Re: [PATCH v1] sched/uclamp: Exclude kernel threads from uclamp logic

[Xuewen Yan]

Hi zihuan,

On Thu, Jul 3, 2025 at 5:15 PM Zihuan Zhang <zhangzihuan@xxxxxxxxxx> wrote:
>
> Kernel threads (PF_KTHREAD) are not subject to user-defined utilization
> clamping. They do not represent user workloads and should not participate
> in any uclamp logic, including:

Indeed, some driver would use set_scheduler() to set some kthread to
improve performance.
It is not a good idea to exclude it.

Thanks！

>
>  - clamp initialization during fork/post-fork
>  - effective clamp value computation
>  - runtime aggregation (uclamp_rq_inc/dec)
>
> Allowing kernel threads into these paths may pollute the rq->uclamp[]
> statistics, mislead schedutil governor's frequency selection, and
> complicate debugging or trace interpretation.
>
> This patch ensures that:
>  - uclamp_fork() and uclamp_post_fork() skip kernel threads
>  - uclamp_eff_value() return default values
>  - uclamp_rq_inc() and uclamp_rq_dec() skip kernel threads
>
> This aligns the semantics of uclamp with its original intent:
> user-space task-specific clamping.
>
> dmesg in uclamp_rq_inc_id:
> [   76.373903] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:0 value:0 kthread:1
> [   76.375905] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:1 value:1024 kthread:1
> [   76.379837] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:0 value:0 kthread:1
> [   76.379839] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:1 value:1024 kthread:1
> [   76.379839] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:0 value:0 kthread:1
> [   76.379841] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:1 value:1024 kthread:1
> [   76.383897] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:0 value:0 kthread:1
> [   76.383897] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:0 value:0 kthread:1
> [   76.383900] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:1 value:1024 kthread:1
> [   76.383901] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:1 value:1024 kthread:1
> [   76.387885] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:0 value:0 kthread:1
> [   76.387885] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:0 value:0 kthread:1
> [   76.387888] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:1 value:1024 kthread:1
> [   76.387889] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:1 value:1024 kthread:1
> [   76.388139] uclamp_rq_inc_id: task:jbd2/sda3-8 pid:316 clamp_id:0 value:0 kthread:1
> [   76.388140] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:0 value:0 kthread:1
> [   76.388142] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:1 value:1024 kthread:1
> [   76.388143] uclamp_rq_inc_id: task:jbd2/sda3-8 pid:316 clamp_id:1 value:1024 kthread:1
> [   76.388169] uclamp_rq_inc_id: task:kworker/u48:6 pid:93 clamp_id:0 value:0 kthread:1
> [   76.388171] uclamp_rq_inc_id: task:kworker/u48:6 pid:93 clamp_id:1 value:1024 kthread:1
> [   76.388891] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:0 value:0 kthread:1
> [   76.388893] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:1 value:1024 kthread:1
> [   76.392900] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:0 value:0 kthread:1
> [   76.392902] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:1 value:1024 kthread:1
> [   76.398850] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:0 value:0 kthread:1
> [   76.398852] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:1 value:1024 kthread:1
> [   76.401880] uclamp_rq_inc_id: task:ksoftirqd/8 pid:67 clamp_id:0 value:0 kthread:1
> [   76.401883] uclamp_rq_inc_id: task:ksoftirqd/8 pid:67 clamp_id:1 value:1024 kthread:1
> [   76.409053] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:0 value:0 kthread:1
> [   76.409054] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:1 value:1024 kthread:1
> [   76.410881] uclamp_rq_inc_id: task:kworker/u48:10 pid:97 clamp_id:0 value:0 kthread:1
> [   76.410884] uclamp_rq_inc_id: task:kworker/u48:10 pid:97 clamp_id:1 value:1024 kthread:1
> [   76.419947] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:0 value:0 kthread:1
> [   76.419949] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:1 value:1024 kthread:1
> [   76.419976] uclamp_rq_inc_id: task:kworker/u48:6 pid:93 clamp_id:0 value:0 kthread:1
> [   76.419979] uclamp_rq_inc_id: task:kworker/u48:6 pid:93 clamp_id:1 value:1024 kthread:1
> [   76.420119] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:0 value:0 kthread:1
> [   76.420121] uclamp_rq_inc_id: task:kworker/2:1H pid:188 clamp_id:1 value:1024 kthread:1
> [   76.420642] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:0 value:0 kthread:1
> [   76.420644] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:1 value:1024 kthread:1
> [   76.434914] uclamp_rq_inc_id: task:kcompactd0 pid:108 clamp_id:0 value:0 kthread:1
> [   76.434916] uclamp_rq_inc_id: task:kcompactd0 pid:108 clamp_id:1 value:1024 kthread:1
> [   76.447689] uclamp_rq_inc_id: task:kworker/3:2 pid:244 clamp_id:0 value:0 kthread:1
> [   76.447691] uclamp_rq_inc_id: task:kworker/3:2 pid:244 clamp_id:1 value:1024 kthread:1
> [   76.447705] uclamp_rq_inc_id: task:ksoftirqd/3 pid:37 clamp_id:0 value:0 kthread:1
> [   76.447707] uclamp_rq_inc_id: task:ksoftirqd/3 pid:37 clamp_id:1 value:1024 kthread:1
> [   76.448809] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:0 value:0 kthread:1
> [   76.448811] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:1 value:1024 kthread:1
> [   76.451260] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:0 value:0 kthread:1
> [   76.451263] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:1 value:1024 kthread:1
> [   76.452806] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:0 value:0 kthread:1
> [   76.452808] uclamp_rq_inc_id: task:rcu_preempt pid:16 clamp_id:1 value:1024 kthread:1
> [   76.488052] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:0 value:0 kthread:1
> [   76.488054] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:1 value:1024 kthread:1
> [   76.488767] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:0 value:0 kthread:1
> [   76.488770] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:1 value:1024 kthread:1
> [   76.490847] uclamp_rq_inc_id: task:kworker/3:2 pid:244 clamp_id:0 value:0 kthread:1
> [   76.490848] uclamp_rq_inc_id: task:kworker/2:1 pid:143 clamp_id:0 value:0 kthread:1
> [   76.490849] uclamp_rq_inc_id: task:kworker/1:3 pid:462 clamp_id:0 value:0 kthread:1
> [   76.490848] uclamp_rq_inc_id: task:kworker/7:2 pid:687 clamp_id:0 value:0 kthread:1
> [   76.490849] uclamp_rq_inc_id: task:kworker/11:1 pid:146 clamp_id:0 value:0 kthread:1
> [   76.490850] uclamp_rq_inc_id: task:kworker/2:1 pid:143 clamp_id:1 value:1024 kthread:1
> [   76.490851] uclamp_rq_inc_id: task:kworker/3:2 pid:244 clamp_id:1 value:1024 kthread:1
> [   76.490851] uclamp_rq_inc_id: task:kworker/11:1 pid:146 clamp_id:1 value:1024 kthread:1
> [   76.490851] uclamp_rq_inc_id: task:kworker/7:2 pid:687 clamp_id:1 value:1024 kthread:1
> [   76.490853] uclamp_rq_inc_id: task:kworker/1:3 pid:462 clamp_id:1 value:1024 kthread:1
> [   76.490857] uclamp_rq_inc_id: task:kworker/5:1 pid:141 clamp_id:0 value:0 kthread:1
> [   76.490859] uclamp_rq_inc_id: task:kworker/5:1 pid:141 clamp_id:1 value:1024 kthread:1
> [   76.491850] uclamp_rq_inc_id: task:kworker/4:2 pid:534 clamp_id:0 value:0 kthread:1
> [   76.491852] uclamp_rq_inc_id: task:kworker/4:2 pid:534 clamp_id:1 value:1024 kthread:1
> [   76.504848] uclamp_rq_inc_id: task:kworker/10:2 pid:228 clamp_id:0 value:0 kthread:1
> [   76.504852] uclamp_rq_inc_id: task:kworker/10:2 pid:228 clamp_id:1 value:1024 kthread:1
> [   76.508785] uclamp_rq_inc_id: task:kworker/9:1 pid:142 clamp_id:0 value:0 kthread:1
> [   76.508787] uclamp_rq_inc_id: task:kworker/9:1 pid:142 clamp_id:1 value:1024 kthread:1
> [   76.514856] uclamp_rq_inc_id: task:kworker/u48:10 pid:97 clamp_id:0 value:0 kthread:1
> [   76.514859] uclamp_rq_inc_id: task:kworker/u48:10 pid:97 clamp_id:1 value:1024 kthread:1
> [   76.522742] uclamp_rq_inc_id: task:kworker/1:1H pid:153 clamp_id:0 value:0 kthread:1
>
> Signed-off-by: Zihuan Zhang <zhangzihuan@xxxxxxxxxx>
> ---
>  kernel/sched/core.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8988d38d46a3..a1e6b4157682 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1630,6 +1630,9 @@ unsigned long uclamp_eff_value(struct task_struct *p, enum uclamp_id clamp_id)
>  {
>         struct uclamp_se uc_eff;
>
> +       if (p->flags & PF_KTHREAD)
> +               return uclamp_none(clamp_id);
> +
>         /* Task currently refcounted: use back-annotated (effective) value */
>         if (p->uclamp[clamp_id].active)
>                 return (unsigned long)p->uclamp[clamp_id].value;
> @@ -1769,6 +1772,9 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p, int flags
>         if (unlikely(!p->sched_class->uclamp_enabled))
>                 return;
>
> +       if (p->flags & PF_KTHREAD)
> +               return;
> +
>         /* Only inc the delayed task which being woken up. */
>         if (p->se.sched_delayed && !(flags & ENQUEUE_DELAYED))
>                 return;
> @@ -1797,6 +1803,9 @@ static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
>         if (unlikely(!p->sched_class->uclamp_enabled))
>                 return;
>
> +       if (p->flags & PF_KTHREAD)
> +               return;
> +
>         if (p->se.sched_delayed)
>                 return;
>
> @@ -1977,6 +1986,8 @@ static void uclamp_fork(struct task_struct *p)
>  {
>         enum uclamp_id clamp_id;
>
> +       if (p->flags & PF_KTHREAD)
> +               return;
>         /*
>          * We don't need to hold task_rq_lock() when updating p->uclamp_* here
>          * as the task is still at its early fork stages.
> @@ -1995,6 +2006,8 @@ static void uclamp_fork(struct task_struct *p)
>
>  static void uclamp_post_fork(struct task_struct *p)
>  {
> +       if (p->flags & PF_KTHREAD)
> +               return;
>         uclamp_update_util_min_rt_default(p);
>  }
>
> --
> 2.25.1
>
>