linux-kernel - Re: [PATCH 5/7] blk-mq: initialize request on allocation

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Message-ID: <CACVXFVNR-FQPkN9j41GKtx71rXR=u0tJZbeWRf7c3setm5m+FA@mail.gmail.com>
Date:	Thu, 17 Apr 2014 23:07:59 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Jens Axboe <axboe@...nel.dk>, Matias Bjorling <m@...rling.me>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux SCSI List <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH 5/7] blk-mq: initialize request on allocation

On Thu, Apr 17, 2014 at 10:57 PM, Christoph Hellwig <hch@....de> wrote:
> On Thu, Apr 17, 2014 at 10:54:23PM +0800, Ming Lei wrote:
>> On Mon, Apr 14, 2014 at 4:30 PM, Christoph Hellwig <hch@....de> wrote:
>> > If we want to share tag and request allocation between queues we cannot
>> > initialize the request at init/free time, but need to initialize it
>> > at allocation time as it might get used for different queues over its
>> > lifetime.
>>
>> Could you explain the use pattern? Looks you mean there are
>> still users of the tag/req even after it is freed, that looks a bit
>> weird since the tag/req can still be reallocated in another path
>> after it is freed.
>
> No difference in use pattern.  But blk_rq_init initializes the rq->q field,
> and a request might get reused for a different queue.

If so, it may be better to only initialize the rq->q in allocation
because Jens said 'It's done that way because of presumed cache
hotness on completion'.


Thanks,
-- 
Ming Lei
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