Re: [PATCH] sched/cache: Don't use fits_capacity() to compare thread and CPU counts

[Tim Chen]

On Mon, 2026-11-02 at 10:15 +0800, Zhan Xusheng wrote:
> From: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> 
> invalid_llc_nr() uses fits_capacity() to compare a thread count with a
> CPU count, only for its 20% margin.  Qais's series changes
> fits_capacity() to take a CPU number as its second argument, and this
> call would then pass the CPU count as a CPU number without a warning.
> 
> Give the check its own helper.  x * 5 < y * 4 is the same comparison as
> x * 1280 < y * 1024, so there is no functional change.
> 
> Link: https://lore.kernel.org/all/20260504020003.71306-5-qyousef@xxxxxxxxxxx/
> Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> ---
> This keeps the 20% margin.  Qais would rather not add hardcoded
> margins, and llc_aggr_tolerance already scales the LLC size here, so
> the margin could go, but that changes when a process stops being
> aggregated.  Tim, Chen Yu, would you drop it?


Xusheng,

We tune the system based on a default llc_aggr_tolerance of 1,
where the number of running threads is 80% of the number of cores,
to prevent over-subscription which could lead to regressions.

This 80% factor is leaning on the conservative side to prevent
regressions for workloads where cache aware scheduling may
not help on multi-threaded process with little shared data.

We set the llc_aggr_tolerance as a default of 1 for more intuitive
tuning.  It varies from 

0 (no aggregation), 
1 (default with conservative aggregation)
100 (always aggregate)

Hope that makes sense.

Otherwise, the patch looks good to me.

Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

Tim

> 
>  kernel/sched/fair.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cfc982205b98..dfd6528d18ee 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1469,6 +1469,12 @@ static bool exceed_llc_capacity(struct sched_cache_group *grp, int cpu)
>  	return false;
>  }
>  
> +/* Are the running threads (in SMT siblings) below ~80% of the LLC's CPUs? */
> +static inline bool fits_llc_nr(u64 nr_threads, unsigned int nr_cpus)
> +{
> +	return nr_threads * 5 < (u64)nr_cpus * 4;
> +}
> +
>  static bool invalid_llc_nr(struct sched_cache_group *grp, struct task_struct *p,
>  			   int cpu)
>  {
> @@ -1485,8 +1491,8 @@ 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));
>  }
>  
>  /*
> 
> base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8