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

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Date:	Thu, 18 Jul 2013 12:14:51 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	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 Thu, 2013-07-18 at 11:51 -0700, 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()..
> 
> Jens..?
> 

Actually sorry, your right.  A call to blk_mq_insert_request() for
REQ_TYPE_BLOCK_PC will not invoke blk_queue_bounce() located near the
top of blk_mq_execute_rq(), which means that only REQ_TYPE_FS is
currently using bounce buffers, if required.

Need to think a bit more about what to do here for REQ_TYPE_BLOCK_PC
bounce buffer special case with blk_execute_rq(), but I'm thinking that
blk_mq_execute_rq() should really not be used here..

Jens..?

--nab

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