Re: [PATCH 19/19] sched/fair: Add user control to adjust the tolerance of cache-aware scheduling

[Chen, Yu C]

On 10/29/2025 4:07 PM, Aaron Lu wrote:
> On Sat, Oct 11, 2025 at 11:24:56AM -0700, Tim Chen wrote:
> ... ...
> > +static inline int get_sched_cache_cap_scale(void)
> > +{
> > +	return (llc_overload_pct / cpu_smt_num_threads);
> > +}
> > +
> ... ...
> > @@ -9749,7 +9811,7 @@ static inline int task_is_ineligible_on_dst_cpu(struct task_struct *p, int dest_
> >    * (default: ~50%)
> >    */
> >   #define fits_llc_capacity(util, max)	\
> > -	((util) * 100 < (max) * llc_overload_pct)
> > +	((util) * 100 < (max) * get_sched_cache_cap_scale())
>
> With this change, fits_llc_capacity() would be false if util is just 1/4
> of max(which is llc's total cpu capacity), is this intended?

Yes, it was changed to this because we want to avoid performance 
regressions
in some systems with a large number of SMTs per core-aggressive task 
aggregation
 is harmful to those systems. However, upon further thought, since we 
have a
user-space knob to control how aggressively users want to enable task 
aggregation,
we can try removing cpu_smt_num_threads and let users decide. I'll do some
tests to check the impact.

thanks,
Chenyu