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

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Date:	Wed, 27 May 2009 10:54:41 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	James.Bottomley@...senPartnership.com, jens.axboe@...cle.com,
	rdreier@...co.com, 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,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense
 buffer

On 05/27/2009 04:36 AM, FUJITA Tomonori wrote:
> On Tue, 26 May 2009 19:05:05 +0300
> Boaz Harrosh <bharrosh@...asas.com> wrote:
> 
>> On 05/26/2009 06:31 PM, FUJITA Tomonori wrote:
>>> Can we just fix some drivers not to do the DMA with the sense buffer in
>>> scsi_cmnd? IIRC, there are only five or six drivers that do such.
>> This is not so.
>> All drivers that go through scsi_eh_prep_cmnd() will eventually DMA through
>> the regular read path. Including all the drivers that do nothing and let
>> scsi-ml do the REQUEST_SENSE
>>
>> Actually I have exact numbers, from the last time I did all that
> 
> Hmm, we discussed this before, I think.
> 

Sure we did I sent these patches. to summarize, 3 types of drivers:
1. Only memcpy into sense_buffer				- 60%
2. Use scsi_eh_prep_cmnd and DMA read into sense.
2.1 Do nothing and scsi-ml does scsi_eh_prep_cmnd		- 30%
3. Prepare DMA descriptors for sense_buffer before execution	- 10%

> scsi-ml uses scsi_eh_prep_cmnd only via scsi_send_eh_cmnd(). There are
> some users of scsi_send_eh_cmnd in scsi-ml but only scsi_request_sense
> does the DMA in the sense_buffer of scsi_cmnd.
> 

Also drivers use scsi_eh_prep_cmnd at interrupt time and proceed to
DMA into the sense_buffer.

> Only scsi_error_handler() uses scsi_request_sense() and
> scsi_send_eh_cmnd() works synchronously. So scsi-ml can easily avoid
> the the DMA in the sense_buffer of scsi_cmnd if we have one sense
> buffer per scsi_host.

Not so. As James explained then, once you have a CHECK_CONDITION return, the
Q-per-host is frozen, yes. But as soon as you send the REQUEST_SENSE the 
target Q is unfrozen again and all in-flight commands can error, much before
the REQUEST_SENSE returns.

Boaz
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