linux-kernel - Re: [PATCH 0/3] blk-mq & nvme: introduce .map

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Message-ID: <CACVXFVMMirA+5dMSK2TyRq4HMfOGBpHBcme5MfLx9iQF25tXWg@mail.gmail.com>
Date:	Wed, 30 Sep 2015 08:08:07 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Keith Busch <keith.busch@...el.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-nvme <linux-nvme@...ts.infradead.org>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 0/3] blk-mq & nvme: introduce .map_changed

On Wed, Sep 30, 2015 at 6:45 AM, Keith Busch <keith.busch@...el.com> wrote:
> On Tue, 29 Sep 2015, Ming Lei wrote:
>>
>> Yes, I thought of that before, but it has the following cons:
>>
>> - some drivers/devices may need different IRQ affinity policy, such as
>> virtio
>> devices which has its own set affinity handler(see
>> virtqueue_set_affinity()),
>
>
> That's not a very good example to support your cause; virtio_scsi's use
> is a perfect example for one that would benefit from letting blk-mq
> handle affinity. virtio_scsi sets affinity only when there is a 1:1
> mapping of cpu's to queue's, but this driver doesn't know the mapping
> that blk-mq used, creating a potentially less than optimal mapping.

The 1:1 mapping is introduced before blk-mq, and that doesn't mean we
have to do that for blk-mq.

Actualy I mean virtio-scsi just lets the 1st CPU of the cpumask handle
the virt-queue's irq, instead of all CPUs mapped to the hw queue(virt-queue).

>
>> - block core has to get the irq vector information which has to be
>> setup/finalized
>> before blk-mq uses that for setting irq affinity, for example, in case
>> NVMe's admin
>> queue, its vector can be changed after admin queue's initialization.
>
>
> Why do you want to put a hint on the admin queue's irq?

No, I don't want, and it is just a example, I mean other drivers/devices
may have this kind of situation too.

-- 
Ming Lei
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