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

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Message-ID: <c67f70c5-1082-47e7-8270-f4b8ae05eace@amd.com>
Date: Fri, 31 Oct 2025 09:02:06 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Tim Chen <tim.c.chen@...ux.intel.com>, "Chen, Yu C" <yu.c.chen@...el.com>
CC: Vincent Guittot <vincent.guittot@...aro.org>, Juri Lelli
	<juri.lelli@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>, "Steven
 Rostedt" <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, "Madadi Vineeth
 Reddy" <vineethr@...ux.ibm.com>, Hillf Danton <hdanton@...a.com>, "Shrikanth
 Hegde" <sshegde@...ux.ibm.com>, Jianyong Wu <jianyong.wu@...look.com>, Yangyu
 Chen <cyy@...self.name>, Tingyin Duan <tingyin.duan@...il.com>, "Vern Hao"
	<vernhao@...cent.com>, Len Brown <len.brown@...el.com>, Aubrey Li
	<aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>, Chen Yu
	<yu.chen.surf@...il.com>, Adam Li <adamli@...amperecomputing.com>, Tim Chen
	<tim.c.chen@...el.com>, <linux-kernel@...r.kernel.org>, Peter Zijlstra
	<peterz@...radead.org>, "Gautham R . Shenoy" <gautham.shenoy@....com>, "Ingo
 Molnar" <mingo@...hat.com>
Subject: Re: [PATCH 15/19] sched/fair: Respect LLC preference in task
 migration and detach

Hello Tim,

On 10/31/2025 1:37 AM, Tim Chen wrote:
> On Thu, 2025-10-30 at 09:49 +0530, K Prateek Nayak wrote:
>> 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
> 
> In the code chunk (with fix I mentioned in last reply):
> 
> +#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 &&
> 		    llc_id(env->dst_cpu) != p->preferred_llc)
> +			break;
> 
> We have already pulled at least one task when we stop detaching because we
> know that all the remaining tasks want to stay in it current LLC.
> "detached" is non zero when we break. So LBF_ALL_PINNED would be cleared.
> We will only exit the detach_tasks loop when there are truly no tasks
> that can be moved and it is truly a LBF_ALL_PINNED case.

So what I was suggesting is something like:

@@ -10251,6 +10252,7 @@ static int detach_tasks(struct lb_env *env)
 	unsigned long util, load;
 	struct task_struct *p;
 	int detached = 0;
+	bool preserve_preferred;
 
 	lockdep_assert_rq_held(env->src_rq);
 
@@ -10268,6 +10270,10 @@ static int detach_tasks(struct lb_env *env)
 
 	tasks = order_tasks_by_llc(env, &env->src_rq->cfs_tasks);
 
+	preserve_preferred = sched_cache_enabled() &&
+			     !(env->sd->flags & SD_SHARE_LLC) &&
+			     !sd->nr_balance_failed;
+
 	while (!list_empty(tasks)) {
 		/*
 		 * We don't want to steal all, otherwise we may be treated likewise,
@@ -10370,16 +10376,15 @@ static int detach_tasks(struct lb_env *env)
 
 #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.
+		 * We've hit tasks that prefer src LLC while balancing between LLCs.
+		 * If previous balances have been successful, pretend the rest of the
+		 * tasks on this CPU are pinned and let the main load balancing loop
+		 * find another target CPU to pull from if imbalance exists.
 		 */
-		if (sched_cache_enabled() && detached && p->preferred_llc != -1 &&
-		    llc_id(env->src_cpu) == p->preferred_llc)
+		if (preserve_preferred && detached && llc_id(env->src_cpu) == p->preferred_llc) {
+			env->flags |= LBF_ALL_PINNED;
 			break;
+		}
 #endif
 

-- 
Thanks and Regards,
Prateek