Re: [PATCH] sched_ext: idle: use active idle domain in WAKE_SYNC check

[Andrea Righi]

Hi Joe,

On Tue, Mar 24, 2026 at 03:58:36PM -0400, Joseph Salisbury wrote:
> The WAKE_SYNC fast path checks whether there are idle CPUs before
> biasing wakeups to the waker CPU.  After per-node idle masks were
> introduced, this path started indexing idle_cpumask() with
> cpu_to_node().
> 
> When per-node idle tracking is disabled, per-node masks are not kept in
> sync and may be stale.  Reading them here can mis-detect system
> utilization and lead to incorrect WAKE_SYNC wakeup placement.
> 
> Use scx_cpu_node_if_enabled() for the waker node lookup so this check
> uses the global idle mask when per-node tracking is disabled and the
> per-node mask only when that mode is enabled.
> 
> Fixes: 48849271e661 ("sched_ext: idle: Per-node idle cpumasks")
> Cc: stable@xxxxxxxxxxxxxxx # v6.15+
> Assisted-by: Codex:GPT-5
> Signed-off-by: Joseph Salisbury <joseph.salisbury@xxxxxxxxxx>

This is good, but it's already fixed:
https://lore.kernel.org/all/20260321105503.869337-1-yphbchou0911@xxxxxxxxx

You can probably use Tejun's branch to search for new issues/bugs:

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git

Thanks,
-Andrea

> ---
>  kernel/sched/ext_idle.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index 8cfe0e9a7319..6b4520ed563d 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -543,7 +543,7 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
>  		 * piled up on it even if there is an idle core elsewhere on
>  		 * the system.
>  		 */
> -		waker_node = cpu_to_node(cpu);
> +		waker_node = scx_cpu_node_if_enabled(cpu);
>  		if (!(current->flags & PF_EXITING) &&
>  		    cpu_rq(cpu)->scx.local_dsq.nr == 0 &&
>  		    (!(flags & SCX_PICK_IDLE_IN_NODE) || (waker_node == node)) &&
> -- 
> 2.47.3