Re: [PATCH v2 2/5] workqueue: add WQ_AFFN_CACHE_SHARD affinity scope

[Tejun Heo <tj@kernel.org>]

On Thu, Mar 26, 2026 at 09:20:15AM -0700, Breno Leitao wrote:
> Assuming wq_cache_shard_size = 8;, we would have the following number of pool
> per number of CPU (not vCPU):
> 
>   - 1–11 CPUs → DIV_ROUND_CLOSEST(n, 8) ≤ 1 → 1 pool containing all CPUs.
>   - 12 CPUs → DIV_ROUND_CLOSEST(12, 8) = 2 → 2 pools of 6 cores each. This is the first split.
>   - 12–19 → 2 pools
>   - 20–27 → 3 pools
>   - 28–35 → 4 pools
>   - 36–43 → 5 pools
>   - 44–51 → 6 pools
>   - 52–59 → 7 pools
>   - 60–67 → 8 pools
>   - 68–75 → 9 pools (e.g. 72-CPU NVIDIA Grace → 9×8)
>   - 76–83 → 10 pools
>   - 84–91 → 11 pools
>   - 92–99 → 12 pools
>   - 100 → 13 pools (9×8 + 4×7)
> 
> Is this what you meant?

Yes.

> +static int __init llc_core_to_shard(int core_pos, int cores_per_shard,
> +				    int remainder)
> +{
> +	int ret;
> +
> +	/*
> +	 * These cores falls within the large shards.
> +	 * Each large shard has (cores_per_shard + 1) cores
> +	 */
> +	if (core_pos < remainder * (cores_per_shard + 1))
> +		return core_pos / (cores_per_shard + 1);
> +
> +	/* These are standard shards */
> +	ret = (core_pos - remainder * (cores_per_shard + 1)) / cores_per_shard;

This is too smart. Any chance you can dumb it down? If you have to go
through intermediate data structures, that's fine too.

Thanks.

-- 
tejun