linux-kernel - Re: [PATCH v1 1/5] block: blk-mq: avoid initializing request during its completion

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Date:	Wed, 1 Jan 2014 12:57:18 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v1 1/5] block: blk-mq: avoid initializing request during
 its completion

Hi Jens,

On Wed, Jan 1, 2014 at 12:38 AM, Jens Axboe <axboe@...nel.dk> wrote:
> On Thu, Dec 26 2013, Ming Lei wrote:
>> One problem is that request->start_time/start_time_ns could be
>> set as wrong.
>>
>> Also it is normal to intialize one data structure just after its
>> allocation.
>>
>> So move the initialization out of its completion path.
>
> It's done that way because of presumed cache hotness on completion,
> since we just touched a lot of the members. Lets just fix the start time
> issue by itself.

But some members of request are already touched in allocation
path(blk-mq core) too, and lots of members will be touched by
driver for starting the transfer.

Also I didn't observe obvious effect on L1 dcache load/store miss rate
after applying the patch when reading/writing on null_blk device.

Considered the theoretic advantage of reinitializing in free path, I
will just fix the start time issue by itself.

Thanks,
-- 
Ming Lei
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