Re: [PATCH v4 14/20] sched/core: Introduce a simple steal monitor

[Yury Norov <yury.norov@gmail.com>]

On Wed, Jun 17, 2026 at 11:11:33PM +0530, Shrikanth Hegde wrote:
> Start with a simple steal monitor.
> 
> It is meant to look at steal time and make the decision to
> reduce/increase the preferred CPUs.
> 
> It has
> - work function to execute the steal time calculations and decision
>   making periodically.
> - low and high thresholds for steal time.
> - sampling period to control the frequency of steal time calculations.
> - cache the previous decision to avoid oscillations

This monitor is the one implementation out of quite many possible,
right? I don't think it should live in the core scheduler files, it
should be a module.
 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
> v3->v4:
> - Drop tmp_mask
> 
>  include/linux/sched.h | 11 +++++++++++
>  kernel/sched/core.c   | 23 +++++++++++++++++++++++
>  kernel/sched/sched.h  |  3 +++
>  3 files changed, 37 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 5f523782ca28..ce6bc8a22eb1 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2517,4 +2517,15 @@ extern void migrate_enable(void);
>  
>  DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
>  
> +#ifdef CONFIG_PREFERRED_CPU
> +struct steal_monitor_t {
> +	struct work_struct  work;
> +	ktime_t prev_time;
> +	u64 prev_steal;
> +	int previous_decision;
> +	unsigned int low_threshold;
> +	unsigned int high_threshold;
> +	unsigned int sampling_period_ms;
> +};
> +#endif
>  #endif
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 24d4abc74241..cc48632dd42d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9138,6 +9138,8 @@ void __init sched_init(void)
>  
>  	preempt_dynamic_init();
>  
> +	sched_init_steal_monitor();
> +
>  	scheduler_running = 1;
>  }
>  
> @@ -11384,4 +11386,25 @@ void sched_push_current_non_preferred_cpu(struct rq *rq)
>  	stop_one_cpu_nowait(rq->cpu, sched_non_preferred_cpu_push_stop,
>  			    push_task, this_cpu_ptr(&npc_push_task_work));
>  }
> +
> +struct steal_monitor_t steal_mon;
> +
> +void sched_init_steal_monitor(void)
> +{
> +	INIT_WORK(&steal_mon.work, sched_steal_detection_work);
> +	steal_mon.low_threshold       = 200;		/* 2% steal time */
> +	steal_mon.high_threshold      = 500;		/* 5% steal time */
> +	steal_mon.sampling_period_ms  = 1000;		/* once per second */
> +}
> +
> +/* This is only a skeleton. Subsequent patches introduce more of it */
> +void sched_steal_detection_work(struct work_struct *work)
> +{
> +	struct steal_monitor_t *sm = container_of(work, struct steal_monitor_t, work);
> +	ktime_t now;
> +
> +	/* Update the prev_time for next iteration*/
> +	now = ktime_get();
> +	sm->prev_time = now;
> +}
>  #endif
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 9cb006c21090..984da3827f19 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -4240,8 +4240,11 @@ static inline bool task_has_preferred_cpus(struct task_struct *p)
>  #ifdef CONFIG_PREFERRED_CPU
>  DECLARE_STATIC_KEY_FALSE(__sched_sm_enable);
>  
> +void sched_init_steal_monitor(void);
> +void sched_steal_detection_work(struct work_struct *work);
>  void sched_push_current_non_preferred_cpu(struct rq *rq);
>  #else	/* !CONFIG_PREFERRED_CPU */
>  static inline void sched_push_current_non_preferred_cpu(struct rq *rq) { }
> +static inline void sched_init_steal_monitor(void) { }
>  #endif
>  #endif /* _KERNEL_SCHED_SCHED_H */
> -- 
> 2.47.3