Subject: Re: [PATCH 1/3] blk-mq: add an API to estimate hardware queue node
To: Shaohua Li <shli@fb.com>, Christoph Hellwig <hch@infradead.org>
References: <68fed570910230ce847f8f3b685eeea399640a7f.1458941500.git.shli@fb.com>
 <20160329072443.GA18920@infradead.org>
 <20160329164722.GA1208161@devbig084.prn1.facebook.com>
CC: <linux-block@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <Kernel-team@fb.com>
From: Jens Axboe <axboe@fb.com>
Message-ID: <56FAB243.5020804@fb.com>
Date: Tue, 29 Mar 2016 10:50:11 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.6.0
MIME-Version: 1.0
In-Reply-To: <20160329164722.GA1208161@devbig084.prn1.facebook.com>
Content-Type: text/plain; charset="windows-1252"; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1143
Lines: 23

On 03/29/2016 10:47 AM, Shaohua Li wrote:
> On Tue, Mar 29, 2016 at 12:24:43AM -0700, Christoph Hellwig wrote:
>> On Fri, Mar 25, 2016 at 02:36:30PM -0700, Shaohua Li wrote:
>>> we allocate most data structure in device's node, but some data
>>> structures are not for DMA and mostly used by specific cpus/node which
>>> could diff from device's node. Allocating such hot data in device's
>>> node doesn't make sense. Add an API to estimate hardware queue node.
>>> This can be used before blk-mq actually establishes the mapping. This
>>> API runs slow, but it only used in initialization time.
>>
>> I think this is the wrong way around.  I've got some proprotype code
>> that just leaves the cpu assignments to the drivers and picks it up
>> in blk-mq.  Give me a few days to post it..
>
> This looks weird, shouldn't the cpu assignment be determined by block
> core (blk-mq) because block core decides how to use the queue?

I agree, that belongs in the blk-mq proper, the driver should just 
follow the rules outlined, not impose their own in this regard. It'll 
also help with irq affinity mappings, once we get that in.

-- 
Jens Axboe