linux-kernel - Re: [RFC PATCH] request: teach the device more intelligent

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Message-ID: <CAH9JG2VhPg-mk36CO_Jh8D0O6SW3RnZyjnBRNH4UqT8SXXWULQ@mail.gmail.com>
Date:	Wed, 10 Aug 2011 17:42:45 +0900
From:	Kyungmin Park <kmpark@...radead.org>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org, arnd@...db.de,
	jh80.chung@...sung.com, linux-mmc@...r.kernel.org
Subject: Re: [RFC PATCH] request: teach the device more intelligent

On Wed, Aug 10, 2011 at 5:08 PM, Jens Axboe <axboe@...nel.dk> wrote:
> On 2011-08-10 01:43, Kyungmin Park wrote:
>> On Wed, Aug 10, 2011 at 3:52 AM, Jens Axboe <axboe@...nel.dk> wrote:
>>> On 2011-08-09 05:47, Kyungmin Park wrote:
>>>> Hi Jens
>>>>
>>>> Now eMMC device requires the upper layer information to improve the data
>>>> performance and reliability.
>>>>
>>>> . Context ID
>>>> Using the context information, it can sort out the data internally and improve the performance.
>>>> The main problem is that it's needed to define "What's the context".
>>>> Actually I expect cfq queue has own unique ID but it doesn't so decide to use the pid instead
>>>>
>>>> . Data Tag
>>>> Using the Data Tag (1-bit information), It writes the data at SLC area when it's hot data. So it can make the chip more reliable.
>>>> First I expect the REQ_META but current ext4 doesn't pass the WRITE_META. only use the READ_META. so it needs to investigate it.
>>>>
>>>> With these characteristics, it's helpful to teach the device. After some consideration. it's needed to pass out these information at request data structure.
>>>>
>>>> Can you give your opinions and does it proper fields at requests?
>>>
>>> You need this to work on all IO schedulers, not just cfq.
>> Of course if the concept is acceptable, I'll add the other IO schedulers also.
>>
>>> And since that's the case, there's no need to add this field since you can just
>>> retrieve it if the driver asks for it. For CFQ, it could look like this:
>>>
>>> static int cfq_foo(struct request *rq)
>>> {
>>>        struct cfq_queue *cfqq = rq->elevator_private[1];
>>>
>>>        if (cfqq)
>>>                return cfqq->pid;
>>>
>>>        return -1;
>>> }
>>
>> The actual user of these information is device driver. e.g.,
>> drivers/mmc/card/block.c
>> So it's not good to use cfq data structure at D/D. some time later
>> these are also used at scsi device drivers.
>
> No, what I'm suggesting above is the CFQ implementation. You would need
> to wire up an elv_ops->get_foo() and have the IO schedulers fill it in.
> If you notice, the above function does not take or output anything
> related to CFQ in particular, it'll just return you the unique key you
> need.
>
> It's either the above, or a field in the request that the schedulers
> fill out. However, it'd be somewhat annoying to grow struct request for
> something that has a narrow scope of use. Hence the suggestion to add a
> strategy helper for this.
Okay, I'll add new elevator function one for getting context or more hints.
BTW, does it okay to call elevator function call at D/D?

The quick-n-dirty call is like this at "drivers/mmc/card/block.c"

                struct elevator_queue *e = md->queue.queue->elevator;
                int context = -1;

                if (e->ops->elevator_get_req_hint_fn && req) {
                        context = e->ops->elevator_get_req_hint_fn(req);

>
>>> As to the hot part, I think that would be better as just a request flag
>>> like eg the meta flag.
>> Yes it can use the JBD_flags at cfq. but same reason it's not proper
>> reference at device drivers. that's reason to make a filed at request.
>>
>> Device driver should or must see the request data structure and don't
>> refer the upper layer data structures.
>
> The device driver sees the struct request, which is where this flag ends
> up.
It's ext{3,4} series specific detection method. if user use the
another filesystem, it's difficult to detect hot/cold.
I'll more think about it.

Thank you for suggestion.
Kyungmin Park
>
> --
> Jens Axboe
>
>
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