linux-kernel - Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense buffer

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Message-Id: <20090526142346O.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 26 May 2009 14:23:38 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	bharrosh@...asas.com
Cc:	jens.axboe@...cle.com, fujita.tomonori@....ntt.co.jp,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, david@...morbit.com, hch@...radead.org,
	akpm@...ux-foundation.org, jack@...e.cz,
	yanmin_zhang@...ux.intel.com
Subject: Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense
 buffer

On Mon, 25 May 2009 12:28:01 +0300
Boaz Harrosh <bharrosh@...asas.com> wrote:

> On 05/25/2009 10:30 AM, Jens Axboe wrote:
> > Fold the sense buffer into the command, thereby eliminating a slab
> > allocation and free per command.
> > 
> > Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> 
> Jens Hi.
> 
> I'm "TO:" this to Tomo.
> 
> This is the way it used to be for a long time. It was only recently changed by
> Tomo because of a bug on none-cache-coherent arches that need to dma-access the
> sense_buffer and also on the other hand change scsi_cmnd members by CPU.
> 
> In my opinion all you need is an __aligned(SMP_CACHE_BYTES) declaration at
> sense_buffer[] and let there be a hole at the end before the array. But Tomo
> did not like that, so he separated the two.

IIRC, it was not my opinion :) I don't think that putting
CACHE_ALIGNMENT here is a good idea though.

If this separated sense buffer allocation actually hurts the
performance, then I prefer the ->alloc_cmnd and ->destroy_cmnd hook
idea. Then most of llds are happy about the current sense buffer
scheme and some can use ->alloc_cmnd and ->destroy_cmnd hooks for the
better performance.
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