linux-kernel - Re: [PATCH v2] sched/fair: Fix non-empty throttled_limbo_list warning in tg_throttle

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Message-ID: <b2adb9ec-b4e3-4744-afee-8b93f2185837@amd.com>
Date: Tue, 28 Oct 2025 08:33:15 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Aaron Lu <ziqianlu@...edance.com>, Hao Jia <jiahao.kernel@...il.com>
CC: <mingo@...hat.com>, <peterz@...radead.org>, <mingo@...nel.org>,
	<juri.lelli@...hat.com>, <vincent.guittot@...aro.org>,
	<dietmar.eggemann@....com>, <rostedt@...dmis.org>, <bsegall@...gle.com>,
	<mgorman@...e.de>, <vschneid@...hat.com>, <linux-kernel@...r.kernel.org>,
	"Hao Jia" <jiahao1@...iang.com>
Subject: Re: [PATCH v2] sched/fair: Fix non-empty throttled_limbo_list warning
 in tg_throttle_down()

Hello Aaron,

On 10/27/2025 5:32 PM, Aaron Lu wrote:
> On Mon, Oct 27, 2025 at 05:05:34PM +0800, Hao Jia wrote:
>> @@ -6403,7 +6407,7 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>>   * expired/exceeded, otherwise it may be allowed to steal additional ticks of
>>   * runtime as update_curr() throttling can not trigger until it's on-rq.
>>   */
>> -static void check_enqueue_throttle(struct cfs_rq *cfs_rq)
>> +static void check_enqueue_throttle(struct cfs_rq *cfs_rq, int flags)
>>  {
>>  	if (!cfs_bandwidth_used())
>>  		return;
>> @@ -6418,6 +6422,13 @@ static void check_enqueue_throttle(struct cfs_rq *cfs_rq)
>>  
>>  	/* update runtime allocation */
>>  	account_cfs_rq_runtime(cfs_rq, 0);
>> +	/*
>> +	 * Do not attempt to throttle on the cfs_rq unthrottle path.
>> +	 * and it must be placed after account_cfs_rq_runtime() to
>> +	 * prevent a possible missed start of the bandwidth timer.
> 
> Hi Prateek and Hao,
> 
> Does it matter to start the bw timer? If no cfs_rq gets throttled, the
> timer doesn't look that useful.

Ack! But if we've reached here with ENQUEUE_THROTTLE set, we are in fact
trying to enqueue a task during unthrottling and we only start the timer
if we don't have any bandwidth since __account_cfs_rq_runtime() would bail
out if "cfs_rq->runtime_remaining > 0"
> Also, account_cfs_rq_runtime() calls
> assign_cfs_rq_runtime() and if assign failed, it will do resched_curr()
> but since we do not throttle cfs_rq here, that resched would be useless.

resched only happens if "cfs_rq->curr" is set andcheck_enqueue_throttle() already bails out if "cfs_rq->curr" is set so
the resched is avoided.

-- 
Thanks and Regards,
Prateek