Re: [PATCH 4/4] sched/rt: Split cpupri_vec->cpumask to per NUMA node to reduce contention

[Chen, Yu C]

On 7/7/2025 10:35 AM, Pan Deng wrote:
> When running a multi-instance FFmpeg workload on HCC system, significant
> contention is observed on bitmap of `cpupri_vec->cpumask`.
>
> The SUT is a 2-socket machine with 240 physical cores and 480 logical
> CPUs. 60 FFmpeg instances are launched, each pinned to 4 physical cores
> (8 logical CPUs) for transcoding tasks. Sub-threads use RT priority 99
> with FIFO scheduling. FPS is used as score.
>
> perf c2c tool reveals:
> cpumask (bitmap) cache line of `cpupri_vec->mask`:
> - bits are loaded during cpupri_find
> - bits are stored during cpupri_set
> - cycles per load: ~2.2K to 8.7K
>
> This change splits `cpupri_vec->cpumask` into per-NUMA-node data to
> mitigate false sharing.
>
> As a result:
> - FPS improves by ~3.8%
> - Kernel cycles% drops from ~20% to ~18.7%
> - Cache line contention is mitigated, perf-c2c shows cycles per load
>    drops from ~2.2K-8.7K to ~0.5K-2.2K

This brings noticeable improvement for RT workload, and it would
be even more convincing if we can have try on normal task workload,
at least not bring regression(schbench/hackbenc, etc).

thanks,
Chenyu

> Note: CONFIG_CPUMASK_OFFSTACK=n remains unchanged.