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

[Peter Zijlstra]

On Tue, Oct 20, 2026 at 04:37:40PM +0800, Li kunyu wrote:
> [Problem]
> 
> It was found that on servers equipped with the Hygon 7447v CPU (Model 7,
> 384 logical cores), migrating tasks across LLC scheduling domains during
> load balancing caused significant performance degradation:
> 
> 1. A low L3 cache hitting rate (due to factors such as non-shared L3
> caches).
> 
> 2. The traversal overhead of higher-level scheduling domains (e.g.,
> cross-NUMA scheduling domains).
> 
> Similar issues were also observed on CPUs from other vendors during
> testing, though the degree of memory bandwidth increase and other
> latency effects varied. Consequently, a series of patches was developed
> to address and improve these issues.
> 
> 
> [Solution]
> 
> This RFC patch series introduces the SMT_BUDDY_ASSIST scheduling
> feature. The main work is as follows:
> 
> 1. Push CFS tasks to SMT sibling CPUs during periodic load balancing.
> 
> 2. Skip SMT-domain balancing after a successful push and avoid pulling
> the pushed task back.
> 
> 3. Restrict newidle balancing so it does not pull from higher-level
> scheduling domains.
> 
> 4. Compensate for the resulting load imbalance with a burst-task wakeup
> path that sends tasks to idle CPUs.

So you're combatting undue LLC migrations with brute force :-( Mostly
this looks like something where cache aware scheduling should help, no?
That ought to make cross-LLC migrations less attractive.

Now, I realize that cache-aware scheduling as is won't help stream; when
build using OpenMP the whole thing is one big process and cache-aware
mostly gives up, when build using MPI its a ton of single threads and
cache-aware will ignore them.

Still, I think you really should look that way, perhaps investigate if
something like:

  https://patch.msgid.link/cover.1787955777.git.tim.c.chen%40linux.intel.com

can help you.




I'm not sure why you're doing the SMT push thing; but there's this
thing:

  https://patch.msgid.link/20261002154415.2270586-8-vincent.guittot%40linaro.org