linux-kernel - Re: [PATCH 05/10] block: remove per-queue plugging

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Message-ID: <4DABF1EA.3070301@fusionio.com>
Date:	Mon, 18 Apr 2011 10:10:18 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	NeilBrown <neilb@...e.de>
CC:	Mike Snitzer <snitzer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On 2011-04-18 09:25, NeilBrown wrote:
> On Mon, 18 Apr 2011 08:38:24 +0200 Jens Axboe <jaxboe@...ionio.com> wrote:
> 
>> On 2011-04-18 00:19, NeilBrown wrote:
>>> On Mon, 11 Apr 2011 14:11:58 +0200 Jens Axboe <jaxboe@...ionio.com> wrote:
>>>
>>>>> Yes.  But I need to know when to release the requests that I have stored.
>>>>> I need to know when ->write_pages or ->read_pages or whatever has finished
>>>>> submitting a pile of pages so that I can start processing the request that I
>>>>> have put aside.  So I need a callback from blk_finish_plug.
>>>>
>>>> OK fair enough, I'll add your callback patch.
>>>>
>>>
>>> But you didn't did you?  You added a completely different patch which is
>>> completely pointless.
>>> If you don't like my patch I would really prefer you said so rather than
>>> silently replace it with something completely different (and broken).
>>
>> First of all, you were CC'ed on all that discussion, yet didn't speak up
>> until now. This was last week. Secondly, please change your tone.
> 
> Yes, I was CC'ed on a discussion.  In that discussion it was never mentioned
> that you had completely changed the patch I sent you, and it never contained
> the new patch in-line for review.   Nothing that was discussed was
> particularly relevant to md's needs so there was nothing to speak up about.
> 
> Yes- there were 'git pull' requests and I could have done a pull myself to
> review the code but there seemed to be no urgency because you had already
> agreed to apply my patch.
> When I did finally pull the patches (after all the other issues had settle
> down and I had time to finish of the RAID side) I found ... what I found.
> 
> I apologise for my tone, but I was very frustrated.
> 
>>
>>> I'll try to explain again.
>>>
>>> md does not use __make_request.  At all.
>>> md does not use 'struct request'.  At all.
>>>
>>> The 'list' in 'struct blk_plug' is a list of 'struct request'.
>>
>> I'm well aware of how these facts, but thanks for bringing it up.
>>
>>> Therefore md cannot put anything useful on the list in 'struct blk_plug'.
>>>
>>> So when blk_flush_plug_list calls queue_unplugged() on a queue that belonged
>>> to a request found on the blk_plug list, that queue cannot possibly ever be
>>> for an 'md' device (because no 'struct request' ever belongs to an md device,
>>> because md doesn't not use 'struct request').
>>>
>>> So your patch (commit f75664570d8b) doesn't help MD at all.
>>>
>>> For md, I need to attach something to blk_plug which somehow identifies an md
>>> device, so that blk_finish_plug can get to that device and let it unplug.
>>> The most sensible thing to have is a completely generic callback.  That way
>>> different block devices (which choose not to use __make_request) can attach
>>> different sorts of things to blk_plug.
>>>
>>> So can we please have my original patch applied? (Revised version using
>>> list_splice_init included below).
>>>
>>> Or if not, a clear explanation of why not?
>>
>> So correct me if I'm wrong here, but the _only_ real difference between
>> this patch and the current code in the tree, is the checking of the
>> callback list indicating a need to flush the callbacks. And that's
>> definitely an oversight. It should be functionally equivelant if md
>> would just flag this need to get a callback, eg instead of queueing a
>> callback on the list, just set plug->need_unplug from md instead of
>> queuing a callback and have blk_needs_flush_plug() do:
>>
>>         return plug && (!list_empty(&plug->list) || plug->need_unplug);
>>
>> instead. Something like the below, completely untested.
>>
> 
> No, that is not the only real difference.
> 
> The real difference is that in the current code, md has no way to register
> anything with a blk_plug because you can only register a 'struct request' on a
> blk_plug, and md doesn't make any use of 'struct request'.
> 
> As I said in the Email you quote above:
> 
>>> Therefore md cannot put anything useful on the list in 'struct blk_plug'.
> 
> That is the heart of the problem.

Hmm, I don't really see a way to avoid the list in that case. You really
do need some way to queue items, a single callback or flag or pointer
will not suffice.

I've added the patch and removed the (now) useless ->unplugged_fn
callback. I suggest you base your md changes on top of my for-linus
branch and tell me when you are confident it looks good, then I'll pull
in your MD changes and submit them later today.

OK with you?

-- 
Jens Axboe

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