Re: [PATCH 07/19] sched/fair: Track LLC-preferred tasks per runqueue

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Chenyu,

On 10/28/2025 8:45 PM, Chen, Yu C wrote:
>> Instead of relying on reset_llc_stats() hack, I think a better approach
>> would be to have a "p->se.llc_sched_active" flag similar to how uclamp
>> has "uc_se->active" and we set this in account_llc_enqueue() which will
>> still check for sched_cache_enabled() but account_llc_dequeue() would
>> only check for "p->se.llc_sched_active" to decrement the stats and then
>> unset the flag.
>>
>> That way, we cannot have an imbalanced accounting. Thoughts?
>>
> 
> I suppose what you mean is to avoid the race condition between
> enabling sched_cache and EQ/DE_LLC, similar to uclamp:
> 
>         enqueue(taskA)
>         // sched_cache gets enabled
>         enqueue(taskB)
>         dequeue(taskA)
>         // Must not decrement rq->llc_pref for taskA
>         dequeue(taskB)

Yup! We can have

  enqueue(p)
    account_llc_enqueue(p)
      if (sched_cache_enabled())
        p->se.llc_sched_active = true;
        rq->nr_llc_running += (p->preferred_llc != -1);
        rq->nr_pref_llc_running += (p->preferred_llc == task_llc(p));

    ...

  dequeue(p)
    account_llc_dequeue(p)
      if (p->se.llc_sched_active)
        rq->nr_llc_running -= (p->preferred_llc != -1);
        rq->nr_pref_llc_running -= (p->preferred_llc == task_llc(p));


We can also have a single bit for "llc_sched_active" in the
task_struct next to the "sched_task_hot" bit instead of using the
hole in sched_entity after "sched_delayed"

-- 
Thanks and Regards,
Prateek