneration):
> The situation is similar to that observed with tbench. Each sample was
> tested five times. The primary regression occurs under bursty task
> scenarios, in which tasks are spread across idle CPUs, resulting in
> increased memory bandwidth contention, cache latency, and other delays
> over a short interval. This is also considered acceptable.
> 
> 
> Li kunyu (6):
>   sched/fair: Add SMT sibling push in periodic load balance
>   sched/fair: Add is_smt_buddy_run() to skip SMT domain
>   sched/fair: SMT sibling exclusion after successful push
>   sched/fair: Restrict newidle balance to LLC domain
>   sched/fair: Add task burst threshold for large-scale bursts
>   sched/docs: Add documentation for SMT Buddy Assist feature
> 
>  Documentation/scheduler/smt-buddy-assist.rst | 126 ++++++++++
>  kernel/sched/fair.c                          | 239 ++++++++++++++++++-
>  kernel/sched/features.h                      |   9 +
>  3 files changed, 364 insertions(+), 10 deletions(-)
>  create mode 100644 Documentation/scheduler/smt-buddy-assist.rst
> 
> -- 
> 2.47.3
>