Re: [PATCH sched_ext/for-7.1] sched_ext: idle: Prioritize idle SMT sibling

[Andrea Righi <arighi@nvidia.com>]

Hi Daniel,

On Fri, Mar 20, 2026 at 11:57:02AM -0400, Daniel Jordan wrote:
> Hi Andrea,
> 
> On Wed, Mar 18, 2026 at 01:38:42AM +0100, Andrea Righi wrote:
> > In the default built-in idle CPU selection policy, when @prev_cpu is
> > busy and no fully idle core is available, try to place the task on its
> > SMT sibling if that sibling is idle, before searching any other idle CPU
> > in the same LLC.
> > 
> > Migration to the sibling is cheap and keeps the task on the same core,
> > preserving L1 cache and reducing wakeup latency.
> 
> Seems reasonable.
> 
> > On large SMT systems this appears to consistently boost throughput by
> > roughly 2-3% on CPU-bound workloads (running a number of tasks equal to
> > the number of SMT cores).
> 
> What workloads out of curiosity?

For the "server" side, I've used an internal benchmark suite, the workload
that is showing the best results (3% speedup) is based on NVBLAS, but doing
pure CPU activity.

I've also tested this locally (AMD Ryzen 9 laptop), usual gaming benchmarks
checking avg fps and tail latency. I noticed small improvements for this
use-case as well (still in the 1-2% range, nothing impressive, but it looks
consistent).

Thanks,
-Andrea