linux-kernel - Re: [PATCH 8/9] padata: unbind parallel jobs from specific CPUs

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Message-ID: <dd31ec02-4a52-995d-801a-95bec39f4944@oracle.com>
Date:   Thu, 22 Aug 2019 18:13:15 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Steffen Klassert <steffen.klassert@...unet.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, linux-crypto@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/9] padata: unbind parallel jobs from specific CPUs

On 8/22/19 12:13 AM, Herbert Xu wrote:
> On Mon, Aug 12, 2019 at 08:52:23PM -0400, Daniel Jordan wrote:
>>
>> @@ -191,22 +184,25 @@ static struct padata_priv *padata_get_next(struct parallel_data *pd)
>>   		padata = list_entry(reorder->list.next,
>>   				    struct padata_priv, list);
>>   
>> -		list_del_init(&padata->list);
>> -		atomic_dec(&pd->reorder_objects);
>> +		/*
>> +		 * The check fails in the unlikely event that two or more
>> +		 * parallel jobs have hashed to the same CPU and one of the
>> +		 * later ones finishes first.
>> +		 */
>> +		if (padata->seq_nr == pd->processed) {
>> +			list_del_init(&padata->list);
>> +			atomic_dec(&pd->reorder_objects);
> 
> Now that you've changed the test for whether there is work to be
> done you also need to update the code at the end of padata_reorder
> that checks whether there is work to do.  Otherwise we can end up
> in a busy loop that just wastes CPU cycles.

So we can, thanks for catching that.