Re: [PATCH v2 04/13] sched/fair: Remove magic hardcoded margin in fits_capacity()

[Chen, Yu C]

On 9/23/2026 2:54 PM, Zhan Xusheng wrote:
> On 05/04/26 02:59, Qais Yousef wrote:
> > -#define fits_capacity(cap, max)	((cap) * 1280 < (max) * 1024)
> > +static inline bool fits_capacity(unsigned long util, int cpu)
> > +{
> > +	return util < cpu_rq(cpu)->fits_capacity_threshold;
> > +}
>
> This does not hold on current tip anymore: fits_capacity() has picked up
> a second caller that does not pass a capacity, and the new signature
> accepts it silently.
>
> kernel/sched/fair.c, invalid_llc_nr():
>
> 	return !fits_capacity((READ_ONCE(grp->nr_running_avg) * cpu_smt_num_threads),
> 			(scale * per_cpu(sd_llc_size, cpu)));
>
> The second argument is a scaled count of the CPUs in an LLC. sd_llc_size
> is a per-CPU int and scale is an int, so the product binds to the new @cpu
> parameter with no conversion and no warning, and the test becomes
>
> 	cpu_rq(scale * per_cpu(sd_llc_size, cpu))->fits_capacity_threshold
>
> With the defaults (CONFIG_SCHED_CACHE=y, sysctl_sched_cache_user=1,
> llc_aggr_tolerance=1) scale is 1, so this is cpu_rq(sd_llc_size), which is
> not a valid CPU id on a machine with a single LLC. invalid_llc_nr() is
> called from account_mm_sched(), task_cache_work() and
> can_migrate_llc_task().
>
> Separating the two uses first makes 04/13 safe to apply. Something like
> the below, against tip/sched/urgent (3cb0243767fd, where the cache-aware
> fixes have just landed; sched/core still has the older mm->sc_stat form).
> It is a no functional change, since x * 1280 < y * 1024 is equivalent to
> x * 100 < y * 80. Built with CONFIG_SCHED_CACHE=y and =n, no new warnings.

How about x * 5 < y * 4 ?  The y * 4 is a left-shift which could be faster.

Thanks,
Chenyu
> I can post it as a standalone patch if you would rather not carry it in
> the series.
>
> ---
>   kernel/sched/fair.c | 19 +++++++++++++++++--
>   1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 57360f5cdde4..97ce56fcaf85 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1526,6 +1526,21 @@ static bool exceed_llc_capacity(struct sched_cache_group *grp, int cpu)
>   	return false;
>   }
>   
> +/*
> + * The margin used when comparing the number of a process' active threads
> + * with the number of CPUs in an LLC.
> + *
> + * Mirrors the ~20% margin of fits_capacity(), but is kept separate because
> + * fits_capacity() describes a utilization versus CPU capacity relation,
> + * which this is not.
> + *
> + * (default: ~80% of the LLC's CPUs)
> + */
> +static inline bool fits_llc_nr(u64 nr_threads, unsigned int nr_cpus)
> +{
> +	return nr_threads * 100 < (u64)nr_cpus * 80;
> +}
> +
>   static bool invalid_llc_nr(struct sched_cache_group *grp, struct task_struct *p,
>   			   int cpu)
>   {
> @@ -1542,8 +1557,8 @@ static bool invalid_llc_nr(struct sched_cache_group *grp, struct task_struct *p,
>   	if (scale == INT_MAX)
>   		return false;
>   
> -	return !fits_capacity((READ_ONCE(grp->nr_running_avg) * cpu_smt_num_threads),
> -			(scale * per_cpu(sd_llc_size, cpu)));
> +	return !fits_llc_nr(READ_ONCE(grp->nr_running_avg) * cpu_smt_num_threads,
> +			    scale * per_cpu(sd_llc_size, cpu));
>   }
>   
>   /*