linux-kernel - Re: [PATCH 1/3] blk-mq: add an API to estimate hardware queue node

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Message-ID: <56FAB243.5020804@fb.com>
Date:	Tue, 29 Mar 2016 10:50:11 -0600
From:	Jens Axboe <axboe@...com>
To:	Shaohua Li <shli@...com>, Christoph Hellwig <hch@...radead.org>
CC:	<linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<Kernel-team@...com>
Subject: Re: [PATCH 1/3] blk-mq: add an API to estimate hardware queue node

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