Re: [PATCH 15/19] sched/fair: Respect LLC preference in task migration and detach

[K Prateek Nayak]

Hello Tim,

On 10/30/2025 2:39 AM, Tim Chen wrote:
>>> I suppose you are suggesting that the threshold for stopping task detachment
>>> should be higher. With the above can_migrate_llc() check, I suppose we have
>>> raised the threshold for stopping "task detachment"?
>>
>> Say the LLC is under heavy load and we only have overloaded groups.
>> can_migrate_llc() would return "mig_unrestricted" since
>> fits_llc_capacity() would return false.
>>
>> Since we are under "migrate_load", sched_balance_find_src_rq() has
>> returned the CPU with the highest load which could very well be the
>> CPU with with a large number of preferred LLC tasks.
>>
>> sched_cache_enabled() is still true and when detach_tasks() reaches
>> one of these preferred llc tasks (which comes at the very end of the
>> tasks list), 
>> we break out even if env->imbalance > 0 leaving
> 
> Yes, but at least one task has been removed to even the load (making forward progress) and
> the remaining tasks all wish to stay in the current LLC and will
> preferred not to be moved. My thought was to not even all the load out
> in one shot and pull more tasks out of their preferred LLC.
> If the imbalance still remain, we'll come to that in the next load balance.

In that case, can we spoof a LBF_ALL_PINNED for the case where we start
hitting preferred task. That way, the main lb loop will goto redo and
try to find another busy CPU to pull tasks from.

> 
> Pulling tasks more slowly when we come to tasks that preferred to stay (if possible)
> would also help to prevent tasks bouncing between LLC.
> 
> Tim
> 

-- 
Thanks and Regards,
Prateek