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

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Date:	Tue, 30 Jul 2013 21:16:02 -0700
From:	Marc C <marc.ceeeee@...il.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Alexander Gordeev <agordeev@...hat.com>,
	Jens Axboe <axboe@...nel.dk>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Mike Christie <michaelc@...wisc.edu>,
	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

>> One thing which would probably be worthwhile tho is getting rid of the
>> bitmap based qc tag allocator in libata.  That one is just borderline
>> stupid to keep around on any setup which is supposed to be scalable.
> Your border might be wider than mine :-). Yes, the bitmap should
> definitely go.
A naive implementation is obviously less-than-efficient. However, what
other problems exist with the libata QC tag allocator? I highly doubt
SATA will change to beyond 32 queue tags, primarily because it would
be a pain to change SDB FIS (it's likely to break the dozens of AHCI
controller implementations out there). Further, it seems like the
industry stopped caring about SATA and is pushing NVMe for future
offerings.

In any event, most modern systems should have instructions to count
leading zeroes and modify bits atomically.

-MC

On Mon, Jul 29, 2013 at 12:19 PM, Nicholas A. Bellinger
<nab@...ux-iscsi.org> wrote:
> On Mon, 2013-07-29 at 13:18 +0200, Alexander Gordeev wrote:
>> On Fri, Jul 26, 2013 at 05:43:13PM -0700, Nicholas A. Bellinger wrote:
>> > On Fri, 2013-07-26 at 14:14 -0700, Nicholas A. Bellinger wrote:
>
> <SNIP>
>
>> I also tried to make a "quick" conversion and hit the same issue(s) as you.
>> Generally, I am concerned with these assumptions in such approach:
>>
>> 1. While libata concept of tags matches nicely with blk-mq (blk_mq_hw_ctx::
>> rqs[] vs ata_port::qcmd[]) right now, it is too exposed to changes in blk-mq
>> in the long run. I.e. ata_link::sactive limits tags to indices, while tags
>> might become hashes. Easily fixable, but still.
>>
>> 2. Unallocated requests in blk-mq are accessed/analized from libata-eh.c as
>> result of such iterations:
>>
>>         for (tag = 0; tag < ATA_MAX_QUEUE; tag++) {
>>                 qc = __ata_qc_from_tag(ap, tag);
>>
>>                 if (!(qc->flags & ATA_QCFLAG_FAILED))
>>                         continue;
>>
>>               ...
>>       }
>>
>> While it is probably okay right now, it is still based on a premise that
>> blk-mq will not change the contents/concept of "payload", i.e. from embedded
>> to (re-)allocated memory.
>>
>> > The thing that I'm hung up on now for existing __ata_qc_from_tag() usage
>> > outside of the main blk_mq_ops->queue_rq -> SHT->queuecommand_mq()
>> > dispatch path, is how to actually locate the underlying scsi_device ->
>> > request_queue -> blk_mq_ctx -> blk_mq_hw_hctx from the passed
>> > ata_port..?
>>
>> I am actually in favor of getting rid of ata_queued_cmd::tag. Converting
>> ata_link::sactive to a list, making ata_link::active_tag as struct
>> ata_queued_cmd *ata_link::active_qc and converting ata_port::qc_allocated to a
>> list seems solves it all, including [2]. Have not checked it though.
>>
>> Anyway, if we need a blk-mq tag (why?), we have qc->scsicmd->request->tag.
>>
>
> Hi Alexander,
>
> So given the feedback from Tejun, I'm going to setup back for the moment
> from a larger conversion, and keep the SHT->cmd_size=0 setting for
> libata in the scsi-mq WIP branch.
>
> I'm happy to accept patches to drop the bitmap piece that Tejun
> mentioned if your interested, but at least on my end right now there are
> bigger fish to fry for scsi-mq.  ;)
>
> Thanks,
>
> --nab
>
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