Re: [PATCH v3 07/21] sched/cache: Introduce per CPU's tasks LLC preference counter

["Chen, Yu C" <yu.c.chen@intel.com>]

On 2/20/2026 6:45 PM, Peter Zijlstra wrote:
> On Tue, Feb 10, 2026 at 02:18:47PM -0800, Tim Chen wrote:
>> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
>> index a4e2fb31f2fd..3aa6c101b2e4 100644
>> --- a/include/linux/sched/topology.h
>> +++ b/include/linux/sched/topology.h
>> @@ -102,6 +102,10 @@ struct sched_domain {
>>   	u64 max_newidle_lb_cost;
>>   	unsigned long last_decay_max_lb_cost;
>>   
>> +#ifdef CONFIG_SCHED_CACHE
>> +	unsigned int *pf;
> 
> So I'm all for short names; but perhaps this could be better. When
> reading this my brain went page-fault, and then WTF :-)
> 

OK, I assume you are suggesting renaming it to llc_counts.

>> +#endif
>> +
>>   #ifdef CONFIG_SCHEDSTATS
>>   	/* sched_balance_rq() stats */
>>   	unsigned int lb_count[CPU_MAX_IDLE_TYPES];
> 
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index ca46b5cf7f78..dae78b5915a7 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
> 
>> @@ -2723,6 +2778,13 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>>   	}
>>   	rcu_read_unlock();
>>   
>> +	/*
>> +	 * Ensure we see enlarged sd->pf when we use new llc_ids and
>> +	 * bigger max_llcs.
>> +	 */
>> +	smp_mb();
>> +	max_llcs = tl_max_llcs;
> 
> This seems wrong. This is *after* cpu_attach_domain() which publishes
> @sd. How about you do something like:
> 
> struct sched_domain {
> 	...
> 
> 	unsigned int llc_max;
> 	unsigned int *llc_counts __counted_by(llc_max);
> }
> 
> Then you always carry matching information that is published together.
>

OK, we will change it accordingly.

Additionally, with this change we should be able to safely read
the data in the sched_domain by verifying whether the target llc_id
falls within the valid range(to avoid a race condition):

   CPU0                                CPU1
   :
   ...
   build_sched_domains                 update_sg_lb_stats
                                         for_each_cpu_and(i, sg)
                                           sd=rq[i]->sd
     per_cpu(sd_llc_id,i)=new_llc
                                           llc=llc_id(i)
                                           if(llc<sd->llc_max)
                                             safe read sd->pf[llc]
     alloc_sd_pref(cpu_map)
       sd->llc_counts=kzalloc()
       sd->llc_max=max_llc

Thanks,
Chenyu