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Wed, 29 Oct 2025 14:23:58 +0000 Message-ID: <91cbbdb7-53fd-483a-8bc3-cd4e63713485@intel.com> Date: Wed, 29 Oct 2025 22:23:43 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 15/19] sched/fair: Respect LLC preference in task migration and detach To: K Prateek Nayak CC: Vincent Guittot , Juri Lelli , Dietmar Eggemann , "Steven Rostedt" , Ben Segall , Mel Gorman , Valentin Schneider , "Madadi Vineeth Reddy" , Hillf Danton , "Shrikanth Hegde" , Jianyong Wu , "Yangyu Chen" , Tingyin Duan , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Tim Chen , , Tim Chen , Peter Zijlstra , "Gautham R . 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>>>>   +#ifdef CONFIG_SCHED_CACHE >>>> +        /* >>>> +         * Don't detach more tasks if the remaining tasks want >>>> +         * to stay. We know the remaining tasks all prefer the >>>> +         * current LLC, because after order_tasks_by_llc(), the >>>> +         * tasks that prefer the current LLC are at the tail of >>>> +         * the list. The inhibition of detachment is to avoid too >>>> +         * many tasks being migrated out of the preferred LLC. >>>> +         */ >>>> +        if (sched_cache_enabled() && detached && p->preferred_llc != -1 && >>>> +            llc_id(env->src_cpu) == p->preferred_llc) >>>> +            break; >>> >>> In all cases? Should we check can_migrate_llc() wrt to util migrated and >>> then make a call if we should move the preferred LLC tasks or not? >>> >> >> Prior to this "stop of detaching tasks", we performed a can_migrate_task(p) >> to determine if the detached p is dequeued from its preferred LLC, and in >> can_migrate_task(), we use can_migrate_llc_task() -> can_migrate_llc() to >> carry out the check. That is to say, only when certain tasks have been >> detached, will we stop further detaching. >> >>> Perhaps disallow it the first time if "nr_balance_failed" is 0 but >>> subsequent failed attempts should perhaps explore breaking the preferred >>> llc restriction if there is an imbalance and we are under >>> "mig_unrestricted" conditions. >>> >> >> 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 > potential imbalance for the "migrate_load" case. > > Instead, we can account for the util moved out of the src_llc and > after accounting for it, check if can_migrate_llc() would return > "mig_forbid" for the src llc. > I see your point, the original decision matrix intends to spread the tasks when both LLCs are overloaded. (src is the preferred LLC, dst is non-preferred LLC) src \ dst 30% 40% 50% 60% 30% N N N N 40% N N N N 50% N N G G 60% Y N G G src : src_util dst : dst_util Y : Yes, migrate N : No, do not migrate G : let the Generic load balance to even the load. I suppose the reason why the code breaks the rule here is because as Tim mentioned in another thread, to inhibit the task bouncing between LLCs. thanks, Chenyu