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

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Date:	Thu, 18 Jul 2013 18:03:45 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Mike Christie <michaelc@...wisc.edu>,
	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>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH RESEND 0/1] AHCI: Optimize interrupt processing

On Thu, 2013-07-18 at 18:30 -0600, Jens Axboe wrote:
> On Thu, Jul 18 2013, Nicholas A. Bellinger wrote:
> > On Thu, 2013-07-18 at 13:12 -0600, Mike Christie wrote:
> > > 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.
> > > 
> > 
> > <nod>, just noticed the blk_queue_bounce() in blk_rq_map_kern().  
> > 
> > Not sure why this doesn't seem to be doing what it's supposed to for
> > libata just yet..
> 
> How are you make the request from the bio? It'd be pretty trivial to
> ensure that it gets bounced properly... blk_mq_execute_rq() assumes a
> fully complete request, so it wont bounce anything.
> 

>From what I gather for REQ_TYPE_BLOCK_PC, scsi_execute() ->
blk_rq_map_kern() -> blk_rq_append_bio() -> blk_rq_bio_prep() is what
does the request setup from the bios returned by bio_[copy,map]_kern()
in blk_rq_map_kern() code.

blk_queue_bounce() is called immediately after blk_rq_append_bio() here,
which AFAICT looks like it's doing the correct thing for scsi-mq..

What is strange here is that libata-scsi.c CDB emulation code is doing
the same stuff for both INQUIRY (that seems to be OK) and READ_CAPACITY
(that is returning zeros), which makes me think that something else is
going on..

Alexander, where you able to re-test using sdev->sdev_mq_reg.queue_depth
= 1 in scsi_mq_alloc_queue()..?

--nab

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