Re: [PATCH 03/13] scsi: unify allocation of scsi command and sensebuffer

[FUJITA Tomonori]

On Wed, 27 May 2009 10:54:41 +0300
Boaz Harrosh <bharrosh@xxxxxxxxxxx> wrote:

> On 05/27/2009 04:36 AM, FUJITA Tomonori wrote:
> > On Tue, 26 May 2009 19:05:05 +0300
> > Boaz Harrosh <bharrosh@xxxxxxxxxxx> 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.

Hmm, I'm not sure what you mean.

Why is 'all in-flight commands can error' a problem? The sense_buffer
per host is used by only scsi_eh kernel thread.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/