Re: [PATCH] sched/fair: Optimize idle core discovery algorithm via LLC-wide bitmask

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 22, 2026 at 10:20:24AM -0500, Qiliang Yuan wrote:
> The current select_idle_cpu() employs a linear O(N) scan to find idle
> cores, which scales poorly on modern high-core-count systems.
> 
> This patch optimizes the discovery algorithm by:
> 1. Adding a per-LLC 'idle_cores_mask' to sched_domain_shared for
>    tracking core-level idle status.
> 2. Converting the linear search into an efficient bitmask iteration,
>    reducing typical search complexity towards O(1) in sparse scenarios.
> 3. Implementing a lazy-clear mechanism during the scan to maintain
>    efficiency without expensive global synchronization.
> 
> This algorithmic refinement minimizes the search space in the critical
> wakeup path, effectively mitigating linear scaling overhead on large
> SMT machines.

Since this is a performance patch; what is missing here is the actual
performance data.

This can be in the form of a micro-benchmark, or perf numbers showing
the improvement for this code-path.

Also, what kind of topologies have you tested this on; typically Intel
has a few very large LLC domains while AMD has more smaller ones.