Re: [RFC PATCH v4 05/28] sched: Add hysteresis to switch a task's preferred LLC

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Aug 09, 2025 at 01:02:18PM +0800, Chen Yu wrote:
> From: Tim Chen <tim.c.chen@linux.intel.com>
> 
> Switching a process's preferred LLC generates lots of task
> migrations across LLCs. To avoid frequent switches
> of home LLC, implement the following policy:
> 
> 1. Require a 2x occ change threshold to switch preferred LLC
> 2. Don't discard preferred LLC for a task
> 
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>  kernel/sched/fair.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a7be5c5ecba3..9e3c6f0eb934 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1175,6 +1175,14 @@ static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
>  #define EPOCH_PERIOD	(HZ/100)	/* 10 ms */
>  #define EPOCH_OLD	5		/* 50 ms */
>  
> +static int llc_id(int cpu)
> +{
> +	if (cpu < 0)
> +		return -1;
> +
> +	return per_cpu(sd_llc_id, cpu);
> +}
> +
>  void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu *_pcpu_sched)
>  {
>  	unsigned long epoch;
> @@ -1307,6 +1315,7 @@ static void __no_profile task_cache_work(struct callback_head *work)
>  	struct task_struct *p = current;
>  	struct mm_struct *mm = p->mm;
>  	unsigned long m_a_occ = 0;
> +	unsigned long last_m_a_occ = 0;
>  	int cpu, m_a_cpu = -1;
>  	cpumask_var_t cpus;
>  
> @@ -1345,11 +1354,13 @@ static void __no_profile task_cache_work(struct callback_head *work)
>  					     per_cpu(sd_llc_id, i), occ, m_occ, m_cpu, nr);
>  			}
>  
> -			a_occ /= nr;
> +			// a_occ /= nr;

This seems broken.

>  			if (a_occ > m_a_occ) {
>  				m_a_occ = a_occ;
>  				m_a_cpu = m_cpu;
>  			}
> +			if (llc_id(cpu) == llc_id(mm->mm_sched_cpu))
> +				last_m_a_occ = a_occ;

Not 'last', 'curr' perhaps?

>  
>  			trace_printk("(%d) a_occ: %ld m_a_occ: %ld\n",
>  				     per_cpu(sd_llc_id, cpu), a_occ, m_a_occ);
> @@ -1363,13 +1374,10 @@ static void __no_profile task_cache_work(struct callback_head *work)
>  		}
>  	}
>  
> -	/*
> -	 * If the max average cache occupancy is 'small' we don't care.
> -	 */
> -	if (m_a_occ < (NICE_0_LOAD >> EPOCH_OLD))
> -		m_a_cpu = -1;
> -
> -	mm->mm_sched_cpu = m_a_cpu;
> +	if (m_a_occ > (2 * last_m_a_occ)) {
> +		/* avoid the bouncing of mm_sched_cpu */
> +		mm->mm_sched_cpu = m_a_cpu;
> +	}


The whole double thing doesn't seem right either. That means that
anything over .5 will never be able to change, even when confronted with
say a .8.

Also, while this is a threshold, this is not in fact hysteresis.