linux-kernel - Re: [PATCH] sched/topology: remove unneeded do while loop in cpu_attach

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Date:   Sun, 25 Jun 2023 09:59:36 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
CC:     Peter Zijlstra <peterz@...radead.org>, <mingo@...hat.com>,
        <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>, <linux-kernel@...r.kernel.org>,
        <yu.c.chen@...el.com>, <tim.c.chen@...el.com>
Subject: Re: [PATCH] sched/topology: remove unneeded do while loop in
 cpu_attach_domain()

On 2023/6/22 2:57, Ricardo Neri wrote:
> On Wed, Jun 21, 2023 at 06:11:59AM -0700, Ricardo Neri wrote:
>> On Wed, Jun 21, 2023 at 10:53:57AM +0800, Miaohe Lin wrote:
>>> On 2023/6/20 22:11, Peter Zijlstra wrote:
>>>> On Sat, Jun 17, 2023 at 04:19:26PM +0800, Miaohe Lin wrote:
>>>>> When sg != sd->groups, the do while loop would cause deadloop here. But
>>>>> that won't occur because sg is always equal to sd->groups now. Remove
>>>>> this unneeded do while loop.
>>>>
>>>> This Changelog makes no sense to me.. Yes, as is the do {} while loop is
>>>> dead code, but it *should* have read like:
>>>>
>>>> 	do {
>>>> 		sg->flags = 0;
>>>> 		sg = sg->next;
>>>> 	} while (sg != sd->groups);
>>
>> Yes, I agree that this is the correct solution.
> 
> I take this back. I think we should do this:
> 
> @@ -758,19 +758,14 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
>  		sd = sd->parent;
>  		destroy_sched_domain(tmp);
>  		if (sd) {
> -			struct sched_group *sg = sd->groups;
> -
>  			/*
>  			 * sched groups hold the flags of the child sched
>  			 * domain for convenience. Clear such flags since
>  			 * the child is being destroyed.
>  			 */
> -			do {
> -				sg->flags = 0;
> -			} while (sg != sd->groups);
> +			sd->groups->flags = 0;
>  
>  			sd->child = NULL;
> -		}
>  	}
>  
>  	sched_domain_debug(sd, cpu);
> 
> A comment from Chenyu made got me thinking that we should only clear the
> flags of the local group as viewed from the parent domain. This is because
> the domain being degenerated defines the flags of such group only.

This looks better to my patch. Thanks.