linux-kernel - Re: [PATCH RESEND 0/1] AHCI: Optimize interrupt processing

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Date:	Thu, 18 Jul 2013 13:12:50 -0600
From:	Mike Christie <michaelc@...wisc.edu>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
CC:	Alexander Gordeev <agordeev@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org,
	Jeff Garzik <jgarzik@...ox.com>, Jens Axboe <axboe@...nel.dk>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH RESEND 0/1] AHCI: Optimize interrupt processing

On 07/18/2013 12:51 PM, Nicholas A. Bellinger wrote:
> On Wed, 2013-07-17 at 18:19 +0200, Alexander Gordeev wrote:
>> On Tue, Jul 16, 2013 at 02:38:03PM -0700, Nicholas A. Bellinger wrote:
>>> [    7.927818] scsi_execute(): Calling blk_mq_free_request >>>
>>> [    7.927826] scsi 0:0:0:0: Direct-Access     ATA      ST9500530NS      CC03 PQ: 0 ANSI: 5
>>>
>>> OK, so INQUIRY response payload is looking as expected here.
>>
>> Yep. It is not on the top of my head, but I remember something like INQUIRYs
>> are emulated and thus do not have payload.
>>
>>> [    7.927960] sd 0:0:0:0: [sda] Sector size 0 reported, assuming 512.
>>> [    7.927964] sd 0:0:0:0: [sda] 1 512-byte logical blocks: (512 B/512 B)
>>> [    7.927965] sd 0:0:0:0: [sda] 0-byte physical blocks
>>>
>>> Strange..  READ_CAPACITY appears to be returning a payload as zeros..?
>>
>> Yep. Because blk_execute_rq() does not put the proper callback and data do
>> not get copied from sg's to bounce buffer. That is why I tried to use
>> blk_mq_execute_rq() instead. Once I do that, data start getting read and
>> booting stops elsewhere.
> 
> Mmmmmm.
> 
> The call to blk_queue_bounce() exists within blk_mq_make_request(), but
> AFAICT this should still be getting invoked regardless of if the struct
> request is dispatched into blk-mq via the modified blk_execute_rq() ->
> blk_execute_rq_nowait() -> blk_mq_insert_request() codepath, or directly
> via blk_mq_execute_rq()..
> 

blk_mq_make_request is not called from the blk insert/execute paths.
blk_mq_make_request takes a bio and tries to merge it with a request and
adds it to the queue. It is only called when the make_request_fn is
called like when generic_make_request is called.

blk_mq_insert_request adds a already formed request to the queue. It is
already formed so that is why that path does not bounce bios. The
bios/pages should already be added within the drivers restrictions. So
for the read_cap path, the call to blk_rq_map_kern in scsi_execute does
the blk_queue_bounce call.

Just saw this while trying out iscsi with the scsi-mq stuff :)

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