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

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Message-Id: <20090526140918I.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 26 May 2009 14:08:57 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	fujita.tomonori@....ntt.co.jp
Cc:	hch@...radead.org, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, david@...morbit.com,
	akpm@...ux-foundation.org, jack@...e.cz,
	yanmin_zhang@...ux.intel.com, linux-scsi@...r.kernel.org
Subject: Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense
	buffer

On Tue, 26 May 2009 13:36:43 +0900
FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:

> On Mon, 25 May 2009 03:50:08 -0400
> Christoph Hellwig <hch@...radead.org> wrote:
> 
> > On Mon, May 25, 2009 at 09:46:47AM +0200, Jens Axboe wrote:
> > > > But that patch looks good to me, avoiding one allocation for each
> > > > command and simplifying the code.  I try to remember why these were
> > > > two slabs to start with but can't find any reason.
> > > > 
> > > > Btw, we might just want to declare the sense buffer directly as a sized
> > > > array in the scsi command as there really doesn't seem to be a reason
> > > > not to allocate it.
> > > 
> > > That is also a workable solution. I've been trying to cut down on the
> > > number of allocations required per-IO, and there's definitely still some
> > > low hanging fruit there. Some of it is already included, like the inline
> > > io_vecs in the bio.
> > 
> > Btw, one thing I wanted to do for years is to add ->alloc_cmnd and
> > ->destroy_cmnd method to the host template which optionally move the
> > command allocation to the LLDD.  That way we can embedd the scsi_cmnd
> > into the drivers per-commad structure and eliminate another memory
> > allocation.  Also this would naturally extend the keep one cmnd pool
> > to drivers without requiring additional code.  As a second step it
> > would also allow killing the scsi_host_cmd_pool byt just having
> > a set of library routines that drivers which need SLAB_CACHE_DMA can
> > use.
> 
> We discussed this idea when I rewrote the sense allocation code, I
> think.
> 
> I like that idea that unifying scsi_cmnd and llds' per-commad
> structure however there is one tricky thing about it.
> 
> Currently, a lld frees (or reuses) its per-commad structure when it
> calls scsi_done(). SCSI-ml uses scsi_cmd after that so we need to
> change the lifetime management (so we need to inspect all the llds,
> e.g. this change will break iscsi ldd).

Oops, as you said, this can be optional (so we don't need to convert
all llds). But as I said, this changes the definition of when
scsi_cmnd is free and ldds don't like that change, I think.


> With that change, we can't tell llds how many per-commad structure are
> possibly necessary. In general, LLDs want to know the maximum number
> of per-commad structure; drivers allocates the number of per-commad
> structure equal to host_template->can_queue.
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