linux-kernel - Re: [PATCH 2/4] sched/fair: Still look for the idlest cpu with no matching cookie

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Message-ID: <20250923073232.1067189-1-sieberf@amazon.com>
Date: Tue, 23 Sep 2025 09:32:31 +0200
From: Fernand Sieber <sieberf@...zon.com>
To: K Prateek Nayak <kprateek.nayak@....com>
CC: <mingo@...hat.com>, <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<bristot@...hat.com>, <vschneid@...hat.com>, <dwmw@...zon.co.uk>,
	<jschoenh@...zon.de>, <liuyuxua@...zon.com>
Subject: Re: [PATCH 2/4] sched/fair: Still look for the idlest cpu with no matching cookie

Hi Prateek,

On 9/23/2025 7:21 AM, K Prateek Nayak wrote:
> Based on the above return in __sched_balance_find_dst_group_cpu(), it
> should always return a valid CPU since "least_loaded_cpu" is initialized
> to "this_cpu".
>
> So, under what circumstance does "cpu" here turns out to be < 0?
> Am I missing something?

Hey Prateek. Thanks for the catch. I'll fix as follows for next rev:

+/*
+ * sched_balance_find_dst_group_cpu - find the idlest CPU among the CPUs in the group.
+ */
+static inline int
+sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
+{
+	int cpu = __sched_balance_find_dst_group_cpu(group, p, -1, true);
+	return cpu >= 0 ? cpu : __sched_balance_find_dst_group_cpu(group, p, this_cpu, false);
+}
+

Thanks,
Fernand



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