linux-kernel - Re: [PATCH v2 05/12] sched: Add ttwu

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Message-ID: <CAKfTPtCKJi_H1MYhfiaAkeNnAyP8p6pTaJurQ5ZM7Q-mUi3wZA@mail.gmail.com>
Date: Thu, 10 Jul 2025 18:51:34 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/12] sched: Add ttwu_queue controls

On Wed, 2 Jul 2025 at 14:12, Peter Zijlstra <peterz@...radead.org> wrote:
>
> There are two (soon three) callers of ttwu_queue_wakelist(),
> distinguish them with their own WF_ and add some knobs on.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20250520101727.874587738@infradead.org
> ---
>  kernel/sched/core.c     |   22 ++++++++++++----------
>  kernel/sched/features.h |    2 ++
>  kernel/sched/sched.h    |    2 ++
>  3 files changed, 16 insertions(+), 10 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3888,7 +3888,7 @@ bool cpus_share_resources(int this_cpu,
>         return per_cpu(sd_share_id, this_cpu) == per_cpu(sd_share_id, that_cpu);
>  }
>
> -static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
> +static inline bool ttwu_queue_cond(struct task_struct *p, int cpu, bool def)
>  {
>         /* See SCX_OPS_ALLOW_QUEUED_WAKEUP. */
>         if (!scx_allow_ttwu_queue(p))
> @@ -3929,18 +3929,19 @@ static inline bool ttwu_queue_cond(struc
>         if (!cpu_rq(cpu)->nr_running)
>                 return true;
>
> -       return false;
> +       return def;
>  }
>
>  static bool ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags)
>  {
> -       if (sched_feat(TTWU_QUEUE) && ttwu_queue_cond(p, cpu)) {
> -               sched_clock_cpu(cpu); /* Sync clocks across CPUs */
> -               __ttwu_queue_wakelist(p, cpu, wake_flags);
> -               return true;
> -       }
> +       bool def = sched_feat(TTWU_QUEUE_DEFAULT);

I'm always confused by this sched feature name because
sched_feat(TTWU_QUEUE_DEFAULT) must be false in order to have the
current (default ?) behaviour
Or you mean queue by default in the wakelist which is disable to keep
current behavior

> +
> +       if (!ttwu_queue_cond(p, cpu, def))
> +               return false;
>
> -       return false;
> +       sched_clock_cpu(cpu); /* Sync clocks across CPUs */
> +       __ttwu_queue_wakelist(p, cpu, wake_flags);
> +       return true;
>  }
>
>  static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
> @@ -3948,7 +3949,7 @@ static void ttwu_queue(struct task_struc
>         struct rq *rq = cpu_rq(cpu);
>         struct rq_flags rf;
>
> -       if (ttwu_queue_wakelist(p, cpu, wake_flags))
> +       if (sched_feat(TTWU_QUEUE) && ttwu_queue_wakelist(p, cpu, wake_flags))
>                 return;
>
>         rq_lock(rq, &rf);
> @@ -4251,7 +4252,8 @@ int try_to_wake_up(struct task_struct *p
>                  * scheduling.
>                  */
>                 if (smp_load_acquire(&p->on_cpu) &&
> -                   ttwu_queue_wakelist(p, task_cpu(p), wake_flags))
> +                   sched_feat(TTWU_QUEUE_ON_CPU) &&
> +                   ttwu_queue_wakelist(p, task_cpu(p), wake_flags | WF_ON_CPU))
>                         break;
>
>                 cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -81,6 +81,8 @@ SCHED_FEAT(TTWU_QUEUE, false)
>   */
>  SCHED_FEAT(TTWU_QUEUE, true)
>  #endif
> +SCHED_FEAT(TTWU_QUEUE_ON_CPU, true)
> +SCHED_FEAT(TTWU_QUEUE_DEFAULT, false)
>
>  /*
>   * When doing wakeups, attempt to limit superfluous scans of the LLC domain.
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2279,6 +2279,8 @@ static inline int task_on_rq_migrating(s
>  #define WF_CURRENT_CPU         0x40 /* Prefer to move the wakee to the current CPU. */
>  #define WF_RQ_SELECTED         0x80 /* ->select_task_rq() was called */
>
> +#define WF_ON_CPU              0x0100
> +
>  static_assert(WF_EXEC == SD_BALANCE_EXEC);
>  static_assert(WF_FORK == SD_BALANCE_FORK);
>  static_assert(WF_TTWU == SD_BALANCE_WAKE);
>
>