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

[Andrea Righi <arighi@nvidia.com>]

Hi Joel,

On Mon, Mar 17, 2025 at 04:28:02AM -0400, Joel Fernandes wrote:
> Consider that the previous CPU is cache affined to the waker's CPU and
> is idle. Currently, scx's default select function only selects the
> previous CPU in this case if WF_SYNC request is also made to wakeup on the
> waker's CPU.
> 
> This means, without WF_SYNC, the previous CPU being cache affined to the
> waker and is idle is not considered. This seems extreme. WF_SYNC is not
> normally passed to the wakeup path outside of some IPC drivers but it is
> very possible that the task is cache hot on previous CPU and shares
> cache with the waker CPU. Lets avoid too many migrations and select the
> previous CPU in such cases.
> 
> This change is consistent with the fair scheduler's behavior as well. In
> select_idle_sibling(), the previous CPU is selected if it is cache
> affined with the target. This is done regardless of WF_SYNC and before
> any scanning of fully idle cores is done.
> 
> One difference still exists though between SCX and CFS in this regard, in CFS
> we first check if the target CPU is idle before checking if the previous CPU is
> idle. However that could be a matter of choice and in the future, that behavior
> could also be unified.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
>  kernel/sched/ext.c | 24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 5a81d9a1e31f..3b1a489e2aaf 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3479,7 +3479,7 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
>  {
>  	const struct cpumask *llc_cpus = NULL;
>  	const struct cpumask *numa_cpus = NULL;
> -	s32 cpu;
> +	s32 cpu = smp_processor_id();
>  
>  	*found = false;
>  
> @@ -3507,22 +3507,20 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
>  			llc_cpus = llc_span(prev_cpu);
>  	}
>  
> +	/*
> +	 * If the waker's CPU is cache affine and prev_cpu is idle, then avoid
> +	 * a migration.
> +	 */
> +	if (cpus_share_cache(cpu, prev_cpu) &&
> +		test_and_clear_cpu_idle(prev_cpu)) {
> +		cpu = prev_cpu;
> +		goto cpu_found;
> +	}
> +

One potential issue that I see is that when SMT is enabled, you may end up
using prev_cpu also when the other sibling is busy. Maybe we should check
if prev_cpu is set in the SMT idle cpumask.

Also, last time I tried a similar change I was regressing a lot of
benchmarks. Maybe we should repeat the tests and get some numbers.

>  	/*
>  	 * If WAKE_SYNC, try to migrate the wakee to the waker's CPU.
>  	 */
>  	if (wake_flags & SCX_WAKE_SYNC) {
> -		cpu = smp_processor_id();
> -
> -		/*
> -		 * If the waker's CPU is cache affine and prev_cpu is idle,
> -		 * then avoid a migration.
> -		 */
> -		if (cpus_share_cache(cpu, prev_cpu) &&
> -		    test_and_clear_cpu_idle(prev_cpu)) {
> -			cpu = prev_cpu;
> -			goto cpu_found;
> -		}
> -
>  		/*
>  		 * If the waker's local DSQ is empty, and the system is under
>  		 * utilized, try to wake up @p to the local DSQ of the waker.
> -- 
> 2.43.0
> 

Thanks,
-Andrea