[PATCH 0/2] sched: small changes

[PATCH 0/2] sched: small changes

[Yafang Shao]

These improments are found in the implementation of BPF-NUMAB series[0].
They can be applied seperately.

[0]. https://lwn.net/Articles/1101442/

Yafang Shao (2):
  sched: fix stale get_pref_llc() stub for !CONFIG_SCHED_CACHE
  sched: use static_branch_likely() for the NUMA balancing tick check

 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

-- 
2.52.0

[PATCH 1/2] sched: fix stale get_pref_llc() stub for !CONFIG_SCHED_CACHE

[Yafang Shao]

Commit b636fef85bda ("sched/cache: Introduce task_struct->sched_cache_grp
to fix UAF") switched get_pref_llc() from taking a struct mm_struct * to
taking the task's struct sched_cache_group *, but only updated the
CONFIG_SCHED_CACHE definition and its caller: the stub in the
!CONFIG_SCHED_CACHE branch kept the old mm_struct parameter.

Fixes: b636fef85bda ("sched/cache: Introduce task_struct->sched_cache_grp to fix UAF")
Cc: <stable@vger.kernel.org> #v7.2.x
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 57360f5cdde4..973647ad6c2d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2194,7 +2194,7 @@ void init_sched_mm(struct task_struct *p) { }
 static void task_tick_cache(struct rq *rq, struct task_struct *p) { }
 
 static inline int get_pref_llc(struct task_struct *p,
-			       struct mm_struct *mm)
+			       struct sched_cache_group *grp)
 {
 	return -1;
 }
-- 
2.52.0

[PATCH 2/2] sched: use static_branch_likely() for the NUMA balancing tick check

[Yafang Shao]

task_tick_fair() checks sched_numa_balancing on every tick. The
static_branch_unlikely() annotation reserves the nop for the disabled case,
so the default configuration (NUMA_BALANCING_DEFAULT_ENABLED=y) pays a
taken jump per tick. Flip the polarity to static_branch_likely() so that
the fast path matches the default.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Gregory Price (Meta) <gourry@gourry.net>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 973647ad6c2d..71dc47d6114d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -15351,7 +15351,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 	if (queued)
 		return;
 
-	if (static_branch_unlikely(&sched_numa_balancing))
+	if (static_branch_likely(&sched_numa_balancing))
 		task_tick_numa(rq, curr);
 
 	task_tick_cache(rq, curr);
-- 
2.52.0

Re: [PATCH 2/2] sched: use static_branch_likely() for the NUMA balancing tick check

[Vincent Guittot]

On Fri, 23 Oct 2026 at 04:31, Yafang Shao <laoar.shao@gmail.com> wrote:
>
> task_tick_fair() checks sched_numa_balancing on every tick. The
> static_branch_unlikely() annotation reserves the nop for the disabled case,
> so the default configuration (NUMA_BALANCING_DEFAULT_ENABLED=y) pays a
> taken jump per tick. Flip the polarity to static_branch_likely() so that
> the fast path matches the default.
>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Acked-by: Gregory Price (Meta) <gourry@gourry.net>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 973647ad6c2d..71dc47d6114d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -15351,7 +15351,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
>         if (queued)
>                 return;
>
> -       if (static_branch_unlikely(&sched_numa_balancing))
> +       if (static_branch_likely(&sched_numa_balancing))

static_branch_unlikely is used by default for other static keys too
like  sched_cluster_active, sched_cache_active, __sched_core_enabled
sched_asym_cpucapacity and others ...
So systems which don't enable them don't pay a jump when not using it.
I don't see why numa should be different

>                 task_tick_numa(rq, curr);
>
>         task_tick_cache(rq, curr);
> --
> 2.52.0
>