Re: [PATCH 05/10] sched_ext: idle: clarify comments

[Yury Norov <yury.norov@gmail.com>]

On Fri, Dec 20, 2024 at 04:11:37PM +0100, Andrea Righi wrote:
> Add a comments to clarify about the usage of cpumask_intersects().
> 
> Moreover, update scx_select_cpu_dfl() description clarifying that the
> final step of the idle selection logic involves searching for any idle
> CPU in the system that the task can use.
> 
> Signed-off-by: Andrea Righi <arighi@nvidia.com>

Reviewed-by: Yury Norov <yury.norov@gmail.com>

> ---
>  kernel/sched/ext_idle.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index dedd39febc88..4952e2793304 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -52,6 +52,10 @@ static bool test_and_clear_cpu_idle(int cpu)
>  		 * scx_pick_idle_cpu() can get caught in an infinite loop as
>  		 * @cpu is never cleared from idle_masks.smt. Ensure that @cpu
>  		 * is eventually cleared.
> +		 *
> +		 * NOTE: Use cpumask_intersects() and cpumask_test_cpu() to
> +		 * reduce memory writes, which may help alleviate cache
> +		 * coherence pressure.
>  		 */
>  		if (cpumask_intersects(smt, idle_masks.smt))
>  			cpumask_andnot(idle_masks.smt, idle_masks.smt, smt);
> @@ -280,6 +284,8 @@ static void update_selcpu_topology(void)
>   * 4. Pick a CPU within the same NUMA node, if enabled:
>   *   - choose a CPU from the same NUMA node to reduce memory access latency.
>   *
> + * 5. Pick any idle CPU usable by the task.
> + *
>   * Step 3 and 4 are performed only if the system has, respectively, multiple
>   * LLC domains / multiple NUMA nodes (see scx_selcpu_topo_llc and
>   * scx_selcpu_topo_numa).
> -- 
> 2.47.1