Re: [PATCH v2 17/23] sched/cache: Record the number of active threads per process for cache-aware scheduling

[Chen, Yu C]

On 12/17/2025 5:40 PM, Aaron Lu wrote:
> On Wed, Dec 03, 2025 at 03:07:36PM -0800, Tim Chen wrote:
> > @@ -1501,6 +1507,7 @@ static void __no_profile task_cache_work(struct callback_head *work)
> >   		mm->mm_sched_cpu = m_a_cpu;
> >   	}
> >   
> > +	update_avg(&mm->nr_running_avg, nr_running);
>
> update_avg() doesn't appear to deal with small numbers well and can have
> an error as large as 7, e.g. when nr_running < 8, nr_running_avg will
> always be 0 and when nr_running >= 8 && < 16, nr_running_avg will be
> 1 - 8, etc.
>
> AMD Genoa has 8 cores per LLC and this will break exceed_llc_nr() there.

Ah, you are right, thanks for pointing this out, dividing by 8 would make
convergence slow for small LLC system. Maybe consider the number of Cores
in the LLC, the smaller the number is, the more we should honor the diff
between two invoking of update_avg()?

static inline void sched_cache_update_avg(u64 *avg, u64 sample)
{
	s64 diff = sample - *avg;
	u32 divisor = clamp_t(u32, nr_cores_llc/4, 2, 8);

	*avg += diff / divisor;
}

For <=8 cores per LLC, the divisor is 2,
for 16 cores per LLC, the divisor is 4,
for >=32 cores per LLC, the divisor is 8

Thanks,
Chenyu

> >   	free_cpumask_var(cpus);
> >   }