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

[Joseph Salisbury]

On 3/25/26 7:24 PM, Andrea Righi wrote:
> 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://urldefense.com/v3/__https://lore.kernel.org/all/20260321105503.869337-1-yphbchou0911@gmail.com__;!!ACWV5N9M2RV99hQ!M88Tr7SYG3FHedTvBFVj_5kAPCVI-WVHWNdHhvaysPHoUD3qbj9bZ8kNj8TwcjHXNvVTnhx0BvGh7pM5R6fC$
>
> 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 for the pointer!  I'll review this branch prior to sending/fixing 
any more bugs.
> 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