linux-kernel - Re: [PATCH] scsi/fcoe: simplify fcoe_select

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Message-ID: <2455a292-5bba-4e6e-ab85-4fed0f917d7d@suse.de>
Date: Thu, 5 Jun 2025 08:01:40 +0200
From: Hannes Reinecke <hare@...e.de>
To: Yury Norov <yury.norov@...il.com>, Bart Van Assche <bvanassche@....org>
Cc: "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
 "Martin K. Petersen" <martin.petersen@...cle.com>,
 linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
 Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] scsi/fcoe: simplify fcoe_select_cpu()

On 6/5/25 02:35, Yury Norov wrote:
> + Tejun, Lai
> 
> On Thu, Jun 05, 2025 at 08:13:53AM +0800, Bart Van Assche wrote:
>> On 6/5/25 7:42 AM, Yury Norov wrote:
>>> diff --git a/drivers/scsi/fcoe/fcoe.c b/drivers/scsi/fcoe/fcoe.c
>>> index b911fdb387f3..07eddafe52ff 100644
>>> --- a/drivers/scsi/fcoe/fcoe.c
>>> +++ b/drivers/scsi/fcoe/fcoe.c
>>> @@ -1312,10 +1312,7 @@ static inline unsigned int fcoe_select_cpu(void)
>>>    {
>>>    	static unsigned int selected_cpu;
>>> -	selected_cpu = cpumask_next(selected_cpu, cpu_online_mask);
>>> -	if (selected_cpu >= nr_cpu_ids)
>>> -		selected_cpu = cpumask_first(cpu_online_mask);
>>> -
>>> +	selected_cpu = cpumask_next_wrap(selected_cpu, cpu_online_mask);
>>>    	return selected_cpu;
>>>    }
>>
>> Why does this algorithm occur in the FCoE driver? Isn't
>> WORK_CPU_UNBOUND good enough for this driver? And if it isn't
>> good enough, shouldn't this kind of functionality be integrated in
>> kernel/workqueue.c rather than having the above algorithm in a
>> kernel driver?
> 
> (I'm obviously not an expert in this driver, and just wanted to cleanup
> the cpumask API usage.)
> 
> It looks like the intention is to distribute the workload among CPUs
> sequentially. If you move this function out of the driver, someone
> else may call the function, and sequential distribution may get
> broken.
> 
> If sequential distribution doesn't matter here, and the real
> intention is just to distribute workload more or less evenly,
> we already have cpumask_any_distribute() for this.
> 
This function is used to distribute incoming skbs onto a work
cpu. And it's actually quite pointless, as the skb already
has a field (skb->sk->sk_incoming_cpu) which tells you exactly
on which CPU this skb was received, so we should use that
here.

I'll send a patch.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich