Re: [PATCH] sched/cache: Avoid publishing partial LLC utilization

[Jemmy Wong <jemmywong512@gmail.com>]

Hi Prateek,

> On Oct 12, 2026, at 11:34 AM, K Prateek Nayak <kprateek.nayak@amd.com> wrote:
> 
> Hello Jemmy,
> 
> On 10/10/2026 5:04 PM, Jemmy Wong wrote:
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -12121,6 +12121,13 @@ static void record_sg_llc_stats(struct lb_env *env,
>> if (env->sd->child != rcu_dereference_all(per_cpu(sd_llc, env->dst_cpu)))
>> return;
>> 
>> + /*
>> +  * Affinity retries can remove CPUs from env->cpus. Do not publish
>> +  * partial utilization against the capacity of the whole group.
>> +  */
>> + if (!cpumask_subset(sched_group_span(group), env->cpus))
>> + return;
>> +
> 
> nit.
> 
> When sched_balance_rq() does a "redo" and arrives here, env->flags
> will always have LBF_ALL_PINNED set.
> 
> That would be a much cheaper check compared to cpumask_subset() to
> ensure env->cpus covers all CPUs.
> 
> Thoughts?
> 
>> /*
>>  * At this point we know this group spans a LLC domain.
>>  * Record the statistic of this group in its corresponding
>> --
>> 2.25.1
> 
> -- 
> Thanks and Regards,
> Prateek

The redo part is right, but LBF_ALL_PINNED reflects the previous
busiest rq, not whether the group being recorded is fully covered.

For example, if cpu1 in LLC0 {cpu0, cpu1} is rejected as all-pinned,
we remove it from env->cpus and redo. The next scan still carries
LBF_ALL_PINNED while recording LLC1 {cpu2, cpu3}, which remains fully
covered. The flag check would skip that update; cpumask_subset()
would not.

It also misses the hotplug window: when cpu1 in LLC0 {cpu0, cpu1}
becomes inactive, a new balance pass can exclude cpu1 from env->cpus
before the domains are rebuilt. The group span and capacity still
cover both CPUs, but utilization is summed only over cpu0.

LBF_ALL_PINNED is clear on this first pass, so the flag check would
allow that partial publication; cpumask_subset() would prevent it.

So I would keep cpumask_subset().

Thanks,
Jemmy