Re: [PATCH RFC] sched_ext: Choose prev_cpu if idle and cache affine without WF_SYNC

[Andrea Righi]

Hi Joel,

On Tue, Mar 18, 2025 at 01:00:47PM -0400, Joel Fernandes wrote:
...
> From: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> Subject: [PATCH] sched/ext: Make default idle CPU selection better
> 
> Currently, sched_ext's default CPU selection is roughly something like
> this:
> 
> 1. Look for FULLY IDLE CORES:
>     1.1. Select prev CPU (wakee) if its CORE is fully idle.
>     1.2. Or, pick any CPU from fully idle CORE in the L3, then NUMA.
>     1.3. Or, any idle CPU from fully idle CORE usable by task.
> 2. Or, use PREV CPU if it is idle.
> 3. Or any idle CPU in the LLC, NUMA.
> 4. Or finally any CPU usable by the task.
> 
> This can end up select any idle core in the system even if that means
> jumping across NUMA nodes (basically 1.3 happens before 3.).
> 
> Improve this by moving 1.3 to after 3 (so that skipping over NUMA
> happens only later) and also add selection of fully idle target (waker)
> core before looking for fully-idle cores in the LLC/NUMA. This is similar to
> what FAIR scheduler does.
> 
> The new sequence is as follows:
> 
> 1. Look for FULLY IDLE CORES:
>     1.1. Select prev CPU (wakee) if its CORE is fully idle.
>     1.2. Select target CPU (waker) if its CORE is fully idle and shares cache
>         with prev. <- Added this.
>     1.3. Or, pick any CPU from fully idle CORE in the L3, then NUMA.
> 2. Or, use PREV CPU if it is idle.
> 3. Or any idle CPU in the LLC, NUMA.
> 4. Or, any idle CPU from fully idle CORE usable by task. <- Moved down.
> 5. Or finally any CPU usable by the task.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> ---
>  kernel/sched/ext.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 5a81d9a1e31f..324e442319c7 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3558,6 +3558,16 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
>  			goto cpu_found;
>  		}
>  
> +		/*
> +		 * If the waker's CPU shares cache with @prev_cpu and is part
> +		 * of a fully idle core, select it.
> +		 */
> +		if (cpus_share_cache(cpu, prev_cpu) &&
> +		    cpumask_test_cpu(cpu, idle_masks.smt) &&
> +		    test_and_clear_cpu_idle(cpu)) {

I think this is always false, because cpu is still in use by the waker and
its state hasn't been updated to idle yet.

> +			goto cpu_found;
> +		}
> +

-Andrea