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

From: Jens Axboe
Date: Mon Apr 11 2011 - 08:12:10 EST

On 2011-04-11 14:05, NeilBrown wrote:
> On Mon, 11 Apr 2011 13:37:20 +0200 Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote:
>> On 2011-04-11 13:26, NeilBrown wrote:
>>> On Mon, 11 Apr 2011 13:04:26 +0200 Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote:
>>>>> I'm sure one of us is missing something (probably both) but I'm not
>>>>> sure what.
>>>>> The callback is central.
>>>>> It is simply to use plugging in md.
>>>>> Just like blk-core, md will notice that a blk_plug is active and will put
>>>>> requests aside. I then need something to call in to md when blk_finish_plug
>>>> But this is done in __make_request(), so md devices should not be
>>>> affected at all. This is the part of your explanation that I do not
>>>> connect with the code.
>>>> If md itself is putting things on the plug list, why is it doing that?
>>> Yes. Exactly. md itself want to put things aside on some list.
>>> e.g. in RAID1 when using a write-intent bitmap I want to gather as many write
>>> requests as possible so I can update the bits for all of them at once.
>>> So when a plug is in effect I just queue the bios somewhere and record the
>>> bits that need to be set.
>>> Then when the unplug happens I write out the bitmap updates in a single write
>>> and when that completes, I write out the data (to all devices).
>>> Also in RAID5 it is good if I can wait for lots of write request to arrive
>>> before committing any of them to increase the possibility of getting a
>>> full-stripe write.
>>> Previously I used ->unplug_fn to release the queued requests. Now that has
>>> gone I need a different way to register a callback when an unplug happens.
>> Ah, so this is what I was hinting at. But why use the task->plug for
>> that? Seems a bit counter intuitive. Why can't you just store these
>> internally?
>>>>> is called so that put-aside requests can be released.
>>>>> As md can be built as a module, that call must be a call-back of some sort.
>>>>> blk-core doesn't need to register blk_plug_flush because that is never in a
>>>>> module, so it can be called directly. But the md equivalent could be in a
>>>>> module, so I need to be able to register a call back.
>>>>> Does that help?
>>>> Not really. Is the problem that _you_ would like to stash things aside,
>>>> not the fact that __make_request() puts things on a task plug list?
>>> Yes, exactly. I (in md) want to stash things aside.
>>> (I don't actually put the stashed things on the blk_plug, though it might
>>> make sense to do that later in some cases - I'm not sure. Currently I stash
>>> things in my own internal lists and just need a call back to say "ok, flush
>>> those lists now").
>> So we are making some progress... The thing I then don't understand is
>> why you want to make it associated with the plug? Seems you don't have
>> any scheduling restrictions, and in which case just storing them in md
>> seems like a much better option.
> 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.

Jens Axboe

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